Fractional distillation and crackingAQA A-Level Chemistry: Subtopic test

1. 1
   A refinery separates crude oil in a fractionating column. The fractions collected, in no particular order, are refinery gas, petrol, kerosene, diesel and fuel oil. Each fraction is a mixture of alkanes.

   (a)

   Which fraction is collected nearest the top of the fractionating column?

   [1 mark]

   • AFuel oil
   • BDiesel
   • CRefinery gas
   • DKerosene

   (b)

   Which statement explains why diesel condenses lower in the column than petrol?

   [1 mark]

   • ADiesel molecules are longer, so the London forces between them are stronger
   • BDiesel molecules are shorter, so their covalent bonds are stronger
   • CDiesel molecules have weaker London forces and so condense more easily
   • DDiesel molecules are ionic, so more energy is needed to separate them

   (c)

   Explain why alkanes are described as saturated hydrocarbons.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Tetradecane, C₁₄H₃₀, is a long-chain alkane that is cracked in an oil refinery. Cracking breaks C–C bonds in long-chain alkanes to form shorter, more useful molecules.

   (a)

   Tetradecane, C₁₄H₃₀, is cracked to give ethene and one other product only. What is the other product?

   [1 mark]

   • AC₁₂H₂₄
   • BC₁₂H₂₆
   • CC₁₄H₂₈
   • DC₂H₆

   (b)

   Which are the conditions used in catalytic cracking?

   [1 mark]

   • ASlight pressure, low temperature, zeolite catalyst
   • BHigh pressure, high temperature, no catalyst
   • CHigh pressure, high temperature, zeolite catalyst
   • DSlight pressure, high temperature, zeolite catalyst

   (c)

   Write an equation for the cracking of tetradecane to form octane and ethene as the only products.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An oil refinery has a large surplus of long-chain hydrocarbons from fractional distillation. At the same time, demand for petrol and for alkenes used in the chemical industry is greater than the amounts the distillation alone provides.

   (a)

   Explain the economic reasons for cracking long-chain alkanes.

   [3 marks]

   (b)

   Compare thermal cracking and catalytic cracking in terms of their conditions and their main products.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A sample of crude oil contains hexane (boiling point 69 °C), octane (boiling point 126 °C), decane (boiling point 174 °C) and hexadecane (boiling point 287 °C). The refinery has a surplus of the hexadecane-rich fraction and a shortage of petrol.

   (a)

   Explain how fractional distillation separates these four hydrocarbons, and state which one is collected nearest the top of the column.

   [6 marks]

   (b)

   Evaluate whether thermal cracking or catalytic cracking is the better choice for the refinery to process its surplus of the hexadecane-rich fraction.

   [6 marks]

   Total for question 4: 12 marks

End of questions