Bonding, Structure and the Properties of Matter (review)

1. Soft drink cans are made from sheets of the Group 3 metal aluminium. The sheets are cut and pressed into the shapes required to make the cans.

a) Explain why sheets of aluminium metal can be pressed into different shapes. Use ideas about the structure and bonding of aluminium in your answer.

Answer: Aluminium has a lattice structure with layers of metal atoms surrounded by delocalised electrons. The layers of atoms are able to slide past each other easily when a force is applied so the metal is malleable and can be shaped.


b) Overhead power cables used in the National Grid are made from aluminium. Explain why aluminium is able to conduct electricity.

Answer: Electricity is the flow of charged particles. Aluminium contains delocalised electrons that can move and carry the charge.


c) The surface of an aluminium power cable is protected from corrosion by a layer of aluminium oxide that forms naturally. Aluminium oxide is an ionic compound with a very high melting point. Describe how an ionic bond forms when aluminium reacts with oxygen.

Answer: Each aluminium atom loses three electrons to form Al3+ ions. Each oxygen atom gains two electrons to form O2− ions. The oppositely charged ions attract each other strongly, and this attraction is the ionic bond.


d) Give the reason for the high melting point of aluminium oxide.

Answer: The ionic bond is very strong so a lot of heat energy is required to overcome it.


e) Explain why the aluminium oxide that protects the power cables does not conduct electricity.

Answer: Electricity is the flow of charged particles. The aluminium oxide is solid so the ions are unable to move and carry the charge.


f) Aluminium oxide nanoparticles can be used to improve the properties of ceramic materials. Explain why nanoparticles of aluminium oxide have different properties to those of the bulk material.

Answer: Nanoparticles have a much higher surface area to volume ratio.


2. Carbon forms many different covalent substances with very different properties. Methane (CH4 ), buckminsterfullerene (C60 ) and silicon carbide (SiC) are three examples. Some properties of methane and silicon carbide are given in the table below.


a) Draw the dot and cross diagram for a molecule of methane.

Answer:


b) Explain the properties of methane shown in the table above. Give your answer in terms of structure and bonding.

Answer: Methane is made up of small molecules with weak intermolecular forces between them. These forces do not require a lot of heat energy to overcome, so methane has a low melting and boiling point. Electricity is the flow of charged particles, but the molecules do not have an overall electric charge, so methane does not conduct electricity.


c) Recent research suggests that methane molecules can be stored inside fullerenes such as buckminsterfullerene. Describe the property of fullerenes that enables them to contain small molecules within their structures.

Answer: Fullerenes have a cage-like/hollow structure that smaller molecules can be put into.


d) Silicon carbide has similar bonding and structure to diamond, but its melting point is significantly lower than for diamond. Describe the structure and bonding of diamond.

Answer: Diamond has a giant covalent structure where every carbon atom is bonded to four other carbon atoms.


e) Why is this difference in melting points of silicon carbide and diamond?

Answer: The bonds between the atoms in silicon carbide are weaker than in diamond.


3. The table below shows some properties of five different substances.


a) Which substance is most likely to be chlorine (Cl2 )? Explain your answer.

Answer: Substance B as it has the lowest melting and boiling point. Substance B as it is a poor conductor of electricity as a solid and as a liquid, so must be simple covalent/small molecules.


b) Identify the particles responsible for carrying the charge when substance D conducts electricity.

Answer: Delocalised electrons (substance D must be a metal).


c) Determine which substance is most likely to consist of oppositely charged ions in a lattice structure. Justify your answer in terms of structure and bonding.

Answer: Substance A. The structure is ionic and the bonds between the oppositely charged ions are strong so a lot of heat energy is requiredto overcome them (giving high melting and boiling points). When the substance is solid the ions are unable to move and carry the charge so it cannot conduct electricity, but when it is a liquid the ions are free to move and carry the charge, so it can conduct electricity.


d) Explain why substance C could be graphite. Answer in terms of structure and bonding

Answer: Substance C has a very high melting point. Graphite has a giant covalent structure with many strong covalent bonds between the carbon atoms. A lot of heat energy is required to overcome these bonds so it has a very high melting point. Graphite contains delocalised electrons that can carry the charge, so it conducts electricity.