General Chemistry Full Course PDF

Faculty of Science Chemistry Department ‫كلية فنىن جميلة المستىي االول برنامج عمارة‬ Prepared by Chemistry Department- Faculty of Science-Helwan University 0202-0202  ‫‪2‬‬ ‫الناشر‪ :‬جهاز نشر وتىزيع الكتاب الجامعي...

Faculty of Science Chemistry Department ‫كلية فنىن جميلة المستىي االول برنامج عمارة‬ Prepared by Chemistry Department- Faculty of Science-Helwan University 0202-0202  ‫‪2‬‬ ‫الناشر‪ :‬جهاز نشر وتىزيع الكتاب الجامعي – جامعة حلىان –‬ ‫حقىق التاليف محفىظة للمؤلف ‪0202 -‬‬  3 Preface General chemistry is the study of matter, energy, and the relations between them. The main topics in general chemistry include the structure of atoms, the periodic table and periodic trends and chemical bonding. This book includes an introduction to Atomic structure, periodicity and quantum numbers. Moreover, Theories that describe the covalent bonding and molecular shape, such as VB, MO and VESPER are also included. Moreover the second part of this book deals with topics like Building and finishing materials, dyes and pigments, and corrosion chemistry and how to prevent corrosion.  4 Table of content Chapter 1: Atomic Structure and Bonding 1.1. Introduction 1.2. Types of Bonds 1.2.1. Covalent Bond 1.2.2. Ionic Bond 1.2.3. Metallic Bond 1.2.4. Coordinate Bond 1.3. Physical Properties of Ionic and Covalent Compounds 1.4. Valence Bond Theory 1.5. Types of Covalent Bonds Sigma () and pi () Bonds 1.6. Valence-shell electron-pair repulsion (VSEPR) Theory 1.7. Hybridization 1.8. Van der Waals Interactions Chapter 2: Polymers 2.1. Introduction 2.2. Types of Polyethylene 2.3. Thermoplastic and Thermosetting Polymers 2.4. Addition Polymerization: 2.5. Processing Polymers 2.6. Rubber and Other Elastomers 2.7. Polymers in Paints 2.8. Condensation Polymers  5 2.9. Composite Materials Chapter 3: Building Materials 3.1. Introduction 3.2. Aggregates 3.3. Bricks 3.4. Lime 3.5. Cement 3.6. Concrete Chapter 4: Finishing Materials 4.1. Importance of Finishes 4.2. Wall finishes 4.2.1. Plastering 4.2.2. Pointing 4.2.3. Distempering 4.2.4. Painting Chapter 5: Dyes and Pigments 5.1. Introduction 5.2. Classification of dyes 5.3. Selection of Dyes 5.4. Considerations in Dye Design 5.5. Toxicological considerations 5.6. Dyes versus pigments  6 Chapter 6: Introduction to Corrosion 6.1. What is corrosion? 6.2. Steps of electrochemical reaction 6.3. Costs of corrosion 6.4. Polarization 6.5. Types of corrosion 6.6. Corrosion Protection and Control  7 CHAPTER 1 CHEMICAL STRUCTUES and BONDING Prepared by Dr. Bahaa Ahmed Salah  8 Chapter 1: Chemical Structure and Bonding 1.3. Introduction 1.4. Types of Bonds 1.2.1. Covalent Bond 1.2.2. Ionic Bond 1.2.3. Metallic Bond 1.2.4. Coordinate Bond 1.3. Physical Properties of Ionic and Covalent Compounds 1.4. Valence Bond Theory 1.5. Types of Covalent Bonds Sigma () and pi () Bonds 4.3. Valence-shell electron-pair repulsion (VSEPR) Theory 1.7. Hybridization 1.8. Van der Waals Interactions 1.9. Questions  9 Learning Objectives 1. Describe the nature of the attraction that leads to formation of various types of bonds. 2. Explain the difference between a covalent bond and an ionic bond. 3. Classify a covalent bond as polar or nonpolar. 4. Predict and explain the shapes of simple molecules 5. Classify a simple molecule as polar or nonpolar from its shape and the polarity of its bonds. 6. Predict and explain the types of hybridization of various molecules  10 Chapter 1: Chemical Structures and Bonding 1.1. Introduction The elements generally do not exist in the form of atoms (except noble gases) but prefer to form group of atoms (called molecules). The force by which the atoms attract each other to form such groups is called a bond. A chemical bond is a force of attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electrostatic force of attraction between opposite charges or between electrons and nuclei or sometimes as a result of a dipole attraction. An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond. Due to the  11 matter wave nature of electrons and their smaller mass, they must occupy a much larger amount of volume compared with the nuclei, and this volume occupied by the electrons keeps the atomic nuclei relatively far apart, as compared with the size of the nuclei themselves. This phenomenon limits the distance between nuclei and atoms in a bond. Normally, in the language of chemical bonding, the distance between two such nuclei of bonded atoms constitute a bond length. In general, strong chemical bonding is associated with the sharing or complete transference of electrons between the participating atoms. The atoms in molecules, crystals, metals and diatomic gases or one may say that most of the physical environment around is held together by chemical bonds, which dictate the structure and the bulk properties of matter. As the noble gases, which have eight electrons in valence shell, rarely form any bond, so the eight  12 electrons in valence shell (octet) was considered to be the most stable state of an element. On the basis of this, it was suggested by Lewis that the atoms combine to complete their octet by losing, gaining or sharing of electrons. But later on it was observed that some combinations of atoms likes SF6, PCl5 etc. also exist in which the central atoms (S and P) contain more than eight electrons in their valence shell. So another reason given for combination of atoms (bond formation) was minimization of energy. In other words, a bond is formed if it results in decrease in energy or increase in stability. 1.2. Types of Bonds As stated, a chemical bond is an attraction between atoms. This attraction may be seen as the result of different behaviors of the outermost electrons of atoms. Although all of these behaviors merge into each other seamlessly in various bonding situations  13 so that there is no clear line to be drawn between them, customarily the chemical bonds are classified into different types. One more point has to be reiterated before discussing the classification of chemical bond. Atom may attain a stable electronic configuration in three different ways by losing electron, by gaining or by sharing electron. Moreover, elements may be divided into following three types as: 1. Electropositive elements: Whose atom gives up one or more electron fairly readily 2. Electronegative elements: Which atom accepts electron. 3. Elements which have little tendency to lose or gain electrons. Three different types of bonds may be formed depending on the electropositive or electronegative character of atom involved.  14 Electropositive element + Electropositive element —> Metallic bond Electropositive element + Electronegative element —» Ionic bond Electronegative element + Electronegative element —> Covalent bond 1.2.1. Covalent Bond In the simplest view of a so-called Covalent bond, one or more electrons (often a pair of electrons) are drawn into the space between the two atomic nuclei. Here the negatively charged electrons are attracted to the positive charges of both nuclei, instead of just their own. This overcomes the repulsion between the two positively charged nuclei of the two atoms, and so this overwhelming attraction holds the two nuclei in a fixed configuration of equilibrium, even though they will still vibrate at equilibrium position. Thus, covalent bonding involves sharing of electrons in  15 which the positively charged nuclei of two or more atoms simultaneously attract the negatively charged electrons that are being shared between them. These bonds exist between two particular same or different atoms, and have a direction in space, allowing them to be shown as single connecting lines between atoms in drawings. For Example: Two chlorine atoms react to form a Cl2 molecule Each chlorine atom gives a share of one of its electrons to other atom. A pair of electrons is shared equally between both atoms and each atom now has eight electrons in its outer shell (stable octet). In a similar way a molecule of tetra chloromethane CC14 is made up of one carbon and four chlorine atoms.  16 The carbon atom is short of four electrons so as to have noble gas structure. Consequently, it forms four bonds with the chlorine atoms which themselves are short of one electron so they each form one bond. By sharing electrons in this way both carbon and all four chlorine atoms attain a noble gas structure. A molecule of ammonia NH3 is made up of one nitrogen and three hydrogen atom Other examples of covalent bonds include water (with two covalent bonds) and hydrogen - fluoride (one covalent bond and three lone pairs). In a polar covalent bond, one or more electrons are  17 unequally shared between two nuclei. Covalent bonds often result in the formation of small collections of better-connected atoms called molecules. These molecules in solid and liquid state are bound to other molecules by intermolecular forces that are often much weaker than the covalent bonds that hold the molecules internally together. Such weak intermolecular bonds give organic molecular substances, such as waxes and oils, their soft bulk character, and their low melting points (in liquids, molecules must cease most structured or oriented contact with each other). When covalent bonds link long chains of atoms in large molecules, however (as in polymers such as nylon), or when covalent bonds extend in networks through solids that are not composed of discrete molecules (such as diamond or quartz or the silicate minerals in many types of rock) then the structures that result may be both strong and  18 tough, at least in the direction oriented correctly with networks of covalent bonds. Also, the melting points of such covalent polymers and networks increase greatly. 1.2.2. Ionic Bond In a simplified view of an Ionic bond, the bonding electron is not shared at all, but completely transferred. In this type of bond, the outer atomic orbital of one atom has a vacancy which allows addition of one or more electrons. These newly added electrons potentially occupy a lower energy- state (effectively closer to more nuclear charge) than they experience in a different atom. Thus, one nucleus offers a more tightly bound position to an electron than does another nucleus, with the result that one atom may transfer an electron to the other. This transfer causes one atom to assume a net positive charge, and the other to assume a net negative charge. The bond thus results from  19 electrostatic attraction between the atoms become positive or negatively charged ions. For example, consider formation of an ionic compound like sodium chloride. When sodium and chlorine react together the outer electron of the sodium atom is transferred to the chlorine atom to produce sodium ion (Na+) and chloride ion (Cl-). Electrostatic attraction between the positive and negative ions holds the ions together in the crystal Lattice. Such bonds have no particular orientation in space, since they result from equal electrostatic attraction of each ion to all ions around them. So, it is crystal clear that ionic bonds are non-directional consequently ionic compounds do not have a shape. Ionic bonds are strong and thus ionic substances require high  20 temperatures to melt. Ionic compounds are brittle since the forces between ions are short-range, and do not easily bridge cracks and fractures. 1.2.3. Metallic Bond A less mentioned type of bonding is the metallic bond. In this type of bonding, all metallic atoms lodes their valence electron to form a pool of electrons, which is mobile. Leaving the valence electrons, the remainder portion of the metal atom is a positive ion called ―kernel‖. For example, in lithium each atom contributes are valence electron to the pool leaving behind Li+ ions; in case of Mg, each atom contributes two valence electrons to the pool leaving behind Mg2+ ions. These positive ions or kernels are held in the three-dimensional space in a definite pattern in the sea of mobile electrons. This model is called electron gas model because the electrons are free to move in all directions like the molecules of a gas.  21 Structure of metals (electron sea model) The simultaneous attraction between the kernels and the mobile electrons which holds the kernels together is called the metallic bond. Metallic bonding may be seen as an extreme example of delocalization of electrons over a large system of covalent bonds, in which every atom participates. This type of bonding is often very strong resulting in the tensile strength of metals. However, metallic bonds are more collective in nature than other types, and so they allow metal crystals to deform more easily, because they are composed of atoms attracted to each other, but not in any particularly-oriented ways. This results in  22 the malleability of metals. The sea of electrons in metallic bonds causes the characteristically good electrical and thermal conductivity of metals, and also their ―shiny‖ reflection of most frequencies of white light. All types of chemical bonds can be explained by quantum theory, but, in practice, simplification rules allow chemists to predict the strength, directionality, and polarity of bonds. The octet rule and VSEPR theory are two examples. More sophisticated theories are valence bond theory which includes orbital hybridization and resonance, and the linear combination of atomic orbitals which is the basis for the extraordinary molecular orbital method. 1.2.4. Coordinate Bond A covalent bond results from sharing of a pair of electrons between two atoms, where each atom contributes one electron to the bond. It is also possible to have an electron pair  23 bond where both the electrons come from one of the two binding atoms and there is no contribution from the other atom. Such bonds are called coordinate bonds or dative bonds. So, coordinate bond is a special type of covalent bond in which both the bonded electrons come from one of the two binding atoms. One common example is formation of ammonium ion. Even though the ammonia molecule has a stable electronic configuration it can react with a hydrogen ion (H+) by donating a lone pair of electrons from N atom to H+ ion forming the ammonium ion NH+4. Covalent bonds are usually shown as a straight line joining the two atoms, and coordinate bonds as arrows indicating which atom is donating the  24 electron. Similarly, ammonia donates its lone pair to boron trifluoride and by this means the boron atom attains noble gas configuration. In a similar way a molecule of BF3 can form a coordinate bond by accepting a lone pair from a F- ion. Double and Triple Bonds Sometimes more than two electrons are shared between a pair of atoms. If four electrons are shared then there are two bonds and this arrangement is called a double bond. If six  25 electrons are shared then there are three bonds and this is called a triple bond. 1.3. Physical Properties of Ionic and Covalent Compounds Melting Point Ionic compounds are typically solids and usually have high melting and boiling points. In contrast covalent compounds are typically gases, liquids or low melting solids. These differences occur because of differences in bonding and structure. Ionic compounds are made up of positive and negative ions arranged in a regular way in a lattice. The attraction between ions is electrostatic and is non-directional, extending equally in all directions. Melting of the compound involves breaking of the lattice. This requires considerable energy and hence melting and boiling points are usually high  26 and the compounds are very hard. Compounds with covalent bonds are usually made up of discrete molecules. The bonds are directional and strong covalent bonding forces hold the atom together to make a molecule. In the solid molecules are held together by weak Van der Waals forces. To melt or boil the compound we simply have to supply small amount of energy to break Van der Waals forces. Hence covalent compounds are often gases, liquids or soft solids with low melting points. Conductivity Ionic compounds conduct electricity when the compound is melted or in aqueous solution. Conduction is achieved by the ions migrating towards the electrodes under the influence of electric potentials. Covalent compounds contain neither ions nor mobile electrons so they are unable to conduct electricity in either solid or  27 gaseous state. Solubility Ionic compounds are usually soluble in polar solvents. There are solvents of high dielectric constant such as water or mineral acids. Covalent compounds are not normally soluble in polar solvents but they are soluble in non-polar solvents of low dielectric constant such as benzene or CCl4. 1.4. Valence Bond Theory Valence Bond Theory is a qualitative method for predicting the behavior of electrons in bonding. It focuses on the overlap of the outermost orbitals where the valence electrons reside. Electrons are thought to be concentrated along the inter nuclear axis, causing a density of negatively charged electrons between both atoms. Attracted by the nuclei, these shared electrons pull together their respective atoms to which the electrons belong,  28 and the result is formation of a covalent bond. In the following diagram, formation of a diatomic molecule is shown : Approaching atoms with their nuclei and electron cloud These hydrogen atoms come together because of the electrostatic attraction between the nuclei and the electron density between them. Based on the principle of Valence Bond Theory, two electrons are required to make a single bond. Hydrogen has only one valence electron, so two hydrogen atoms are needed to make a bond. This allows accurate predictions about the shapes of simple molecules. In case of H2, the shape is simply linear. The valence bond theory was proposed by Heitler and  29 London to explain the formation of covalent bond quantitatively using quantum mechanics. Later on, Linus Pauling improved this theory by introducing the concept of hybridization. The main postulates of this theory are as follows: 1. A covalent bond is formed by the overlapping of two half-filled valence atomic orbitals of two different atoms. 2. The electrons in the overlapping orbitals get paired and confined between the nuclei of two atoms. 3. The electron density between two bonded atoms increases due to overlapping. This confers stability to the molecule. 4. Greater the extent of overlapping, stronger is the bond formed. 5. The direction of the covalent bond is along the region of overlapping of the atomic orbitals  30 so that it can be said that covalent bond is directional in nature. 1.5. Types of Covalent Bonds Sigma () and pi () Bonds Depending upon the type of overlapping, the covalent bonds are mainly of two types. Sigma () Bond When a bond is formed between two atoms by the overlap of their atomic orbitals along the internuclear axis the resulting bond is called sigma bond. Such type of overlap is also known as end to end or head on overlap. It is a strong bond and cylindrically symmetrical. The overlapping along the internuclear axis can take place in any of the following ways: 1. s-s Overlapping: This type of overlapping takes place between atoms having half-filled s-orbitals in their outer most energy shell.  31 For example, in the formation of hydrogen molecule, ‫ه‬s orbital of one hydrogen atom overlaps with Is orbital of other hydrogen atom thus forming a sigma bond. overlap of s-s orbital 2. s-p Overlapping: In this case, half-filled s- orbital of one atom overlaps with the half- filled p-orbital of another atom. A simple example of this type is the formation of hydrogen fluoride. Here Is orbitals of hydrogen overlaps with 2pz orbital of fluorine.  32 Overlap of s-p orbital 3. p-p Overlapping: This type of overlapping occurs when p-orbital of one atom overlaps with the p-orbital of the other as in case of fluorine molecule. The molecule of fluorine is produced by the overlapping of 2pz orbitals of the two fluorine atoms. Overlap of p-p orbital Pi (p) Bond Pi-bond is formed by lateral or sidewise overlapping of p-orbitals. Sideways overlap means overlapping of p-orbitals in a direction  33 perpendicular to the inter nuclear axis. A p-bond is not formed between two bonded atoms unless the two are held together with a s-bond. It is relatively a weaker bond since the electrons are not strongly attracted by the nuclei of bonding atoms. For example, 1. In case of oxygen molecule each oxygen atom having electronic configuration, (Is2 2s2 2px2 2py2 2pz1), the two atoms are held together by one s-bond and one p-bond. Formation of O2 molecule  34 2. In the molecule of nitrogen both nitrogen atoms are held together by one s-bond and 2p-bonds. Nitrogen atom has an electronic configuration, 1s2 2s2 2px1 2py1 2pz1. Formation of N2 molecule It is important to remember that the ‗s‘ orbitals can only form s-bonds, whereas the p, d and f-orbitals can form both s and p-bonds. Bonding in Molecules Explained by Valence Bond Theory H2 Molecule The electronic configuration of hydrogen atom in the ground state is 1s1. In the formation of  35 hydrogen molecule, two half-filled 1s orbitals of hydrogen atoms overlap along the inter-nuclear axis and thus forming by as ss-s bond. The formation of H2. Formation of H2 molecule In this diagram, H A and HB both have one electron that can be accounted for. It is known that the electron on HA belongs to HA and the electron on HB belongs to HB. However, when these two hydrogen atoms bond together, it is impossible to know which electron belongs to H A or HB. Also, since these two molecules are the same, they have equal attraction on the electrons they share. This is  36 because their orbitals overlap and they now share the electrons. The electrons are allowed to spin in their respective orbitals. Cl2 Molecule The electronic configuration of Cl atom in the ground state is [Ne]3s2 3px2 3py2 3pz1. The two half-filled 3pz atomic orbitals of two chlorine atoms overlap along the inter-nuclear axis and thus by forming a pp-p bond. Formation of Cl2 molecule HCl Molecule In the ground state, the electronic configuration of hydrogen atom is Is1 and the ground state  37 electronic configuration of Cl atom is [Ne] 3s2 3px2 3py2 3pz1. The half-filled 1s orbital of hydrogen overlap with the half-filled 3pz atomic orbital of chlorine atom along the inter-nuclear axis to form a ss-p bond. Formation of HCl molecule O2 Molecule The electronic configuration of O in the ground state is [He] 2s2 2px2 2py1 2pz1. The half-filled 2py orbitals of two oxygen atoms overlap along the inter-nuclear axis and form a bond. The remaining  38 half filled 2pz orbitals overlap laterally to form a pp-p bond. Thus a double bond (one pp.p and one pp.p) is formed between two oxygen atoms. Formation of O2 molecule N2 Molecule The ground state electronic configuration of N is [He] 2s2 2px1 2py1 2pz1. A sp.p bond is formed between two nitrogen atoms due to overlapping of half filled 2px atomic orbitals along the inter- nuclear axis. The remaining half filled 2py and 2pz orbitals form two pp.p bonds due to lateral overlapping. Thus a triple bond (one and two) is formed between two nitrogen atoms.  39 Drawbacks of Valence Bond Theory Although Valence Bond Theory (together with hybridization and VSEPR) predicts simple structures relatively well, it becomes difficult to predict accurate energies of a bond. It also fails to predict aspects of magnetism that can be explained by Molecular Orbital Theory. However the old version of valence bond theory is limited to diatomic molecules only. It could not explain the structures and bond angles of molecules with more than three atoms. e.g., It could not explain the structures and bond angles of H2O, NH3 etc., However, in order to explain the structures and bond angles of molecules, Linus Pauling modified the valence bond theory using hybridization concept.  40 1.6. Valence-shell electron-pair repulsion (VSEPR) Theory VSEPR Theory predicts the shapes and geometries of molecules which may or may not obey the octet rule but have only single bonds. The model extensively uses the number of electron pairs surrounding the central atoms to predict the geometry of individual molecules and ions. It was being developed by Gillespie and Nyholm. VSEPR theory is usually compared with valence bond theory, which addresses molecular shape through orbitals that are energetically accessible for bonding. Valence bond theory concerns itself with the formation of o and it bonds. Molecular orbital theory is another model for understanding how atoms and electrons are assembled into molecules and polyatomic ions. VSEPR theory has been criticized for not being quantitative, and therefore,  41 limited to the generation of crude though structurally accurate molecular geometries of covalently-bonded molecules. When the Central Atom is Surrounded by Bond Pair of Electrons The premise of VSEPR is that the valence shell electron pairs surrounding an atom tend to repel each other, and will therefore adopt an arrangement that minimizes this repulsion and maximize their stability. This particular arrangement of electron pairs fixes their positions in three-dimensional space giving rise to a fixed shape of the molecule. The sum of the number of bond pairs and the number of lone pairs formed by its nonbonding valence electrons is important in determining the shape of the molecule. The number of electron pairs in the valence shell of a central atom is determined after drawing the Lewis structure of the molecule, and expanding it to show  42 all electron-pair bonds and lone pairs of electrons. For the purposes of VSEPR theory, the multiple electron pairs in a double bond or triple bond are treated as though they were a bond with single pair of electrons. In cases where a molecule can be depicted by two or more resonance structures, these structures generally differ only by the interchange of double and single bonds, so that they have the same VSEPR model. The electron pairs are assumed to lie on the surface of a sphere centered on the central atom and tend to occupy positions that minimizes their mutual repulsions by maximizing the distance between them. The number of electron pairs, therefore, determine the overall geometry that they will adopt. For example, when there are two electron pairs surrounding the central atom, their mutual repulsion is minimal when they lie at opposite poles of the sphere. Therefore, the central atom is  43 predicted to adopt a linear geometry. If there are 3 electron pairs surrounding the central atom, their repulsion is minimized by placing them at the vertices of an equilateral triangle centered on the atom. Therefore, the predicted geometry is trigonal planar. Likewise, for 4 electron pairs, the optimal arrangement is tetrahedral. When the Central Atom is Surrounded by Bond Pair and Lone Pairs of Electrons The overall geometry of a covalently bonded molecule is further refined by distinguishing between bonding and nonbonding electron pairs or bond pairs and lone pairs. The bonding electron pair shared in a sigma bond with an adjacent atom lies further from the central atom than a nonbonding or lone pair of that atom, which is held close to its positively-charged nucleus. VSEPR theory therefore views repulsion by the lone pair to be greater than the repulsion by a bonding pair.  44 Lone pair-lone pair (lp-lp) repulsions are considered stronger than lone pair-bonding pair (lp-bp) repulsions, which in turn are considered stronger than bonding pair-bonding pair (bp-bp) repulsions. As a matter of fact, the presence of lone pair of electrons always distorts a regular geometry. Some of the examples are considered to explain the impact of lone pairs of electrons on the molecular geometries. For instance, when 5 ligands or lone pairs surround a central atom, a trigonal bipyramidal molecular geometry is specified. In this geometry, the 2 collinear ―axial‖ positions lie 180° apart from one another, and 90° from each of 3 adjacent ―equatorial‖ positions; these 3 equatorial positions lie 120° apart from each other. The axial positions therefore experience more repulsion than the equatorial positions; hence, when there are lone pairs, they tend to occupy equatorial positions.  45 The difference between lone pairs and bonding pairs may also be used to rationalize deviations from idealized geometries. For example, the H2O molecule has four electron pairs in its valence shell: two lone pairs and two bond pairs. The four electron pairs are spread so as to point roughly towards the apices of a tetrahedron. However, the bond angle between the two O-H bonds is only 104.5°, rather than the 109.5° of a regular tetrahedron, because the two lone pairs whose density or probability envelopes lie closer to the oxygen nucleus exert a greater mutual repulsion than the two bond pairs. The AXE Method: The “AXE method‖ of electron counting is commonly used when applying the VSEPR theory. The A represents the central atom and always has an implied subscript one. X represents the number of ligands atoms bonded to A. The E represents the number of lone  46 electron pairs surrounding the central atom. Based on the different types of distribution of X‘s and E’s around A, VSEPR theory makes the predictions in the following table. For example the description of AX2E1 as a bent molecule means that the three atoms AX2 are not in one straight line, although the lone pair helps to determine the geometry.  47 Table 1: Shapes of molecules on the basis of VSEPR theory When the substituent (X) atoms are not all the same, the geometry is still approximately valid, but the bond angles may be slightly different from the ones where all the outside atoms are the same. For example, the double-bond carbons in alkenes like  48 C2H4 are AX3E0, but the bond angles are not all exactly 120°. Likewise, SOCl2 is AX3E1 but because the X substituents are not identical, the XAX angles are not all equal. Following steps are given to find out the shape of a molecule: 1. Identify the central atom and count the number of valence electrons. 2. To this add number of other atoms. 3. If it is an ion, add negative charges and subtract positive charges. Represent the total number which we have calculated above as N. 4. On dividing N by 2 and comparing the result we obtain the shapes of molecule as given in table 2.  49 Table 2: Shapes of molecules Exmaple 1: Find the shapes of the following molecules using the VSEPR theory. (I) BF3, (II) BF4-, (III) Ammonia NH3, (IV) PC15, (V) ClF3, (VI) SF4, (VII) I3, (VIII) SF6  50 Solution : (I) BF3: Central atom is B. There are 3 fluorine atoms. Valence electrons = 3 Negative Charge is = 0 Add all of these, 3 + 3 + 0 = 6 Number of bonds (N/2) = 6/2 = Shape is a Triangular planar. (II) BF-4 : Central atom is B. There are 4 fluorine atoms. Valence electrons = 3 Negative charge is = -1 Add all these, 3 + 4 + 1=8 Number of bonds (N/2) =8/2 = 4 Shape is a Tetrahedral.  51 (III) Ammonia NH3 : Central atom is N. Valence electrons = 5 There are three hydrogen atoms. Add these, 5 + 3 = 8 Number of bonds (N/2) = 8/2 = 4 Shape is Trigonal pyramidal. (IV) PCl5 : Central atom is P. Valence electrons = 5 There are 5-Cl atoms, so the sum (N) is 10. Number of bonds = 10/2 = 5 Bonding electrons = 5. Non-bonding = 0 Shape is Trigonal bipyramidal.  52 (V) C1F3 : Central atom is Cl. Valence electrons = 7 There are three fluorine atoms, so the number of electron pairs shared is = 3. Sum is 10. No. of bonds = 10/2 = 5 A It is T-shaped. Note : Even though 3 possible structures are possible Only (c) is possible because Ip-ip repulsion is at 120°, and two Ip-bp are at 120°. This is because in (a) though the Ip-lp repulsion is at 180°, Ip-bp repulsion is at 90° each, (b) is not possible because  53 Ip-Ip repulsion is at 90°. Hence in a trigonal bipyramidal structure, the lone pairs will always occupy the equatorial position. (VI) SF4 : (Sulphur tetrafluoride) Central atom is S. Valence electrons = 6 Number of Fluorine atoms = 4 Total (N) = 10 Number of bonds (N/2) = 10/2 = 5 shape is Irregular Tetrahedral. (VII) I3- Central atom is I. Valence electrons = 7 Number of I atoms = 2 Negative charge = 1  54 Total (N) = 7 + 2 + 1 = 10 N/2 = 10/2 = 5 shape is Linear. (VII) SF6 (Sulphur hexafluoride): Central atoms is S. Valence electrons = 6 Number of Fluorine atoms = 6 Total (N) = 12 N/2 = 12/2 = 6 It is Octahedral in shape Example 2: Use the Lewis structure of the NO2 molecule shown in the figure below to predict the shape of this molecule. Solution: Bent shape  55 Example 3: Use the Lewis structure of the ICI ‫؛‬ ion shown in the figure below to predict the shape of this ion. Solution: It is a linear molecule as it belongs to AX2E2 category. Example 4: What is the use of VSEPR in chemistry? Solution: It is used to predict the molecular shape of molecules (I) How to predict a molecular structure using VSEPR theory? First step is to count the total number of valence electrons. After the total number of electrons is determined, this number is divided by two to give the total number of electron pairs. With the electron pairs of the molecule, the shape of the molecule is determined  56 (II) What is the shape of PF5? It is trigonal bipyramidal because it has total of 20 electron pairs. Each Fluorine atom give 1 electron to the Phosphorus central atom which creates total of 5 pairs. Also, each Fluorine atom has 3 electron pairs. With the presence of 5 Fluorine atom, there are 15 more electron pairs so there are 20 electron pairs total. Example 5: What is VSEPR theory? Solution: The geometric arrangement of atoms in molecules and ions may be predicted by means of the valence-shell electron-pair repulsion (VSEPR) theory. This theory predicts the shapes of molecules which may or may not obey the octet rule but have only single bonds. VSEPR theory may be summarized as follows: 1. The shape of the molecule is determined by the repulsions between all of the electron pairs  57 present in the valence shell. 2. A lone pair of electrons takes up more space around the central atom than a bond-pair, since the lone-pair is one occupied by atom while the bond pair is shared by two nuclei. It follows that repulsion between two lone pairs is greater than repulsion between a lone pair and a bond pair, which in turn is greater than the repulsion between two bond pairs. Thus, the presence of lone pairs on the central atom causes slight distortion of the bond angles from the ideal shape. If the angle between a lone pair, the central atom and a bond pair is increased, it follows that the actual bond angle between the atoms must be decreased. 3. The magnitude of repulsions between bonded pairs of electrons depends on the electro negativity difference between the central atom and the other atoms.  58 4. Triple bond causes more repulsion than double bond and double bond causes more repulsion than single bond. With very few exceptions, the predictions based on the VSEPR theory have been shown to be correct. 1.7. Hybridization In chemistry, hybridization is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory. Hybrid orbitals are very useful in the explanation of molecular geometry and atomic  59 bonding properties. Although sometimes taught together with the valence shell electron-pair repulsion (VSEPR) theory, valence bond and hybridization are in fact not related to the VSEPR model. It is the process of intermixing of atomic orbitals of same atom, having almost similar energies, followed by redistribution of energies to form new orbitals of identical energies and sizes. The new orbitals formed are called hybrid orbitals. The number of hybrid orbitals formed is always equal to the number of pure atomic orbitals employed for hybridization. It is not known whether hybridization actually takes place or not but it is a concept which is used to explain certain observed behavior of molecules. The following points are to be remembered about hybridization. 1. Orbitals belonging to the same atom or ion having almost similar energies get hybridized.  60 2. Number of hybrid orbitals is equal to the number of pure atomic orbitals taking part in hybridization. 3. The hybridization takes place to produce equivalent hybrid orbitals which are degenerate and which give maximum symmetry. 4. Hybrid orbitals are always involved in head on overlap, so the type of bonding resulted is always sigma (s bond. sp Hybridization In this type of hybridization, one ‗s‘ and three ‗p‘ orbitals of the same value of n (principal quantum number) mix up to form four sp3 hybridized orbitals. The mixing of orbitals is shown below,  61 Formation of sp3 hybrid orbitals The sp3 hybrid orbitals have 25% ‗s‘ character and 75% ‘p character. These orbitals orient themselves towards the corners of a regular tetrahedron. The angle between the orbitals is 109°28'. The commonest example is CH4 as shown below,  62 Shape of methane sp2 Hybridization This is a second type of hybridization involving pure one ‗s‘ and two ‗p‘ atomic orbitals of the same principle quantum number. These pure atomic orbitals mix together to form three sp hybridized orbitals. The mixing of orbitals can be shown as :  63 Formation of sp0 hybrid orbitals The sp2 hybrid orbitals have 33.3% ‗s‘ character and 66.6% ‗p‘ character. These orbitals orient themselves towards the corners of an equilateral triangle, as shown previous. It is important to remember that one p-orbital perpendicular to the molecular plane is unhybridized. For example, in BF3‫ ؛‬the central boron atom is sp hybridized.  64 In cases like ethene (C2H4), the bonding can be shown as : Bond line notation C2H4 Upon exhaustive analysis, it is found that the two carbon atoms have one p-orbital each with an unpaired electron. Hybrid orbital arrangement of C2H4  65 These two p-orbitals overlap in sideways manner to give rise to a double bond. Both and framework bonding is shown: s and p it bond arrangement of C2H4 It is always important to remember that when a bond is formed with head-to-head overlap, it is called a sigma (s) bond. Similarly, when a bond is formed with sideways overlap, it is called a pi (p) bond. sp Hybridization In this type, one ‗s‘ and one p orbital of the same  66 principal quantum number mix to form two sp hybridized orbitals, for example in BeCl2, Be is sp hybridized. The mixing of s and p orbitals can be shown as : Formation of sp hybrid orbitals They possess 50% ‗s‘ and 50% ‗p‘ character. In cases like ethyne C2H2, the central carbon atom is sp hybridized. Ethyne has the following Lewis structure.  67 each carbon atom is sp hybridized having two perpendicular p orbitals with two unpaired electrons present on each carbon atom. These overlap in sideways fashion and produce a triple bond. All the overlaps are shown below: Formation of Ethyne sp3d Hybridization In this type of hybridization, one ‗s‘, three ‗p‘ and one ‗d‘ orbital of the same principle quantum number mix to give five sp3d hybridized orbitals. These orbitals orient themselves toward the corners of a trigonal bipyramidal structure. Among  68 them, three are arranged in trigonal plane and the remaining two orbitals are present above and below the trigonal plane at right angles. The sp3d hybrid orbitals have 20% ‗s‘, 60% ‗p‘ and 20% ‗d‘ characters. The orientation of various lobes with respect to each other is shown below: As an example, in PCl5, P atom is undergoing sp3d hybridization. sp3d2 Hybridization In this type of hybridization, one ‗s‘ three ‗p‘ and two 'd‘ orbitals mix to form six sp3d2 hybridized orbitals. The lobes in sp3d2 hybridization orient themselves towards the corners of a regular  69 octahedron as shown below : An example is SF6 where the central S atom is sp3d2 hybridized. The compound is shown below: F sp3d3 Hybridization In sp3d3 hybridization, one ‗s‘, three ‘p and three ‗d‘ orbitals of almost same energy intermix to give seven sp3d3 hybrid orbitals, which are oriented in pentagonal bipyramidal symmetry. Five among the sp3d3 orbitals are arranged in a pentagonal plane by making 72° of angles. The remaining are arranged perpendicularly above and below this pentagonal plane. In the example IF7 the central iodine atom is undergoing sp3d3 hybridization. The bonding in IF7 is shown below:  70 Table : List of different species and their shapes with hybridization state  71 Example 6: Find the shapes of the following species on the basis of hybridization states of centra] atom: (i) ClO3- (ii) SO2 (iii) CO3 (iv) SO3 (v) SO42- (vi) CO32- (vii) CO32- (viii) C1O2- (ix) CIO4- (x) SO32- (xi) PO43- (xii) XeOF2 (xiii) XeOF4 (xiv) XeOF6 Solution: (i) ClO3- Electronic configuration of Cl = 1s2 2s2 2p6 3s2 3p5 Oxidation state of Cl = 5 Electronic configuration after excitation Cl atom is sp3 hybridized. Shape is Trigonal pyramidal.  72 ii) SO2 Electronic configuration of S = 1s2 2s2 2p6 3s2 3p4 O.S. of S = 4 Electronic configuration after excitation Shape of the molecule is Angular. (iii) CO2 Electronic configuration of C = 1s2 2s2 2p2 O.S. of C = 4 Electronic configuration after excitation Shape of the molecule is Linear.  73 (iv) SO3 Electronic configuration of S = 1s2 2s2 2p6 3s2 3p4 O.S. of S = 6 Electronic configuration after excitation Shape of the molecule is Trigonal planar. (v) SO42- Electronic configuration of S = 1s2 2s2 2p63s2 3p4 O.S. of S = 6 Electronic configuration after excitation Shape is Tetrahedral  74 (vi) CO32- Electronic configuration of C = 1s2 2s2 2p2 O.S. of C = 4 Electronic configuration after excitation Shape is Trigonal planar (vii) ClO- Electronic configuration of Cl = 1s2 2s2 2p6 3s2 3p5 O.S. of Cl = 1 Electronic configuration after excitation Shape is Linear.  75 (viii) C1O2- Electronic configuration of Cl = 1s2 2s2 2p6 3s2 3p5 O.S. of Cl = 3 Electronic configuration after excitation. A Shape is Angular. (ix) ClO4- Electronic configuration of Cl = Is2 2s2 2p6 3s2 3p5 O.S. of Cl. = 7 Electronic configuration after excitation  76 Shape is Tetrahedral. (x) SO32- Electronic configuration of S = 1s2 2s2 2p6 3s2 3p4 O.S. of S = 4 Electronic configuration after excitation Shape is Trigonal pyramidal. (xi) PO43- Electronic configuration of P = Is2 2s2 2p6 3s2 3p3 O.S. of P = 5 Electronic configuration after excitation  77 Shape is Tetrahedral. (xii) XeOF2 Outermost electronic configuration of Xe = 5s2 5p6 O.S. of Xe = 4 Electronic configuration after excitation Shape is T-Shaped. (xiii) XeOF4 Outermost electronic configuration of Xe = 5s2 5p6 O.S. of Xe = 6  78 Electronic configuration after excitation Shape is Square pyramidal. (xiii) XeOF6 Outermost electronic con-figuration of Xe = 5s2 5p6 O.S. of Xe = 8 Electronic configuration after excitation Shape is Pentagonal bipyramidal.  79 Example 7: (i) What is hybridization? (ii) What is intermixing? (iii) What are the requirements for atomic orbitals to undergo hybridization? (iv) Do the orbitals of different atoms undergo hybridization? (v) What are hybrid orbitals? And what are its characteristics? (vi) How many hybrid orbitals are formed? (vii) How do the electrons are going to be filled in the hybrid orbitals? Solution : (i) The intermixing of two or more pure atomic orbitals of an atom with almost same energy to give same number of identical and degenerate new type of orbitals is known as hybridization. The new orbitals formed are also known as hybrid orbitals. (ii) The intermixing or hybridization of atomic  80 orbitals is a mathematical concept based on quantum mechanics. During this process, the wavefunctions, y of atomic orbitals of same atom are combined to give new wavefunctions corresponding to hybrid orbitals. (iii) The atomic orbitals of same atom with almost same energy can only participate in the hybridization. The full filled or half-filled or even empty orbitals can undergo hybridization provided they have almost equal energy. (iv) No! The hybridization is the mixing of orbitals of same atom only. The combination of orbitals belonging to different atoms is called bonding. (v) The new orbitals that are formed due to intermixing of atomic orbitals are also known as hybrid orbitals, which have mixed characteristics of atomic orbitals. The shapes of hybrid orbitals are identical. Usually they  81 have one big lobe associated with a small lobe on the other side. The hybrid orbitals are degenerate i.e., they are associated with same energy. (vi) The number of hybrid orbitals formed is equal to the number of pure atomic orbitals undergoing hybridization. If three atomic orbitals intermix with each other, the number of hybrid orbitals formed will be equal to 3. (vi) The hybrid orbitals are filled with those electrons which were present in the pure atomic orbitals forming them. The filling up of electrons in them follows Pauli‘s exclusion principle and Hund‘s rule. Example 7: predict the shape of the following xenon compounds XeF2, XeF4, XeO2F2, XeO3, XeO3F2, XeO2F2, XeO4 Solution  82 1. XeF2 It is a linear compound 2. XeF4 The shape of the molecule is square planar 3. XeF6 The shape of the molecule is octahedral 4. XeO2F2 The shape of the molecule is distorted tetrahedral  83 5. XeO3 The shape of the molecule is trigonal pyramide 6. XeO3F2 The shape of the molecule is trigonal bipyramide 7. XeO2F4 : The shape of the compound is octahedral 8. XeO4 : The shape of the compound is tetrahedral as  84 1.8. Van der Waals Interactions In chemistry, the Van der Waals force or Van der Waals interaction is the sum of the attractive or repulsive forces between molecules or between parts of the same molecule other than those due to covalent bonds, or the electrostatic interaction of ions with one another, with neutral molecules, or with charged molecules. Van der Waals forces include attractions and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces. As such, Van der Waals forces define many properties of organic compounds, including their solubility in polar and non-polar solvents etc. For example, in low molecular weight alcohols, the hydrogen-bonding  85 properties of the polar hydroxyl group dominate the weaker Van der Waals interactions. In higher molecular weight alcohols, the properties of the nonpolar hydrocarbon chain(s) dominate and define the solubility. Van der Waals forces quickly vanish at longer distances between interacting molecules. Van der Waals forces are relatively weak compared to covalent bonds which is quite expected of them being the intermolecular forces and not a type of chemical bond. However, these forces play a fundamental role in chemistry and its different offshoots like supramolecular chemistry, polymer science, nanotechnology, and surface science. All Van der Waals forces are anisotropic except those between two noble gas atoms. It means that the magnitude of these forces depend on the relative orientation of the molecules. The induction and  86 dispersion interactions are always attractive, irrespective of orientation, but the electrostatic interaction changes sign upon rotation of the molecules. That is, the electrostatic force can be attractive or repulsive, depending on the mutual orientation of the molecules. The main characteristics are: 1. They are weaker than normal covalent or ionic bonds. 2. Van der Waals forces are additive and cannot be saturated. 3. They have no directional characteristics. 4. They are all short - range forces and hence only interactions between nearest need to be considered instead of all the particles. The greater is the attraction if the molecules are closer due to Van der Waals forces.  87 5. Van der Waals forces are independent of temperature except dipole -dipole interactions. Types of Van Der Waals Forces Van der Waals forces include a number of interactions. These are discussed below. 1. Dipole-dipole interaction: A force between two permanent dipoles is known as dipole-dipole interaction or Keesom force. It can be diagrammatically shown below, Dipole-dipole interaction Dipole-Dipole interactions result when two polar molecules approach each other in space. When this occurs, the partially negative portion of one of the polar molecules is attracted to the partially positive  88 portion of the second polar molecule. This type of interaction between molecules accounts for many physically and biologically significant phenomena. An example is shown below: Dipole—dipole interaction in HCI 2. Dipole - induced dipole interaction: A force between a permanent dipole and a corresponding induced dipole is also known as dipole - induced dipole or Debye force. This type of attractive interaction also depends on the presence of a polar molecule. However, the second participating molecule need not be polar as shown below:  89 Example of dipole induced dipole interaction In the dipole-induced-dipole interaction, the presence of the partial charges of the polar molecule causes a polarization, or distortion, of the electron distribution of the other molecule. As a result of this distortion, the second molecule acquires regions of partial positive and negative charge, and thus it becomes polar. The partial charges so formed behave just like those of a permanently polar molecule and interact favourably with their counterparts in the polar molecule that originally induced them. Hence, the two molecules attract as shown below:  90 Dipole-induced dipole interaction This interaction also contributes to the intermolecular forces that are responsible for the condensation of hydrogen chloride gas. 3. Induced dipole-induced dipole interaction : It is  91 a force between two instantaneously induced dipoles also known as London dispersion force. This type of interaction acts between all types of molecule, polar or not. It is the principal force responsible for the existence of the condensed phases of certain molecular substances, such as benzene, other hydrocarbons, bromine, and the solid elements phosphorus (which consists of tetrahedral P4 molecules) and sulfur (which consists of crown- shaped S8 molecules). The interaction is called the dispersion interaction or, less commonly, the induced-dipole-induced-dipole interaction. Two nonpolar molecules of argon are considered near each other as shown below,  92 Example of induced-dipole induced dipole interaction Although there are no permanent partial charges on either molecule, the electron density can be thought of as ceaselessly fluctuating. As a result of these fluctuations, regions of equal and opposite partial charge arise in one of the molecules and give rise to a transient dipole. This transient dipole can induce a dipole in the neighbouring molecule, which then interacts with the original transient dipole as shown here, Induced dipole induced dipole interaction Although the latter continuously flickers from one direction to another (with an average of zero dipole  93 overall), the induced dipole follows it, and the two correlated dipoles either attract or repel with one another.  94 CHAPTER (2) POLYMERS Prepared by Dr. Bahaa Ahmed Salah  95 Contents 2.1. Introduction 2.2. Types of Polyethylene 2.3. Thermoplastic and Thermosetting Polymers 2.4. Addition Polymerization: 2.5. Processing Polymers 2.6. Rubber and Other Elastomers 2.7. Polymers in Paints 2.8. Condensation Polymers 2.9. Composite Materials  96 Learning Objectives 1. Define polymer and monomer. List several natural polymers, including a chemically modified one. 2. Describe the structure and properties of the two main types of polyethylene. 3. Write the structural formula for a polymer from its monomer structure(s). 4. Define cross-linking and explain how it changes the properties of a polymer. 5. Differentiate between addition and condensation polymerization. 6. Write the structures of the monomers that form polyesters and polyamides.  97 Chapter 2: Polymers 2.1. Introduction Polymers are macromolecules (from the Greek makros, meaning "large" or "long"). Macromolecules are in fact not very large, as most such "giant" molecules are invisible to the human eye, but compared with other molecules, they are enormous. A polymer (from the Greek poly, meaning "many," and meros, meaning "part") is made from much smaller molecules called monomers (from the Greek monos, meaning "one"). Hundreds, even thousands, of monomer units combine to make one polymer molecule. A monomer is a small-molecule building block from which a polymer is made. These monomers generally have a functional group, or groups, that  98 undergo multiple reactions that link the many units. The process by which monomers are converted to polymers is called polymerization. Natural Polymers Polymers have served humanity for millennia by providing the starches and proteins in our food, the wood we use for shelter, and the wool, cotton, and silk we make into clothing. Starch is a polymer made up of glucose (C6H12O6, a simple sugar) units. Cotton is made of cellulose, also a glucose polymer; wood is largely cellulose as well. Proteins are polymers made up of amino acid monomers. Wool and silk are two of the thousands of different kinds of proteins found in nature. Semisynthetic Polymers It didn't take long for the chemical industry to recognize the potential of synthetic polymers.  99 Scientists found ways to make macromolecules from small molecules rather than simply modifying large ones. The first such truly synthetic polymers were phenol-formaldehyde resins (such as Bakelite), initially made in 1909. The earliest synthetic attempt to improve on natural polymers involved chemical modification of a common macromolecule. The semisynthetic material celluloid was derived from natural cellulose (from cotton and wood, for example). When cellulose is treated with nitric acid, a derivative called cellulose nitrate is formed. Celluloid was used in movie film as well as for stiff shirt collars, which didn't require laundering and repeated starching. Today, movie film is made mainly from polyethylene terephthalate, a polyester. And high, stiff collars on men's shirts are  100 definitely out of fashion, although clerical collars are still made of plastic. Synthetic Polymers The prevalent plastic polyethylene is the simplest and least expensive synthetic polymer. It is familiar to us in the form of plastic bags used for packaging fruit and vegetables, garment bags for dry- cleaned clothing, garbage-can liners, and many other items. Polyethylene is made from ethylene (CH2=CH2). Ethylene is produced in large quantities from the cracking of petroleum, a process by which large hydrocarbon molecules are broken down into simpler hydrocarbons. With pressure and heat, and in the presence of a catalyst, ethylene monomers join together to form long chains. This process involves breaking one of the double bonds in the monomer, leaving lone electrons on each carbon atom of the  101 original double bond as well as a single bond between the carbon atoms. Because electrons like to be paired in forming bonds, random collisions of those molecules cause bonds between carbons with lone electrons in two monomers to be formed, and this continues along the chain. Polyethylene proved to be tough and flexible, excellent as an electric insulator, and able to withstand both high and low temperatures. Polyethylene was used for insulating cables in radar equipment.  102 The raw materials for most synthetic polymers are petroleum and natural gas. These are not renew- able sources, and we not only have a limited supply of these fossil fuels, but we are literally burning up over 95% for energy. Comparatively little is left for making other products such as polymers. 2.2. Types of Polyethylene Today, there are three main types of polyethylene that are widely used, although other kinds do exist. High-density polyethylene (HDPE) has mostly linear molecules that pack closely together and can assume a fairly well-ordered, crystalline structure. HDPEs therefore are rather rigid and have good tensile strength. As the name implies, the densities of HDPEs (0.94—0.96 g/cm3) are high compared with those of other polyethylenes. HDPEs  103 are used for such items as threaded bottle caps, toys, detergent bottles, and milk jugs. Low-density polyethylene (LDPE), on the other hand, has many side chains branching off the polymer molecules. The branches prevent the molecules from packing closely together and assuming a crystalline structure. LDPEs are waxy, bendable plastics that have lower densities (0.91— 0.94 g/cm3) and lower melting temperatures than high-density polyethylene. LDPEs are used to make plastic bags and film, squeeze bottles, electric wire insulation, and many common household products for which flexibility is important. The third type of polyethylene, called linear low- density polyethylene (LLDPE), is actually a copolymer, a polymer formed from two (or more) different monomers. LLDPEs are made by  104 polymerizing ethylene with a branched-chain alkene such as 4-methyl-l-pentene. LLDPEs are used to make such things as plastic films for use as landfill liners, trash cans, tubing, and automotive parts. 2.3. Thermoplastic and Thermosetting Polymers Polyethylene is one of a variety of thermoplastic polymers. Because its molecules can slide past one another when heat and pressure are applied, a thermoplastic polymer can be softened and then reshaped. It can be repeatedly melted down and remolded. Not all polymers can be readily melted.  105 Thermosetting polymers, harden permanently when formed. They cannot be softened by heat and remolded. Instead, strong heating causes them to discolor and decompose. The permanent hardness of thermosetting plastics is due to cross-linking (side-to- side connection) of the polymer chains. 2.4. Addition Polymerization: There are two general types of polymerization reactions: addition polymerization and condensation polymerization. In addition, polymerization (also called chain-reaction polymerization), the monomer molecules add to one another in such a way that the polymeric product contains all the atoms of the starting monomers. The polymerization of ethylene to form polyethylene is an example. In polyethylene, the two carbon atoms and the four hydrogen atoms of each monomer molecule are incorporated into the  106 polymer structure. In condensation polymerization, some part of each monomer molecule is not incorporated in the final polymer. Polypropylene Most of the many familiar addition polymers are made from derivatives of ethylene in which one or more of the hydrogen atoms are replaced by another atom or group. Replacing one of the hydrogen atoms with a methyl group gives the monomer propylene (propene). Polypropylene molecules look much like polyethylene molecules, except that there is a methyl group (—CH3) attached to every other carbon atom. The chain of carbon atoms is called the polymer backbone. Groups attached to the backbone, such as  107 the —CH3 groups of polypropylene, are called pendant groups, shown in green. Polypropylene is a tough plastic material that resists moisture, oils, and solvents. It is molded into hard-shell luggage, battery cases, and various kinds of appliance parts. It is also used to make packaging material, fibers for textiles such as upholstery fabrics and carpets, and ropes that float. Because of polypropylene's high melting point (121 °C), objects made from it can be sterilized with steam Polystyrene Replacing one of the hydrogen atoms in ethylene with a benzene ring gives a monomer called styrene, which has the formula CH2=CHC6H5, where C6H5 represents the benzene ring. Polymerization of styrene produces polystyrene, which has benzene rings as pendant groups, shown in green  108 Polystyrene is the plastic used to make transparent disposable drinking cups. With color and filler added, it is the material of thousands of inexpensive toys and household items. When a gas such as air is blown into polystyrene liquid, it foams and hardens into the familiar material (called Styrofoam) of some disposable ice chests and coffee cups. The polymer can easily be formed into shapes as packing material  109 for shipping instruments and appliances, and it is widely used for home insulation. Vinyl Polymers Would you like a tough synthetic material that looks like leather at a fraction of the cost? Perhaps a clear, rigid material from which unbreakable bottles could be made? Do you need an attractive, long-lasting floor covering? Or lightweight, rustproof, easy- to- connect plumbing? Polyvinyl chloride (PVC) has all these properties, and more Replacing one of the hydrogen atoms of ethylene with a chlorine atom gives vinyl chloride (CH2=CHCl), a compound that is a gas at room temperature. Polymerization of vinyl chloride yields the tough thermoplastic material PVC. A segment of the PVC molecule is shown here.  110 PVC is readily formed into various shapes. The clear, transparent polymer is used in plastic wrap and clear plastic bottles. Adding color and other ingredients to a vinyl plastic yields artificial leather. Most floor tile and shower curtains are made from vinyl plastics, which are also widely used to simulate wood in home siding panels and window frames. About 40% of the PVC produced is molded into pipes. Vinyl chloride, the monomer from which vinyl plastics are made, is a carcinogen. A number of people who worked closely with this gas later developed a kind of cancer called angiosarcoma.  111 PTFE: The Nonstick Coating In 1938, a young American chemist at DuPont, Roy Plunkett (1910-1994), was working with the gas tetrafluoroethylene (CF2=CF2). He opened the valve on a tank of the gas-and nothing came out. Rather than discarding the tank, he decided to investigate. The tank was found to be filled with a waxy, white solid. He attempted to analyze the solid but ran into a problem: It simply wouldn't dissolve, even in hot concentrated acids. Plunkett had discovered polytetrafluoroethylene (PTFE), the polymer of tetrafluoroethylene, best known by its trade name, Teflon®. In this case, the fluorine atoms are pendant groups  112 Because its C—F bonds are exceptionally strong and resistant to heat and chemicals, PTFE is a tough, unreactive, non-flammable material. PTFE is used to coat the sole plates of irons used for pressing clothes and to make electrical insulation, bearings, and gaskets. Medical catheters are often coated with PTFE to help prevent bacteria from adhering to them and causing infections. However, it is most widely known as a coating for surfaces of cookware to eliminate sticking of food. PTFE begins to decompose at temperatures above 260 °C (500 °F), but most frying is done at much lower temperatures than that. 2.5. Processing Polymers In everyday life, many polymers are called plastics. In chemistry, a plastic material is one that can be made to flow under heat and pressure. The material  113 can then be shaped in a mold or in other ways. Plastic products are often made from granular polymeric material. In compression molding, heat and pressure are applied directly to such plastic grains in the mold cavity. In transfer molding, the polymer is softened by heating before being poured into molds to harden. There also are several methods of molding molten polymers. In injection molding, the plastic is melted in a heating chamber and then forced by a plunger into cold molds to set. In extrusion molding, the melted polymer is extruded through a die in continuous form to be cut into lengths or coiled. Bottles and similar hollow objects often are blow- molded; a "bubble" of molten polymer is blown up like a balloon inside a hollow mold. Table lists some of the more important addition polymers, along with a few of their uses.  114 Table Some addition polymers  115 2.6. Rubber and Other Elastomers As noted in the introduction to this chapter, the search for a material to replace natural rubber when the supply was cut off during World War II was the basis for much of the development of the synthetic polymer industry. Natural rubber can be broken down into simple hydrocarbon units called isoprene. Isoprene is a volatile liquid, whereas rubber is a semisolid, elastic material. Chemists can make polyisoprene, a substance identical to natural rubber, except that the isoprene comes from petroleum refineries rather than from the cells of rubber trees.  116 Chemists have also developed several synthetic rubbers and devised ways to modify these polymers to change their properties. Vulcanization: Cross-linking The long-chain molecules that make up rubber are coiled and twisted and intertwined with one another. When rubber is stretched, its coiled molecules are straightened. Natural rubber is soft and tacky when hot. It can be made harder by reaction with sulfur. In this process, called vulcanization, sulfur atoms cross- link the hydrocarbon chains side-to-side.  117 Its cross-linked structure makes vulcanized rubber a harder, stronger substance that is suitable for automobile tires. Surprisingly, cross-linking also improves the elasticity of rubber. With just the right degree of cross-linking, the individual chains are still free to uncoil and stretch somewhat. When stretched vulcanized rubber is released, the cross-links pull the chains back to their original arrangement. Rubber bands owe their snap to this sort of molecular structure. Materials that can be extended and will return to their original size are called elastomers. Synthetic Rubber Natural rubber is a polymer of isoprene, and some synthetic elastomers are closely related to natural rubber. For example, polybutadiene is made from the monomer butadiene (CH2=CHCH=CH2), which differs from isoprene only in that it lacks a methyl  118 group on the second carbon atom. Polybutadiene is made rather easily from this monomer. However, this polymer has only fair tensile strength, and has poor resistance to gasoline and oils. These properties limit its value for automobile tires, the main use of elastomers. Another synthetic elastomer, polychloroprene (Neoprene), is made from a monomer similar to isoprene but with a chlorine (shown in green) in place of the methyl group on isoprene. Neoprene is more resistant to oil and gasoline than other elastomers are. It is used to make gasoline pump hoses and similar items used at automobile service stations.  119 Styrene-butadiene rubber (SBR) is a copolymer of styrene (about 25%) and butadiene (about 75%). A segment of an SBR molecule might look something like this. SBR is more resistant to oxidation and abrasion than natural rubber, but it has poorer physical strength and resilience. Like those of natural rubber, SBR molecules contain double bonds and can be cross- linked by vulcanization. SBR accounts for about a third of the total U.S. production of elastomers, and is used mainly for making tires.  120 2.7. Polymers in Paints A surprising use for elastomers is in paints and other coatings. The substance in a paint that hardens to form a continuous surface coating-often called the binder, or resin-is a polymer, usually an elastomer. Paint made with elastomers is resistant to cracking. Various kinds of polymers can be used as binders, depending on the specific qualities desired in the paint. Latex paints, which have polymer particles dispersed in water and thus avoid the use of organic solvents, are most common. Brushes and rollers can easily be cleaned in soap and water. This replacement of the hazardous organic solvents historically used in paints with water is a good example of green chemistry.  121 2.8. Condensation Polymers The polymers considered so far are all addition polymers. All the atoms of the monomer molecules are incorporated into the polymer molecules. In a condensation polymer, part of the monomer molecule is not incorporated in the final polymer. During condensation polymerization, also called step- reaction polymerization, a small molecule-usually water but sometimes methanol, ammonia, or HCl is formed as a by-product. Nylon and Other Polyamides As an example, let's consider the formation of nylon. The monomer in one type of nylon, called nylon 6, is a six-carbon carboxylic acid with an amino group on the sixth carbon atom: 6-aminohexanoic acid (HOOCCH2CH2CH2CH2CH2NH2). (There are several different nylons, each prepared from a different  122 monomer or set of monomers, but all share certain common structural features.) In this polymerization reaction, a carboxyl group of one monomer molecule forms an amide bond with the amine group of another. For each amide bond made, a water molecule is formed as a by-product. This formation of a nonpolymeric by-product distinguishes condensation polymerization from addition polymerization. Note that the formula of a repeat unit of a condensation polymer is not the same as that of the monomer.  123 Because the linkages holding the polymer together are amide bonds, nylon 6 is a polyamide. Another nylon is made by the condensation of two different monomers: 1,6-hexanediamine (H2NCH2CH2CH2CH2CH2CH2NH2) and adipic acid (HOOCCH2CH2CH2CH2COOH). Each monomer has six carbon atoms; this polymer is called nylon 66. This was the original nylon polymer. Note that one monomer has two amino groups and the other has  124 two carboxyl groups, but the product is still a polyamide, quite similar to nylon 6. Silk and wool, which are protein fibers, are natural polyamides. Although nylon can be molded into various shapes, most nylon is made into fibers. Some is spun into fine thread to be woven into silk-like fabrics, and some is made into yarn that is much like wool. Carpets were once made mainly from wool or cotton, but now at least 90% of carpets are made from nylon. Polyethylene Terephthalate and Other Polyesters A polyester is a condensation polymer made from molecules with alcohol and carboxylic acid functional groups. The most common polyester is made from ethylene glycol and terephthalic acid. It is called polyethylene terephthalate (PET)  125 The hydroxyl groups in ethylene glycol react with the carboxylic acid groups in terephthalic acid to produce long chains held together by many ester linkages. PET can be molded into bottles for beverages and other liquids. It can also be formed into a film that is used to laminate documents and to make tough packaging tape. Polyester finishes are used on premium wood products such as guitars, pianos, and the interiors of vehicles and boats. Polyester fibers are strong, quick drying, and resistant to mildew,  126 wrinkling, stretching, and shrinking. They are used in home furnishings and products such as carpets, curtains, sheets and pillowcases, and upholstery. Because polyester fibers do not absorb water, they are ideal for outdoor clothing to be worn in wet and damp environments and for insulation in boots and sleeping bags. For other clothing, they are often blended with cotton for a more natural feel. A familiar use of polyester film (Mylar) is to make the shiny balloons that are filled with helium to celebrate special occasions. Phenol-Formaldehyde and Related Resins Let's go back to Bakelite, the original synthetic polymer. Bakelite, a phenol formaldehyde resin. Phenolic resins are no longer important as industrial polymers, but they are used as a substitute for  127 porcelain and in board and tabletop game pieces such as billiard balls, dominoes, and checkers. Phenol-formaldehyde resins are formed in a condensation reaction that also yields water molecules, the hydrogen atoms coming from the benzene ring of phenol and the oxygen atoms from the aldehyde. The reaction proceeds stepwise, with formaldehyde first adding to the 2 or 4 position of the phenol molecule. The substituted phenol molecules then link up as water molecules are formed. (Remember that there  128 are hydrogen atoms at all the unsubstituted corners of a benzene ring.) The hookup of molecules continues until an extensive network is achieved. Water produced by the reaction is driven off by heat as the polymer sets. The structure of the polymer is an extremely complex three-dimensional network somewhat like the framework of a giant building. Note that the phenolic rings are joined together by CH2 units from the formaldehyde. These network  129 polymers are thermosetting resins; they cannot be melted and remolded. Instead, they decompose when heated to high temperatures. Formaldehyde can also be condensed with urea [H2N(C = O)NH] to make urea-formaldehyde resins and with melamine to form melamine formaldehyde resins. (Melamine is formed by condensation of three molecules of urea. These resins, like phenolic resins, are thermosetting. The polymers are complex three-dimensional networks formed by the condensation reaction of formaldehyde (H2C=O) molecules and amino (— NH2) groups. Urea-formaldehyde resins are used to bind wood chips together in panels of particle board. Melamine formaldehyde resins are used in plastic (Melmac) dinnerware and laminate countertops.  130 Other Condensation Polymers There are many other kinds of condensation polymers, but we will look at only a few. Polycarbonates are "clear-as-glass" polymers tough enough to be used in bulletproof windows. They are also used in protective helmets, safety glasses, clear plastic water bottles, baby bottles, and even dental crowns. One polycarbonate is made from bisphenol- A (BPA) and phosgene (COC12). The products will have a carbonate group.  131 Polyurethanes are similar to nylon polymers in structure, except that they have an isocyanate group (— N=C=O) that reacts with an alcohol group (— OH) to form a —NHCOO— bond rather than an amide bond. The repeat unit in one common polyurethane is Polyurethanes may be elastomeric or tough and rigid, depending on the monomers used. They are common in foamed padding (foam rubber) in cushions, mattresses, and padded furniture. They are also used for skate wheels, in running shoes, and in protective gear for sports activities, as well as for hard lacquer- like coatings for wood.  132 Epoxies make excellent surface paints and coatings. They are used to protect steel pipes and fittings from corrosion. The insides of metal cans are often coated with an epoxy to prevent rusting, especially in cans for acidic foods like tomatoes. A common epoxy is made from epichlorohydrin and BPA. Epoxies also make powerful adhesives. These adhesives usually have two components that are mixed just before use. The polymer chains become cross-linked, and the bonding is extremely strong.  133 2.9. Composite Materials Composite materials are made up of high-strength fibers (of glass, graphite, synthetic polymers, or ceramics) held together by a polymeric matrix, usually a thermosetting condensation polymer. The fiber reinforcement provides the support, and the surrounding plastic keeps the fibers from breaking. Among the most commonly used composite materials are polyester resins reinforced with glass fibers. These are widely used in boat hulls, molded chairs, automobile panels, and sports gear such as tennis rackets. A notable example is their use in poles used for pole vaulting. Some composite materials have the strength and rigidity of steel but with only a fraction of the weight of steel.  134 Silicones Not all polymers are based on chains of carbon atoms. A good example of a different type of polymer is silicone (polysiloxane), in which the chains have a series of alternating silicon and oxygen atoms. (In simple silicones, R represents a hydrocarbon group, such as methyl, ethyl, or butyl.) Silicones can be linear, cyclic, or cross-linked networks. They are heat-stable and resistant to most chemicals, and are excellent waterproofing materials. Depending on chain length and amount of cross-linking, silicones can be oils or greases, rubbery compounds, or solid  135 resins. Silicone oils are used as hydraulic fluids and lubricants. Other silicones are used in such products as sealants, auto polish, shoe polish, and waterproof sheeting. Fabrics for raincoats and umbrellas are frequently treated with silicone. Perhaps the most remarkable silicones are the ones used for synthetic human body parts, ranging from finger joints to eye sockets. Artificial ears and noses are also made from silicone polymers. These can even be specially colored to match the surrounding skin.  136 CHAPTER (3) Building Materials Prepared by Dr. Ahmed A. Younes  137 Chapter 3: Building Materials 3.1. Introduction 3.2. Aggregates 3.3. Bricks 3.4. Lime 3.5. Cement 3.6. Concrete  138 Chapter 3: Building Materials 3.1. Introduction Construction or building materials are any materials that are used in the construction work. They are of two major categories: natural and synthetic. Natural materials are like aggregates, sand, stones and wood are natural, while cement, bricks, steel, concrete, or plastics are synthetic. 3.2. Aggregate Aggregates give the concrete its form and reduce its shrinkage. Aggregates represent about 70 to 80 % of concrete volume. Aggregates are classified according to shape and size. The shape of the aggregates is determined not only by the parent rock but also by the crushing machine used. According to shape, aggregates can be rounded, irregular or partly  139 rounded, angular, flaky, elongated, and flaky and elongated aggregates. According to size, Aggregates are either fine (sand) or coarse (gravel) aggregates 3.3. Bricks Bricks are building materials used to build walls or paving roads. They can be connected using mortar, adhesives or by interlocking them. Bricks can be classified according to quality or the building process. Based on quality there are first, second and third class bricks. First class bricks have standard size, regular shape, uniform yellow or red color, and  140 well burnt. Second class bricks almost have the same characteristics like first class except that the burning temperature is slightly lower than first class. Unlike first and second class, the third class has irregular shape and size, the color is soft and light red and is under burnt. According to building process, Bricks are classified into unburnt bricks, burnt bricks and over burnt bricks. 3.4. Lime Lime is used in construction works as lime mortar. Lime can be hydraulic or non-hydraulic lime. The difference between these two types is that hydraulic lime sets under water but non-hydraulic lime do not set underwater. Quick Lime is a non-hydraulic lime manufactured by burning calcium carbonate  141 containing lime stones. Lime used in construction works should exhibit good plasticity, should be flexible and easily workable, and should harden in short time. 3.5. Cement Cement is a fine powder which sets after a few hours when mixed with water, and then hardens in a few days into a solid, strong material. Cement is mainly used to bind fine sand and coarse aggregates together in concrete. Cement is a hydraulic binder, i.e. it hardens when water is added. Portland cement is made of lime stone (CaCO3) and clay (Kaolin Al2O3SiO2.2H2O) and some iron oxides (Fe2O3). Cements are classified as non-hydraulic or hydraulic, based on the ability of the cement to set in the presence of water. Non-hydraulic cement does not set  142 in the presence of water. Oppositely, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting. Hydraulic cements (e.g., Portland cement) set in the presence of water because of the formation of water- insoluble metal hydrates. 3.5.1.Production of Portland cement Basically, the cement production process involves two main steps; clinker formation step and cement formulation step. Clinker is produced by mixing the cement raw materials at high temperatures, up to 2000 oC, in a rotary kiln. The presence of some ferric oxide in the mixture of the raw materials helps in the formation of the clinker at lower temperatures (around 1300oC). Afterwards, the formed clinker is crushed and grinded in a cement grinding mill. Some additives,  143 such as calcium sulphate or limestone, are grinded in a cement grinding mill too, leading to a fine and homogenous cement powder. The cement is then stored in storage tower before being shipped either in bulk or bagged. Clinker consists of four main phases: 1. C3S: Tri Calcium Silicate (Alite) (3CaO·SiO2) 2. C2S: Di Calcium Silicate (Belite) (2CaO·SiO2) 3. C3A: Tri Calcium Aluminate (celite) (3CaO·Al2O3) 4. C4AF: Tetra Calcium Alumino Ferrite (Brownmillerite) (4CaO·Al2O3·Fe2O3). The silicates are responsible for the cement's mechanical properties, the tricalcium aluminate and Tetra Calcium Alumino Ferrite are essential for the formation of the liquid phase during the burning process of clinker in the kiln.  144 First, the limestone (calcium carbonate) is burned to eliminate its carbon, creating lime (calcium oxide) in what is known as a calcination reaction. This single chemical reaction is a main source of global carbon dioxide emissions. CaCO3 → CaO + CO2 The lime reacts with SiO2 to yield dicalcium silicate and tricalcium silicate. 2CaO + SiO2 → 2CaO·SiO2 3CaO + SiO2 → 3CaO·SiO2 The lime also joins with aluminum oxide to produce tricalcium aluminate. 3CaO + Al2O3 → 3CaO·Al2O3 The lime also unifies with aluminum oxide, and ferric oxide to give cement. 4CaO + Al2O3 + Fe2O3 → 4CaO·Al2O3·Fe2O3 (cement)  145 Fig. 2 Cement production steps 3.5.2.Main tests for cement quality: 5. Magnesium oxide percentage: This percentage should not exceed 5%, since its increase means that the quality of the used limestone is poor. Magnesia is rather refractory and does not take part in the cement reactions.  146 6. Powder size ‫النعومة‬ The prepared cement should be very fine, such that at least 98% of it should pass through mesh 200 (200 pinch per linear inch). 7. Setting time: This test is done to test the speed of cement solidification. For this purpose, samples of cement are mixed with standard amounts of water. In this test, the time at which a standard needle loaded with a standard weight cannot penetrate the prepared sample is measured. 8. Date of Manufacturing It is very important to check the manufacturing date because the strength of cement decreases with time. It's better to use cement before 3 months from the date of manufacturing.  147 3.6. Concrete A good quality concrete is basically a heterogeneous mixture of cement, coarse and fine aggregates and water which combines into a hard mass due to chemical action between the cement and water. In concrete production, each component has its specific role. The coarser aggregates (gravel) acts as fillers. The fine aggregates (sand) occupy the holes between the paste and the gravel. The cement with water acts as a binder. 3.6.1. Properties of Fresh Concrete: Concrete should preserve its fresh form the time it mixed until the time it compacted. The properties of the fresh concrete are very critical because it affects its quality after being hardened. Concrete consistency, workability, and settlement and bleeding  148 are important properties that should be taken into account when working with concrete. 1. Concrete consistency Concrete consistency reflects the stiffness or sloppiness or the concrete fluid. For good handling, placing and compacting of the concrete, consistency must be the same for each batch. 2. Concrete workability The workability of a concrete is a measure of how easy a concrete can be placed, compacted and finished without parting of the individual materials. Workability is not the same thing as consistency. Workability is size dependent property; concrete mixes made up with smaller stones are more workable than that with larger stones.  149 3. Settlement and Bleeding Cement and aggregate particles are three times denser than water. Therefore, in concrete mix they have a tendency to to settle and displace mixing water which moves upward the concrete surface. This upward movement of mixing water is known as bleeding; water that splits from the rest of the concrete is called bleed water.  150 CHAPTER (4) Finishing Materials Prepared by Dr. Ahmed A. Younes  151 Chapter 4: Finishing Materials 8.1. Importance of Finishes 8.2. Wall finishes 8.2.1.Plastering 8.2.2.Pointing 8.2.3.Distempering 8.2.4.Painting  152 Chapter 4: Finishing Materials 4.1. Importance of Finishes Surface finishes not only make products look nice, but they also ensure the product performance for long time. Moreover, finishes protect the products from outside damaging reactions, such as corrosion, wear and rust. For example, the paint works as a protective film for the body of your car. If your car is left scratched for long time, this paint off area will likely be corroded and this eventually will damage your car body. Certain types of finishes, such as aerospace coatings, can influence the performance of the product itself. These coatings guard the aircraft from weather resistance and sun destruction, ensuring its safety in the sky.  153 4.2. Wall finishes 4.2.1.Plastering )‫(محارة‬ Plastering is a protective and decorative layer over walls or concrete surface to protect them against the atmospheric effect and give them nice appearance. The plaster is prepared by mixing sand and lime or cement concrete along with water. Plastering can be applied for various purposes. These include increasing the durability of the wall, decorating the structures of the walls, covering the uneven surface and rough walls, preventing water entrance into brick-work, and making a proper base ready for further painting works.  Requirement of Good Plaster The surface of a good plaster should be smooth, non- absorbent, not wash by water, and paintable.  154 Moreover, good plaster should be firmly attached to the base surface, not shrink when it dries, fire resistant, and sound insulated.  Plaster Defects and their Solution: If the plaster quality is not good enough it can cause one of the problems listed below. a. Plaster De-bonding Plaster de-bonding means that a plaster is disconnected from the wall. The reasons behind this phenomenon may be a thick plaster layer, insufficient substrate preparation or the use of dusty, oily or dry substrate. To avoid plaster de-bonding, we should remove any dusts or oils from the substrates, prepare the plaster in a good manner and finally add bonding chemical.  155 b. Cracks on Plastered Surface Cracks on plastered surfaces are very common problem that can be observed. Cracks are of different forms and due to different reasons. Crazing cracks are fine cracks like spider web. They happen because of the presence of excess fine content in the sand or due to dry wall on which plaster is applied, when the wall absorbs the water and fines gather on the surface, it leads to crazing. Separation cracks at joints occur at joints of two different materials because of differential thermal movement. Crack with Hollowness occurs due to hollowness in plaster because of extra water in the plaster mix or due to poor workmanship. c. Efflorescence on Plastered Surface When a newly built wall dries out, the soluble salts get out to the surface and appear as whitish  156 crystalline substances. This is called efflorescence. Efflorescence is formed on plasters when soluble salts exist in plaster itself or in one of the building materials such as bricks, sand, cement etc. It badly affects the adhesion of paints with the wall surface. To avoid this problem, all construction materials as well as plaster materials should be free from salt. d. Falling Out of Plaster Falling out of plaster from the plastered walls is of two types, flaking or peeling off. Flaking of plaster means that a small loose mass on the plastered surface is formed due to bad bonding between successive coats of plaster. Peeling off plaster means the formation of a patch in the plastered wall because plaster comes off from the surface. This is also because bond failure between successive coats of plaster. Both forms of falling out of plasters can be  157 prevented by proper material selection, surface preparation and good workmanship. e. Popping of Plaster Popping is the formations of holes that break out of the plaster. It is produced due to the existence of contaminant particles such as burnt lime or other organic constituents in the mix of mortar. Removal of any contaminants from the mortar mix will prevent popping of plaster. 4.2.2.Pointing )‫(ترتشة‬ Pointing is the finishing of mortar joints in brick or stone masonry construction. Pointing is the implementing of joints to a depth of 10 mm to 20 mm and filling it with better quality mortar in desired shape. It is done for cement mortar and lime mortar joints. Pointing finishing is applied to protect the  158 exposed surface from adverse effects due to atmospheric action like rain, sun, wind and snow, or to enhance the appearance. Flush pointing is the most available type of pointing and is generally employed in brick masonry and stone masonry. In flush pointing, mortar is pushed into the gathered joints and joints are made flush with the edge of the stone or brick to provide a uniform appearance. After that, with the help of a trowel and straight edge, edges are precisely trimmed. This type of pointing doesn‘t have a good appearance, but it doesn‘t have any space for dust and water which make it long-lasting. Recessed Pointing is another form of pointing. It has a vertical pointing face and offers a better appearance. A recessed pointing mortar is pushed back inside the wall surface using a proper pointing tool.  159 4.2.3.Distempering (‫(طالء مائي‬ Distemper is a water based paint in which the binding medium consists mainly of either glue or casein. The main ingredients of distemper are chalk, lime, water and some coloring agents if required. They are also known as cement paint; because it can be applied directly on cement walls without any other coating on them. The distempers are offered in powder form or paste form. They are to be mixed with hot water before use. As the water dries, the oil provides a hard washable surface.  Process of Distempering: The application of distemper is carried out in three successive steps (1) surface preparation, (2) prime coating and (3) distemper coating.  160 a. Surface preparation The surface to be distempered should be carefully rubbed and cleaned. The new plastered surfaces should be kept exposed for a period of two months or so to dry out before distemper is applied on them. If distemper is to be applied on previously distempered surfaces, the old distemper should be detached. b. Priming coating After preparing the surface, the surface is coated with priming coat is allowed dry. c. Distemper coating Normally two layers of distemper are applied. The first layer should be light color and applied with great care. The second coat of distemper is applied after the first coat has dried and become hard.  161 4.2.4.Painting Paints are coatings of fluid materials which are applied as a final finish to surfaces like walls, wood and metal works. Painting is done to protect the surface from the effects of weathering, to prevent wood from decay and metal from corrosion, to provide a decorative finish. Painting process can be applied to new or old wood work, new or old iron and steel surfaces, galvanized iron surface, metals and plastered surfaces. For painting new wood surfaces the following steps should be followed (1) surface preparation, the surface to be painted should be clean, dry and free of dusts or spots, (2) knotting, knots in the wood surface must be killed or covered, (3) priming, applying a prime or a first coat on the wood surface to make the surface smooth, (4) stopping, in this step nail holes and cracks are filled using putty then the entire  162 surface is rubbed with glass paper, (5) under coating, the process by which second and third coats are applied, and (6) finishing, applying the last coat on the wood surface.  163 CHAPTER (5) Dyes and Pigments Prepared by Dr. Ahmed A. Younes  164 Chapter 5: Dyes and Pigments 5.1. Introduction 5.2. Classification of dyes 5.3. Selection of Dyes 5.4. Considerations in Dye Design 5.5. Toxicological considerations 5.6. Dyes versus pigments  165 Chapter 5: Dyes and Pigments 5.1 Introduction If a molecule absorbs ligh