Chemistry of Engineering Materials PDF

CHEMISTRY OF ENGINEERING MATERIALS CHARACTERISTIC PROPERTIES OF GASES, LIQUIDS, AND SOLIDS In gases, the distances between molecules are so great (compared with their diameters) that at ordinary temperatures and pressures (25°C and 1 atm), there is no appreciable interaction between the molec...

CHEMISTRY OF ENGINEERING MATERIALS CHARACTERISTIC PROPERTIES OF GASES, LIQUIDS, AND SOLIDS In gases, the distances between molecules are so great (compared with their diameters) that at ordinary temperatures and pressures (25°C and 1 atm), there is no appreciable interaction between the molecules. Because there is a great deal of empty space in a gas—that is, space that is not occupied by molecules—gases can be readily compressed. The lack of strong forces between molecules also allows a gas to expand to fill the volume of its container. Furthermore, the large amount of empty space explains why gases have very low densities under normal conditions. Liquids and solids are quite a different story. The principal difference between the condensed states (liquids and solids) and the gaseous state is the distance between molecules. In a liquid, the molecules are so close together that there is very little empty space. Thus, liquids are much more difficult to compress than gases, and they are also much denser under normal conditions. Molecules in a liquid are held together by one or more types of attractive forces. A liquid also has a definite volume, because molecules in a liquid do not break away from the attractive forces. The molecules can, however, move past one another freely, and so a liquid can flow, can be poured, and assumes the shape of its container. In a solid, molecules are held rigidly in position with virtually no freedom of motion. Many solids are characterized by long-range order; that is, the molecules are arranged in regular configurations in three dimensions. There is even less empty space in a solid than in a liquid. Thus, solids are almost incompressible and possess definite shape and volume. INTERMOLECULAR FORCES Intermolecular Forces – are attractive forces between molecules that exert more influence in the condensed phases of matter – liquids and solids. Intramolecular Forces – are forces that hold atoms together in a molecule (chemical bonding). Intramolecular forces stabilize individual molecules, whereas intermolecular forces are primarily responsible for the bulk physical properties of matter. 1|Page  Types of Intermolecular Forces 1. Van der Waals forces – are attractive forces between molecules that exert more influence in the condensed phases of matter – liquids and solids. a. Dipole-Dipole Forces – are attractive forces between polar molecules, that is, between molecules that possess dipole moments (or uneven distribution of electrons in a molecule forming partial positive or negative charges). b. Ion-Dipole Forces – are attractive forces between an ion (either cation or anion) and a polar molecule. Hydration is one example of ion-dipole interaction between cations and anions in an ionic compound with water. c. Dispersion Forces – or London forces, are attractive forces between nonpolar molecules that arise as a result of temporary dipoles induced in atoms or molecules. Induced dipole – the separation of (+) and (-) charges in a nonpolar molecule due to the proximity of an ion or a polar molecule. i. Ion-induced dipole forces ii. Dipole-induced dipole forces Dispersion forces usually increase with molar mass because larger molar mass molecules tend to have more electrons, and dispersion forces increase in strength with the number of electrons. Also, larger molar mass often means a bigger atom whose electron distribution is more easily disturbed because the outer electrons are less tightly held by the nuclei. 2. Hydrogen Bond – a special type of dipole-dipole interaction between the hydrogen atom in a polar bond, such as N-H, O-H, or F-H, and an electronegative N, O, or F atom. N, O, and F atoms all possess at least one lone pair that can interact with the hydrogen atom in hydrogen bonding. Intermolecular forces, although they are attractive in nature, the molecules also exert repulsive forces on one another. The magnitude of the repulsive force rises very steeply as the distance separating the molecules in a condensed phase decreases. This is the reason that liquids and solids are so hard to compress since the molecules are already in close contact with one another. PROPERTIES OF LIQUID 1. Surface Tension – is a measure of the elastic force in the surface area of a liquid. It is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Molecules of liquid at the surface are pulled downward and sideways by other molecules, but not upward away from the surface. These intermolecular attractions thus tend to pull the molecules into the liquid and cause the surface to tighten like an elastic film minimizing the surface area of a liquid. Liquids that have strong intermolecular forces also have high surface tensions. Water, because of hydrogen bonding, has a considerably greater surface tension than most other liquids. 2|Page  Capillary action – an example of surface tension. Water rising spontaneously in a capillary tube. Two types of forces that bring about capillary action: i. Cohesion – the intermolecular attraction between like molecules (i.e., between water molecules) ii. Adhesion – an attraction between unlike molecules (i.e., water molecules and the sides of a glass tube) 2. Viscosity – is a measure of a fluid’s resistance to flow. The greater the viscosity, the more slowly the liquid flows. The viscosity of a liquid usually decreases as temperature increases. Liquid that have strong intermolecular forces have higher viscosities than those that have weaker intermolecular forces. CATEGORIES OF SOLID Two Categories of Solid: 1. Crystalline Solid – possesses rigid and long-range order; its atoms, molecules, or ions occupy specific positions. The arrangement of such particles in a crystalline solid is such that the net attractive intermolecular forces are at their maximum. 2. Amorphous Solid – lack a well-defined arrangement and long-range molecular order. CRYSTAL STRUCTURE Unit Cell – the basic repeating structural unit of a crystalline solid. Any of the unit cells, when repeated in space in all three dimensions, forms the lattice structure (or crystal lattice) characteristic of a crystalline solid. 3|Page PACKING SPHERES To understand the general geometric requirements for crystal formation is by considering the different ways of packing a number of identical spheres (Ping-Pong balls, for example) to form an ordered three-dimensional structure. The way the spheres are arranged in layers determines what type of unit cell. coordination number - is the number of atoms (or ions) surrounding an atom (or ion) in a crystal lattice. Its value gives us a measure of how tightly the spheres are packed together—the larger the coordination number, the closer the spheres are to each other. Types of cubic cells: 1. simple cubic cell – the basic, repeating unit in the array of spheres with a coordination number of 6 (each sphere is in contact with four spheres in its own layer, one sphere in the layer above, and one sphere in the layer below). 2. body-centered cubic cell (bcc) – an arrangement that differs from a simple cube, in that the second layer of spheres fits into the depressions of the first layer and the third layer into the depressions of the second layer. The coordination number of each sphere in this structure is 8 (each sphere is in contact with four spheres in the layer above and four spheres in the layer below). 3. face-centered cubic cell (fcc) – an arrangement where there are spheres at the center of each of the six faces of the cube, in addition to the eight corner spheres. The coordination number of each sphere in this structure is 12 (each sphere is in contact with six spheres in its own layer, three spheres in the layer above, and three spheres in the layer below). 4|Page Closest packing – is the most efficient arrangement of spheres 1. cubic close-packed (ccp) structure – an ABC layer arrangement which corresponds to the face-centered cube. 2. hexagonal close-packed (hcp) structure – an ABAB layer arrangement where the spheres in every other layer occupy the same vertical position. In both structures, each sphere has a coordination number of 12. Both the hcp and ccp structures represent the most efficient way of packing identical spheres in a unit cell, and there is no way to increase the coordination number to beyond 12. 5|Page TYPES OF CRYSTALS 1. Ionic Crystals – have two important characteristics: (a) They are composed of charged species and (b) anions and cations are generally quite different in size. Most ionic crystals have high melting points, an indication of the strong cohesive forces holding the ions together. A measure of the stability of ionic crystals is the lattice energy; the higher the lattice energy, the more stable the compound. These solids do not conduct electricity because the ions are fixed in position. However, in the molten state (that is, when melted) or dissolved in water, the ions are free to move and the resulting liquid is electrically conducting. 6|Page 2. Covalent Crystals – atoms are held together in an extensive three-dimensional network entirely by covalent bonds. Well-known examples are the two allotropes of carbon: diamond and graphite. The strong covalent bonds in three dimensions contribute to diamond’s unusual hardness (it is the hardest material known) and very high melting point (3550°C). In graphite, carbon atoms are arranged in six-membered rings. The covalent bonds in graphite account for its hardness; however, because the layers can slide over one another, graphite is slippery to the touch and is effective as a lubricant. It is also used in pencils and in ribbons made for computer printers and typewriters. Another covalent crystal is quartz (SiO2). The arrangement of silicon atoms in quartz is similar to that of carbon in diamond, but in quartz there is an oxygen atom between each pair of Si atoms. Because Si and O have different electronegativities, the Si¬O bond is polar. Nevertheless, SiO2 is similar to diamond in many respects, such as hardness and high melting point (1610°C). 3. Molecular Crystals – the lattice points are occupied by molecules, and the attractive forces between them are van der Waals forces and/or hydrogen bonding. An example of a molecular crystal is solid sulfur dioxide (SO 2), in which the predominant attractive force is a dipole-dipole interaction. Intermolecular hydrogen bonding is mainly responsible for maintaining the three- dimensional lattice of ice. 7|Page  4. Metallic Crystals – the structure of metallic crystals is the simplest because every lattice point in a crystal is occupied by an atom of the same metal. Metallic crystals are generally body-centered cubic, face-centered cubic, or hexagonal close-packed. Consequently, metallic elements are usually very dense. CONDUCTIVITY PROPERTY OF METALS Band Theory – states that delocalized electrons in metals move freely through “bands” formed by overlapping molecular orbitals. Valence Band – are the set of closely spaced filled molecular orbitals Conduction Band – are the set of closely spaced empty molecular orbitals that are higher in energy. Conductors (metals) – the valence and conduction bands have no energy gap between them, so electrons flow when a tiny electrical potential difference is applied. When the temperature is raised, greater random motion of the atoms hinders electron movement: conductivity decreases when a metal is heated. Insulators (nonmetals) – a large energy gap exists between the valence and conduction bands: no current is observed even when the substance is heated. Semiconductors (metalloids) – a small energy gap exists between the valence and conduction bands. Thermally excited electrons can cross the gap, allowing a small current to flow: conductivity increases when a semiconductor is heated. SEMICONDUCTORS A number of elements are semiconductors, that is, they normally are not conductors, but will conduct electricity at elevated temperatures or when combined with a small amount of certain other elements. The Group 4A elements silicon and germanium are especially suited for this purpose. The use of semiconductors in transistors and solar cells, to name two applications, has revolutionized the electronic industry in recent decades, leading to increased miniaturization of electronic equipment. The ability of a semiconductor to conduct electricity can also be enhanced by adding small amounts of certain impurities to the element, a process called doping. Let us consider what happens when a trace amount of boron or phosphorus is added to solid silicon (semiconductor). 8|Page The structure of solid silicon is similar to that of diamond; each Si atom is covalently bonded to four other Si atoms. Phosphorus ([Ne]3s23p3) has one more valence electron than silicon ([Ne]3s23p2), so there is a valence electron left over after four of them are used to form covalent bonds with silicon. This extra electron can be removed from the phosphorus atom by applying a voltage across the solid. The free electron can move through the structure and function as a conduction electron. Impurities of this type are known as donor impurities, because they provide conduction electrons. n-type semiconductors – are solids containing donor impurities, where n stands for negative (the charge of the “extra” electron). The opposite effect occurs if boron is added to silicon. A boron atom has three valence electrons (1s22s22p1). Thus, for every boron atom in the silicon crystal there is a single vacancy in a bonding orbital. It is possible to excite a valence electron from a nearby Si into this vacant orbital. A vacancy created at that Si atom can then be filled by an electron from a neighboring Si atom, and so on. In this manner, electrons can move through the crystal in one direction while the vacancies, or “positive holes,” move in the opposite direction, and the solid becomes an electrical conductor. Impurities that are electron deficient are called acceptor impurities. p-type semiconductors – are semiconductors that contain acceptor impurities, where p stands for positive. In both the p-type and n-type semiconductors, the energy gap between the valence band and the conduction band is effectively reduced, so that only a small amount of energy is needed to excite the electrons. Typically, the conductivity of a semiconductor is increased by a factor of 100,000 or so by the presence of impurity atoms. SUPERCONDUCTORS Superconductivity – another type of electrical conductivity where a metal conducts with no energy loss or resistive heating by minimizing the atomic vibrational movement through cooling using either liquid helium or liquid nitrogen. 9|Page POLYMERS Polymer – is an extremely large molecular compound, or macromolecule, distinguished by a high molar mass, ranging into thousands and millions of grams, and made up of a covalently linked chain of smaller repeating units of molecules, called monomers. Two Types of Polymers: 1. Naturally occurring polymers include proteins, nucleic acids, cellulose (polysaccharides), and rubber (polyisoprene). 2. Synthetic polymers are organic compounds. Familiar examples are nylon which is poly(hexamethylene adipamide); Dacron which is poly(ethylene terephthalate); and Lucite or Plexiglas which is poly(methyl methacrylate). SYNTHETIC ORGANIC POLYMERS Synthetic polymers are created by joining monomers together, one at a time, by means of addition reactions and condensation reactions. 1. Addition reactions involve unsaturated compounds containing double or triple bonds, particularly C=C and C≡C. Hydrogenation and reactions of hydrogen halides and halogens with alkenes and alkynes are examples of addition reactions. 10 | P a g e Polyethylene is an example of a homopolymer, which is a polymer made up of only one type of monomer. The individual chains of polyethylene pack together well and so account for the substance’s crystalline properties. Polyethylene is mainly used in films, in frozen food packaging and other product wrappings. A specially treated type of polyethylene called Tyvek is used for home insulation. Other homopolymers that are synthesized by the radical mechanism are Teflon, polytetrafluoroethylene and poly(vinyl chloride) (PVC): 11 | P a g e 2. Condensation reactions – two functional groups on the monomer react together to form small molecules. Those small molecules then split off, and the remaining parts of the two monomers are joined together. Two common condensation reactions are the production of Nylon and Dacron, which are important in fiber manufacturing. Nylon is referred to as polyamide while Dacron is a polyester. 12 | P a g e PHYSICAL PROPERTIES OF POLYMERS There are two key reasons for the importance of polymers as engineering materials. First, polymers offer physical properties such as strength and elasticity that can be desirable in a wide range of uses. Second, those properties can be controlled or tailored to a greater degree than is usually possible in metals or other classes of matter. THERMAL PROPERTIES OF POLYMERS Two categories of polymers in terms of its thermal property: 1. Thermoplastic Polymers – polymers that melt or deform on heating. Depending on their complexity, products made from thermoplastic polymers are typically extruded or formed in molds or presses that allows shaping into the desired form. Once cooled, the polymer solidifies and regains its structural properties. 2. Thermosetting Polymers – polymers that can maintain their shape and strength when heated. 13 | P a g e  The name “thermosetting” comes from the fact that these polymers must be heated to set or “lock in” their structures. But once this has been done, the materials offer increased strength and do not lose their shape upon further heating. Rather than being extruded, most thermosetting polymers are molded. An important example of the engineering importance of thermosetting polymers forming cross-linking bonds is the discovery of vulcanization. In vulcanization, natural rubber is heated in the presence of sulfur. This produces cross-linking and leads to a harder material that is markedly more resistant to heat. Until vulcanization was discovered, natural rubber was difficult to use in applications such as automobile tires because it would become sticky when heated. CONDUCTIVITY PROPERTIES OF POLYMERS Applications of conducting polymers are currently being pursued in several areas, most notably as organic light-emitting diodes (OLEDs). Different preparations can produce polymers that will display different colors. Because the polymers can be dissolved, they can be manipulated in intriguing ways. A polymer solution can be deposited through a process similar to ink-jet printing, and adding contacts can produce a highly customized display that can be applied to almost any surface. The development of conducting polymers has seen important contributions from both chemists and engineers, as is usually the case. The interplay between the discoveries of the scientist, driven largely by curiosity, and the advances of the engineer seeking a solution to a specific problem is synergistic. Progress on either side fuels the further thinking of both groups and spurs the next generation of advances. NANOTUBES For decades, it was known that there were two forms of the element carbon: graphite and diamond. But in 1985, a team of chemists at Rice University discovered a new form of carbon, whose 60 atoms formed a framework that looks like a tiny soccer ball. Because the structure resembled the geodesic domes popularized by the architect Buckminster Fuller, it was given the whimsical name buckminsterfullerene. The discovery of C60 and other related molecules, now collectively known as the fullerenes, helped usher in a new form of carbon and also a new branch of science— nanotechnology. 14 | P a g e New carbon structures are certainly not the only area where nanotechnology research is flourishing, but the story of C60 is an excellent example of the way the study of chemistry affects the development of new materials. What do the new discoveries about C60 hold for materials that engineers might use in the 21st century? Although the small sphere of C60 may not have any immediate uses, manipulation of experimental conditions allows the growth of tubes of carbon, called nanotubes. These materials are not yet viable for large-scale design projects, but they have remarkable properties. Their tensile strength is significantly higher than that of steel, and futurists have already begun to contemplate their potential for things as speculative as space elevators. Carbon nanotubes grown with a metallic element enclosed within them might represent a way to build wires that are one molecule wide. The implications for miniaturization of electronic devices are among the most immediate and exciting opportunities promised by new nanotechnologies. So, scientists and engineers are actively exploring fundamental questions related to nanotechnology. Development of new materials has followed a number of different pathways, depending on both the nature of the problem being pursued and the means of investigation. Breakthroughs in the discovery of new materials have ranged from pure serendipity, to trial-and-error approaches, to design by analogy to existing systems. These methodologies will remain important in the development of materials but as the challenges and requirements for new materials become more complex, the need to design and develop new materials from the molecular scale through the macroscopic final product will become increasingly important. The use of molecular modeling and the engineering of new materials into useable forms or devices are of particular importance. 15 | P a g e