1. 1
   A refinery separates crude oil into fractions by heating it and passing the vapours up a tall fractionating column. Crude oil is mainly a mixture of hydrocarbon molecules, most of which are alkanes. The column is hot at the bottom and cools towards the top, so fractions with different boiling points condense at different heights and are drawn off separately.

   (a)

   Which general formula correctly represents an alkane, where n is a whole number?

   [1 mark]

   • ACnH2n+2
   • BCnH2n
   • CCnH2n-2
   • DCnHn

   (b)

   Inside the fractionating column, the fraction collected near the top (where it is coolest) contains molecules with the shortest carbon chains. Compared with the long-chain fraction collected near the bottom, the short-chain fraction has:

   [1 mark]

   • AA higher boiling point and higher flammability
   • BA lower boiling point and higher flammability
   • CA lower boiling point and lower flammability
   • DA higher boiling point and lower viscosity

   (c)

   Explain, in terms of the size of the hydrocarbon molecules and the strength of intermolecular forces between them, why different fractions condense at different heights in the column.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A camper uses a small stove that burns propane gas (C3H8) to heat food. The stove is designed to mix the gas with a plentiful supply of air before it burns, and the camper compares propane's burning behaviour with that of butane (C4H10), which is used in a different camping stove.

   (a)

   A camping stove burns propane gas (C3H8) with a plentiful supply of air. Assuming complete combustion occurs, which pair of products is formed?

   [1 mark]

   • ACarbon dioxide and hydrogen
   • BCarbon monoxide and water
   • CCarbon and hydrogen
   • DCarbon dioxide and water

   (b)

   The camper notices that butane (C4H10) is stored as a liquid under pressure in its canister, while propane (C3H8) is often used in colder conditions because it vaporises more easily. This difference is best explained by the fact that, compared with propane, butane has:

   [1 mark]

   • AA longer carbon chain and therefore a lower boiling point
   • BA shorter carbon chain and therefore a higher boiling point
   • CA longer carbon chain and therefore a higher boiling point
   • DThe same boiling point, since both are alkanes

   (c)

   Write the balanced symbol equation for the complete combustion of propane (C3H8).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the nineteenth century, town gas made from coal was widely used for lighting and heating, but during the twentieth century most homes and industries in many countries switched to fuels refined from crude oil, choosing different fractions for different purposes.

   (a)

   Describe how the boiling point, viscosity and flammability of hydrocarbon fractions change as the molecular size of the hydrocarbons increases.

   [3 marks]

   (b)

   Explain why fractions containing shorter hydrocarbon molecules, such as those used for portable camping gas, are more suitable as fuels for equipment that must ignite quickly and burn cleanly, whereas fractions containing longer hydrocarbon molecules, such as heavy fuel oil, are used for large ships' engines instead.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A tanker carrying crude oil runs aground near a coastline, spilling its cargo into the sea. Environmental scientists explain to local residents that crude oil is a finite resource made mainly of alkane hydrocarbons, and that modern life depends heavily on the many fuels and feedstocks obtained from it.

   (a)

   Explain what crude oil is made of, why it is described as a finite resource, and why the range of hydrocarbon fractions obtained from it is so important to modern society.

   [6 marks]

   (b)

   The scientists also explain how the spilled crude oil would normally have been processed at a refinery before the accident. Explain, in detail, how fractional distillation is used to separate crude oil into fractions, referring to evaporation, condensation and the relationship between molecular size and boiling point.

   [6 marks]

   Total for question 4: 12 marks

End of questions