Crude oil, fuels and cracking – IGCSE Chemistry exam questions with mark scheme

1Grade 1–34 marks

Crude oil is separated into fractions by fractional distillation.

(a) Give one use for each of the fractions bitumen and kerosene. [2]

(b) State how the boiling point and the viscosity change as the number of carbon atoms in the molecules increases. [2]

(a) bitumen: road surfaces / roofing (allow roads / tar for roads)(1)
(a) kerosene: aircraft fuel (accept jet fuel; allow fuel for domestic heating / paraffin lamps / stoves)(1)
(b) boiling point increases(1)
(b) viscosity increases (allow becomes thicker / less runny / flows less easily)(1)
Answer: (a) bitumen – roads; kerosene – aircraft fuel (b) both increase
Examiner tip: Match each use to the right fraction — fuel for cars (petrol/gasoline) and aircraft (kerosene) are often swapped.

2Grade 6–74 marks

Long-chain alkanes are cracked to make shorter, more useful molecules.

(a) Complete the equation for the cracking of tetradecane. $$\text{C}_{14}\text{H}_{30} \rightarrow \text{C}_8\text{H}_{18} + \ldots\text{C}_2\text{H}_4$$ [1]

(b) State the conditions used for catalytic cracking. [2]

(a) 3 (C₂H₄)(1)
(b) catalyst of silica / alumina (accept zeolite / aluminium oxide / silicon dioxide)(1)
(b) temperature of 600–700 °C (accept any value in this range)(1)
(c) demand for short-chain hydrocarbons (e.g. petrol) is greater than supply / to make alkenes for polymers(1)
Answer: (a) 3 (b) silica/alumina catalyst, 600–700 °C (c) supply of short chains is less than demand
Examiner tip: Give BOTH conditions — catalyst name and a temperature in the correct range.

Still losing marks on this topic?

Book a free introductory session and we'll find exactly which step is costing you.

Book a free intro