1. 1
   Pauling electronegativity values are: Cs 0.8, Mg 1.3, Al 1.6, Si 1.9, Cl 3.2, O 3.4 and F 4.0. In the bonding triangle the average electronegativity of the two elements is plotted horizontally and the difference in electronegativity vertically. The ionic vertex is at the top (large difference), the metallic vertex at the bottom left (low average, small difference) and the covalent vertex at the bottom right (high average, small difference).

   (a)

   What are the average electronegativity and the electronegativity difference for aluminium chloride?

   [1 mark]

   • AAverage 4.8, difference 1.6
   • BAverage 2.4, difference 1.6
   • CAverage 1.6, difference 2.4
   • DAverage 2.4, difference 4.8

   (b)

   Which compound lies closest to the ionic vertex of the bonding triangle?

   [1 mark]

   • AMgO
   • BAlCl₃
   • CSiO₂
   • DCsF

   (c)

   Deduce the predominant type of bonding in silicon dioxide, SiO₂, from its position in the bonding triangle, and predict one physical property that follows from it.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Poly(chloroethene), PVC, is made from chloroethene, CH₂=CHCl, and is used for window frames and pipes. Poly(propene) is made from propene, CH₂=CHCH₃, and is used for food containers. A sample of poly(propene) has an average molar mass of 2.10 × 10⁵ g mol⁻¹. Relative atomic masses: C 12.01, H 1.01.

   (a)

   Which is the repeating unit of PVC?

   [1 mark]

   • A–[CH=CHCl]–
   • B–[CHCl–CHCl]–
   • C–[CH₂–CHCl]–
   • D–[CH₂=CHCl]–

   (b)

   Approximately how many propene monomers make up one average chain in the sample?

   [1 mark]

   • A5.0 × 10³
   • B1.5 × 10⁴
   • C1.8 × 10⁴
   • D2.5 × 10³

   (c)

   Outline what happens to the carbon–carbon double bond when propene polymerises, and state the empirical formula of poly(propene).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two forms of poly(ethene) are made from ethene. Low-density poly(ethene), LDPE, has chains with many side branches; it has a density of 0.92 g cm⁻³ and softens at about 105 °C. High-density poly(ethene), HDPE, has mostly unbranched chains; it has a density of 0.96 g cm⁻³ and softens at about 130 °C. Both are electrical insulators, and both last for hundreds of years in landfill sites.

   (a)

   Explain, in terms of structure and bonding, why poly(ethene) is an electrical insulator and is not biodegradable.

   [3 marks]

   (b)

   Explain the differences in density and softening temperature between LDPE and HDPE.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Pure iron is soft and rusts easily. Steel is iron containing 0.2–1.5 % carbon and is much harder and stronger. Pure aluminium is soft, but duralumin (aluminium with about 4 % copper and small amounts of magnesium) is used for aircraft frames. Bridges and buildings are made from reinforced concrete: concrete, made largely of ionic and covalent network solids such as calcium silicates, is cast around steel bars. Concrete is strong when squeezed (compression) but cracks easily when stretched (tension); steel is strong in tension.

   (a)

   Explain why alloys such as steel and duralumin are harder and stronger than the pure metals, and why they still conduct electricity.

   [6 marks]

   (b)

   Evaluate reinforced concrete as a building material, explaining how the bonding in each component accounts for its properties and why the combination performs better than either component alone.

   [6 marks]

   Total for question 4: 12 marks

End of questions