Chemical Bonding and Structure Past Paper PDF

Nanyang Girls’ High School Name: Science Department (Chemistry) Class: Chemical Bonding and Structure Learning Outcomes Candidates should be able to: Structure and properties of materials (a) describe the differences between elements...

Nanyang Girls’ High School Name: Science Department (Chemistry) Class: Chemical Bonding and Structure Learning Outcomes Candidates should be able to: Structure and properties of materials (a) describe the differences between elements, compounds and mixtures (b) compare the structures of the following substances in order to deduce their properties: (i) simple molecular substances, e.g. methane; iodine (ii) macromolecules, e.g. poly(ethene) (will be covered under Organic Chemistry) (iii) giant covalent substances, e.g. sand (silicon dioxide); diamond; graphite (c) compare the bonding and structures of diamond and graphite in order to deduce their properties such as electrical conductivity, lubricating or cutting action (candidates will not be required to draw the structures) (d) deduce the physical and chemical properties of substances from their structures and bonding and vice versa. Ionic bonding (a) describe the formation of ions by electron loss/gain and that these ions usually have the electronic configuration of a noble gas (b) describe, including the use of ‘dot-and-cross’ diagrams, the formation of ionic bonds between metals and non-metals, e.g. NaCl; MgCl2 (c) state that ionic materials contain a giant lattice in which the ions are held by electrostatic attraction, e.g. NaCl (candidates will not be required to draw diagrams of ionic lattices) (d) relate the physical properties (including electrical property) of ionic compounds to their lattice structure. Covalent bonding (a) describe the formation of a covalent bond by the sharing of a pair of electrons and that the atoms in the molecules usually have the electronic configuration of a noble gas (b) describe, using ‘dot-and-cross’ diagrams, the formation of covalent bonds between non- metallic elements, e.g. H2; O2; H2O; CH4; CO2 (c) deduce the arrangement of electrons in other covalent molecules (d) relate the physical properties (including electrical property) of covalent substances to their structure and bonding. Metallic bonding (a) describe metals as a lattice of positive ions in a ‘sea of electrons’ (b) describe the general physical properties of metals as solids having high melting and boiling points, malleable, good conductors of heat and electricity in terms of their structure. 1  Chemical Bonding 1) What is Bonding?  It is the way in which atoms are joined together and combine with one another.  The arrangement of the resulting particles is known as the structure.  The aim of every atom during bonding is to achieve a fully-filled valence electron shell which is very stable. 2) Why do elements react? To become more stable when their atoms obtain fully-filled valence electron shells (not including transition elements and 3d orbitals)  Atoms of these elements can do so by losing/gaining/sharing electrons 3) Why are some elements unreactive?  These elements are mainly found in Group 18 of the Periodic Table and is known as the Noble gases  Nobel gases are inert (unreactive) atoms as they already have fully- filled valence electron shells. Overview of Bonding and Structure Chemical Bonding bonding Ionic Bonding Covalent Bonding Metallic Bonding structure Giant ionic Simple covalent Giant covalent Giant metallic structure structure structure structure All types of bonds consist of electrostatic forces of attraction, the force of attraction that arises between positive and negative charges. 2 Ionic bonding Formation of ions and transfer of electrons  All atoms are electrically neutral (overall charge = 0) as the number of protons and electrons in an atom are the same.  Ions are charged particles formed when atoms lose/gain electrons. o Metals tend to lose electrons to form cations o Non-metals tend to gain electrons to form anions.  Example: Transfer of electrons from sodium to chlorine to form sodium chloride dot-and-cross diagram showing all electrons Definition of ionic bonds  Strong electrostatic forces of attraction between oppositely charged ions. o In the above example, the electrostatic forces of attraction between sodium cations and chloride anions are the ionic bonds. Dot-and-cross diagrams for ionic substances  Typically, dot-and-cross diagrams showing only valence shells are required (shown below), rather than the diagram drawn above for sodium chloride showing all electrons.  Practice: Draw the dot-and-cross diagram for potassium sulfide, K2S aluminium oxide, Al2O3 magnesium nitride, Mg3N2 3 Structure of ionic compounds  Ionic compounds exist in a giant ionic (lattice) structure as shown below, not just with one cation and one anion. Example: structure of sodium chloride A giant structure is a three-dimensional network of atoms or ions in an extended regular pattern. 4 Properties of substances with Giant Ionic Structure How do we explain the following properties with reference to structure and bonding? Property Explanation Melting and boiling point  It has a giant ionic / crystal lattice structure  High melting & boiling point   Eg: sodium chloride Large amount of energy needed to (m.p. 801 oC, b.p. 1465 oC) overcome strong electrostatic forces of attraction / strong ionic bonds between the  Generally exist as solids at r.t.p. oppositely-charged ions. Hardness  Ionic compounds are hard. Electrical Conductivity  It has giant ionic structure  Unable to conduct electricity in  In solid state, the oppositely-charged ions solid state are held in place by strong electrostatic forces of attraction. Hence, there are no free-moving / mobile ions to conduct electricity.  Able to conduct electricity in  In the aqueous / molten state, there are molten and aqueous state. free-moving / mobile ions to conduct electricity. Solubility  Many ionic compounds are soluble in water.  Usually insoluble in organic solvents (eg. ethanol, hexane). Brittleness  When the lattice is hit, a layer of ions is shifted so 5 Ionic compounds are brittle. that ions with the same charges are lined up together.  These like charges repel each other and split the ionic lattice causing it to shatter. 6 Covalent bonding  Covalent bond is formed by the sharing of electrons positively-charged positively-charged nucleus nucleus shared electrons used in bonding Dot-and-cross diagrams for covalent substances  Example: dot-and-cross diagram of chlorine molecule  Example: dot-and-cross diagram of carbon dioxide molecule Note: Use only dots and crosses, and alternate dots and crosses for neighbouring atoms. 7  Practice: Draw the dot-and-cross diagram for the following substances: nitrogen gas, N2 ammonia, NH3 ethene, C2H4 water, H2O methanone, CH2O 8 Types of covalent structures  Simple covalent structure (or simple molecular structure)  Giant covalent structure (or giant molecular structure) Simple covalent structure Strong covalent bonds Weak between atoms intermolecular H Cl forces of Cl H attraction H Cl In general, small, discrete molecules are present in simple covalent structures. In a simple covalent structure, there are:  strong covalent bonds between atoms in each molecule  weak intermolecular (i.e. between molecules) forces of attraction Strength of intermolecular forces substance Mr boiling point / °C H2 2.0 –252.9 N2 28.0 –195.8 O2 32.0 –183 Consider the boiling points of the substances shown above. What is the trend observed? Explain.  Boiling point increases from H2 to N2 to O2  H2, N2 and O2 all exist as small discrete molecules with simple covalent structures  Strength of intermolecular forces of attraction increases as the relative molecular mass increases from H2 to N2 to O2.  More energy required to overcome the stronger intermolecular forces of attraction. 9 Properties of substances with Simple Covalent Structure How do we explain the following properties with reference to structure and bonding? Property Explanation Melting and boiling point  Low melting & boiling point  It consists of small, discrete molecules with a  Many substances with simple simple covalent / molecular structure covalent structures are liquids and  Only a small amount of energy needed to gases at room temperature. overcome the weak intermolecular forces of  E.g. CO2, H2, N2, CH4 etc. attraction Note: Strong covalent bonds between atoms are not broken (eg. water does not change chemically when boiled) during changes of state. Electrical Conductivity  It consists of small, discrete molecules with  Generally* unable to conduct a simple covalent / molecular structure electricity in any state  There are no mobile / free-moving ions or delocalised electrons to conduct electricity. *There are exceptions to this property that will be covered in the topic, Acids and Bases. Solubility  Many molecules with simple covalent structures are insoluble in water.  Examples of exceptions: ammonia, hydrogen chloride, sulfur dioxide (refer to Experimental Chemistry notes)  Generally soluble in organic solvents (eg. ethanol, hexane). 10 Giant covalent structure  They are made up of giant covalent molecules.  In certain molecules, atoms are joined together by strong covalent bonds to give a 3-D lattice. An example of substances with giant covalent structures are allotropes of carbon (diamond and graphite). Diamond and graphite are allotropes of carbon, which are different forms of an element in the same physical state. Diamond and graphite have similar chemical properties but different physical properties. Diamond  Each carbon atom is covalently bonded to 4 other carbon atoms in a repeated tetrahedral arrangement.  Melting points of some giant molecules with diamond-like structures: substance melting point / oC diamond (C) 3550 silicon carbide (SiC) 2700 boron nitride (BN) 3000 silicon dioxide (SiO2) 1610 silicon (Si) 1410 Graphite  Each carbon atom is covalently bonded to 3 other carbon atoms in a repeated hexagonal arrangement. They form parallel layers of carbon atoms, which are held by weak intermolecular forces of attraction. Layers of carbon atoms weak intermolecular forces of attraction between layers  Since only 3 out of 4 valence electrons of each carbon atom are involved in covalent bonding, the fourth valence electron is not used in bonding, and is delocalised and able to conduct electricity.  Melting point of graphite: 3625 °C 11 Properties of substances with Giant Covalent Structure How do we explain the following properties with reference to structure and bonding? Property Explanation Melting and boiling point Diamond and graphite  Very high melting & boiling  It has a giant covalent / molecular structure. point  Large amount of energy needed to overcome strong covalent bonds between carbon atoms. Electrical Conductivity  Diamond is unable to Diamond conduct electricity  It has a giant covalent / molecular structure.  Each carbon atom uses all four of its valence electrons to form covalent bonds with four other carbon atoms.  It does not have any delocalised electrons to conduct  Graphite is able to conduct electricity. electricity. Graphite  It has a giant covalent structure.  Each carbon atom uses 3 out of 4 of its valence electrons to form covalent bonds with 3 other carbon atoms.  Each carbon atom has one valence electron not used in bonding; it is delocalised and is able to conduct electricity. Hardness Diamond  Diamond is very hard.  It has a giant covalent / molecular structure. o Used as a cutting tool or  Large amount of energy needed to overcome strong drill bit to cut hard covalent bonds between carbon atoms. objects like glass and rocks. Graphite  Graphite is soft and slippery.  It has a giant covalent / molecular structure. o Used as a lubricant  Graphite is soft and slippery because the layers of o Used in pencil lead carbon atoms are held by weak intermolecular forces and can slide over each other easily. Solubility o Insoluble in water and organic solvents. 12 Metallic bonding Definition of metallic bonds  Strong electrostatic forces of attraction between metallic cations and ‘sea’ of delocalised electrons. Structure  Giant metallic structure metal cation + + + + + delocalised (positively- electron charged) + + + + + (negatively- charged) + + + + +  In metals, each metal atom contributes its valence electrons to a ‘sea’ of delocalised electrons which are free-moving and can move throughout the metallic structure. o Note: metals are electrically neutral (overall charge = 0) 13 Properties of substances with Giant Metallic Structure Property Explanation Melting and boiling point  It has a giant metallic structure.  Generally high melting &  Large amount of energy needed to overcome the strong boiling point electrostatic forces of attraction between the metallic  Exceptions: Group 1 metals cations and ‘sea’ of delocalised electrons. (low m.p. and b.p.) Electrical Conductivity  It has giant metallic structure.  Able to conduct electricity  The ‘sea’ of delocalised electrons are mobile / free- moving and therefore metals can conduct electricity. Pure metals are malleable and ductile.  Pure metals consist of orderly layers of cations of the same size. When a force is applied, these layers of cations can slide over each other easily. Solubility  Insoluble in water. 14 Summary of types of structures and bonding type of type of particles Forces of melting electrical electrica bonding structure involved attraction point conductivity l between when solid conducti particles vity when liquid oppositely- ionic giant ionic charged strong high poor good bonding structure ions simple covalent covalent molecules weak low poor poor bonding structure giant covalent covalent atoms strong high poor poor bonding structure metal giant cations and metallic metallic ‘sea’ of strong high good good bonding structure delocalised electrons 15