Crude oil, alkanes, fractional distillation & cracking – GCSE Chemistry exam questions with mark scheme

1Grade 1–3Foundation4 marks

Crude oil is a mixture of hydrocarbons. Most of these hydrocarbons are alkanes.

(a) What is a hydrocarbon? [1]

(b) The general formula of the alkanes is C_nH_2n+2. Give the molecular formula of the alkane with 10 carbon atoms. [1]

(d) Describe how the boiling point of an alkane changes as the molecules get bigger. [1]

(a) (a compound / molecule) containing hydrogen and carbon (atoms) only; 'only' needed; do not accept 'a mixture of hydrogen and carbon'(1)
(b) C₁₀H₂₂(1)
(c) propane(1)
(d) (boiling point) increases; allow gets higher(1)
Answer: (a) compound of hydrogen and carbon only (b) C₁₀H₂₂ (c) propane (d) increases
Examiner tip: Without the word 'only' the definition of a hydrocarbon does not score.

2Grade 4–54 marks

Crude oil is separated into fractions by fractional distillation.

Describe how fractional distillation separates crude oil into fractions. [4]

crude oil is heated so it evaporates / vaporises / boils(1)
the vapour rises up the (fractionating) column(1)
the column is hotter at the bottom and cooler at the top / has a temperature gradient; allow vapour cools as it rises(1)
hydrocarbons / fractions condense at different heights (where the temperature falls below their boiling points) and are collected / removed; allow fractions have different boiling points so condense at different levels; must refer to condensing(1)
Answer: Oil vaporised, vapours rise up a column that is cooler at the top, each fraction condenses at the level where the temperature is below its boiling point and is collected.
Examiner tip: 'Condense' not 'evaporate' at the different levels — mixing these up is the most common lost mark.

3Grade 4–53 marks

Propane is used as a fuel.

(a) Complete and balance the equation for the complete combustion of propane.

C₃H₈ + ___O₂ → ___CO₂ + ___H₂O [2]

(b) Name the toxic gas that can form when propane burns in a limited supply of oxygen. [1]

(a) 3CO₂ and 4H₂O(1)
(a) 5O₂ (equation fully balanced); allow correct multiples of the whole equation(1)
(b) carbon monoxide; allow CO; ignore carbon / soot(1)
Answer: (a) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (b) carbon monoxide
Examiner tip: Balance C, then H, then O last — oxygen atoms on the right are 6 + 4 = 10, so 5O₂.

4Grade 4–55 marks

Dodecane, C₁₂H₂₆, can be cracked to produce octane and ethene.

(a) Complete the equation. [1]

C₁₂H₂₆ → C₈H₁₈ + ___C₂H₄

(b) Give two conditions used for catalytic cracking. [2]

(a) 2(1)
(b) high temperature / heat / vaporise the hydrocarbon(1)
(b) (pass over a hot) catalyst; allow named catalyst e.g. zeolite / aluminium oxide; do not credit steam (that is steam cracking, not catalytic cracking)(1)
(c) add bromine water(1)
(c) (from orange) to colourless / decolourised; allow yellow / brown / orange-brown to colourless; do not accept 'clear'; dependent on bromine water(1)
Answer: (a) 2(C₂H₄) (b) high temperature (vaporise the hydrocarbon) and a hot catalyst (c) add bromine water: it changes from orange to colourless
Examiner tip: Bromine water goes colourless, not 'clear' — clear and colourless are not the same in chemistry.

5Grade 1–3Foundation6 marks

Crude oil is a fossil fuel.

(a) Complete the sentence. Use words from the box.

coal

mud

plankton

salt

volcanic ash

Crude oil formed over millions of years from the remains of ancient biomass, mainly __________, that was buried in __________. [2]

(b) Crude oil is a finite resource. What does this mean? [1]

(i) Which molecule, A, B or C, is an alkane? [1]

(ii) Name the alkane. [1]

(iii) Give the reason why molecule C is not a hydrocarbon. [1]

(a) plankton (first gap)(1)
(a) mud (second gap); mark each gap independently(1)
(b) it will run out / it is not being made (or replaced) as fast as it is used; allow it takes millions of years to form / there is a limited amount; ignore non-renewable on its own(1)
(c)(i) A(1)
(c)(ii) butane; do not accept butene / butanol / C₄H₁₀ (a name is required)(1)
(c)(iii) it contains oxygen (atoms) / it contains an element other than carbon and hydrogen; allow it contains O(1)
Answer: (a) plankton; mud (b) it will run out / is not being replaced as fast as it is used (c)(i) A (ii) butane (iii) it contains oxygen as well as carbon and hydrogen
Examiner tip: For 'finite', 'non-renewable' just repeats the idea – say it will run out because it forms far more slowly than we use it.

6Grade 4–55 marks

The diagram shows a fractionating column used to separate crude oil into fractions A to E.

(a) Give one way in which the hydrocarbon molecules in fraction C are similar to each other. [1]

(b) Which fraction, A to E, is the most viscous? Give a reason for your answer. [2]

Suggest one reason why fraction B is a better fuel for cars than fraction D. [1]

(d) Some fractions are used as feedstock in the petrochemical industry.

Give one useful product, other than a fuel, made from this feedstock. [1]

(a) similar number of carbon atoms (per molecule) / similar (size of) molecules / similar boiling points; allow 10–14 carbon atoms; ignore 'same boiling point' / 'all hydrocarbons' / 'all alkanes'(1)
(b) E(1)
(b) (it has the) largest / longest molecules / most carbon atoms per molecule; allow highest boiling point; dependent on E or on a fraction with the most carbon atoms(1)
(c) B is more flammable / ignites (or burns) more easily; allow B is less viscous / flows more easily / evaporates more easily; ignore 'B burns better' / 'B is cleaner'(1)
(d) any one from: polymers / plastics; solvents; lubricants; detergents; allow named example e.g. poly(ethene); ignore medicines / chemicals unqualified(1)
Answer: (a) similar numbers of carbon atoms (similar boiling points) (b) E – it has the most carbon atoms / largest molecules (c) B is more flammable (less viscous) (d) e.g. polymers / solvents / lubricants / detergents
Examiner tip: 'Same boiling point' loses the mark – a fraction contains molecules with similar boiling points, not one boiling point.

7Grade 6–7Higher5 marks

This question is about alkanes. The general formula of the alkanes is C_nH_2n+2.

Relative atomic masses (A_r): H = 1 C = 12

(a) An alkane has a relative formula mass (M_r) of 72.

Determine the molecular formula of the alkane. Show your working. [2]

(b) Hexane, C₆H₁₄, is an alkane.

Write a balanced equation for the complete combustion of hexane. [2]

What does 'oxidised' mean in this reaction? [1]

(a) 12n + (2n + 2) = 72 / 14n + 2 = 72 / n = 5; allow evidence of testing formulae, e.g. M_r of C₄H₁₀ = 58 and C₅H₁₂ = 72(1)
(a) C₅H₁₂; correct answer with no working scores 1 mark only(1)
(b) correct formulae: C₆H₁₄ + O₂ → CO₂ + H₂O (CO₂ and H₂O the only products)(1)
(b) balanced: 2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O; allow C₆H₁₄ + 9½O₂ → 6CO₂ + 7H₂O; allow correct multiples; balancing mark dependent on correct formulae; ignore state symbols(1)
(c) (carbon and hydrogen) gain oxygen / combine with oxygen; allow lose electrons; ignore 'burn' on its own(1)
Answer: (a) 14n + 2 = 72 so n = 5: C₅H₁₂ (b) 2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O (c) they gain oxygen
Examiner tip: With an odd number of oxygen atoms needed (19), double the whole equation rather than leave it unbalanced.

8Grade 4–55 marks

The bar chart shows the supply of different fractions from crude oil and the demand for them, as percentages.

(a) Calculate the difference between demand and supply, in percentage points, for the fraction with 5–9 carbon atoms per molecule. [1]

(b) Oil companies crack hydrocarbons with more than 20 carbon atoms per molecule.

Use the bar chart to explain why. [2]

C₁₄H₃₀ → C₈H₁₈ + 2X

Give the molecular formula of X. [1]

(d) X is an alkene. Give one use of alkenes. [1]

(a) 16 (percentage points); allow 16%(1)
(b) supply (48%) is greater than demand (32%) for hydrocarbons with more than 20 carbon atoms / there is more of this fraction than is needed; allow large molecules are less useful (as fuels)(1)
(b) cracking breaks them into smaller molecules / fractions for which demand is greater than supply (e.g. 5–9 carbon atoms: demand 28%, supply 12%); allow to make more petrol / more useful fuels; must have the idea of smaller molecules or of a fraction in short supply(1)
(c) C₃H₆; allow CH₂=CHCH₃; do not accept C₆H₁₂(1)
(d) to make polymers / plastics; allow named polymer e.g. poly(propene); allow as a starting material to make other chemicals; ignore as a fuel(1)
Answer: (a) 16 (b) supply of >20 C fraction (48%) exceeds demand (32%); cracking turns it into smaller molecules such as 5–9 C, where demand (28%) exceeds supply (12%) (c) C₃H₆ (d) making polymers
Examiner tip: In 'use the chart' questions, quote the numbers – e.g. 48% supply vs 32% demand – to show you have used the data.

9Grade 6–76 marks

A teacher uses the apparatus in the diagram to crack liquid paraffin in the laboratory. Liquid paraffin is a mixture of alkanes.

Describe how the apparatus is used to crack the liquid paraffin and to show that an alkene is produced.

In your answer you should explain:

why the aluminium oxide is heated strongly
the steps taken to work safely.

[6]

heat the aluminium oxide (catalyst) strongly first(1)
then heat the mineral wool / paraffin (gently or occasionally) so the paraffin evaporates / vaporises(1)
paraffin vapour passes over the hot catalyst, which breaks the long alkane molecules into smaller molecules (alkanes and alkenes)(1)
the catalyst must be very hot for cracking to happen (high temperature provides the energy to break the long molecules); the catalyst speeds up cracking / allows it at a lower temperature than without a catalyst(1)
gas is collected over water in an inverted (water-filled) test tube(1)
discard the first tube(s) of gas because it is mainly air from the apparatus(1)
add bromine water to a tube of the gas and shake(1)
bromine water changes from orange to colourless, showing an alkene is present(1)
control: liquid paraffin itself does not decolourise bromine water (it is an alkane)(1)
remove the delivery tube from the water before stopping heating, to prevent suck-back (cold water would crack the hot boiling tube)(1)
the gas is flammable: keep collected tubes away from flames / stopper them(1)
wear eye protection; avoid skin contact with bromine water (gloves)(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, logically ordered description that could be followed to crack the paraffin and test the product. Explains the role of the hot catalyst and how the gas is collected and tested with bromine water, with the correct result linked to an alkene. Includes at least one safety step with its reason, such as preventing suck-back.
Level 2	3–4	Describes most of the main steps, including heating the catalyst and paraffin and collecting the gas, and gives the bromine water test or its result. Some reasons (role of catalyst, safety) may be missing or unclear.
Level 1	1–2	Simple, separate statements about heating, collecting gas or testing with bromine water. The order may be unclear and few reasons are given.
No relevant content	0	Nothing worthy of credit.

Answer: Heat the Al₂O₃ catalyst strongly, warm the paraffin so its vapour passes over the hot catalyst and cracks; collect gas over water, discard the first tube, shake gas with bromine water – orange to colourless shows an alkene; lift the delivery tube out of the water before removing the heat to stop suck-back.
Examiner tip: Most answers lose Level 3 by not explaining suck-back: take the delivery tube out of the water before you stop heating.

10Grade 8–9Higher8 marks

Decane, C₁₀H₂₂, can be cracked to produce ethene, propene and one other hydrocarbon, Y.

C₁₀H₂₂ → C₂H₄ + C₃H₆ + Y

(a) Give the molecular formula of Y and state whether Y is an alkane or an alkene. [2]

(b) A student says:

'An alkane molecule can be cracked into just two molecules that are both alkanes.'

Use the general formula of the alkanes, C_nH_2n+2, to explain why the student is wrong. [3]

Relative atomic masses (A_r): H = 1 C = 12 [3]

(a) C₅H₁₂(1)
(a) alkane; allow ecf: correct classification of the student's formula for Y (fits C_nH_2n+2 → alkane); no reason required(1)
(b) two alkanes with a total of n carbon atoms would need (2a + 2) + (2b + 2) = 2n + 4 hydrogen atoms; allow a correct numerical example, e.g. two alkanes from C₁₀H₂₂ such as C₆H₁₄ + C₄H₁₀ need 24 H atoms(1)
(b) but the alkane only has 2n + 2 hydrogen atoms / there are 2 hydrogen atoms too few (e.g. only 22 H in C₁₀H₂₂)(1)
(b) so at least one product must have fewer hydrogen atoms / be an alkene (C_nH_2n); allow there are not enough hydrogen atoms for both products to be saturated(1)
(c) M_r(C₁₀H₂₂) = 142 and M_r(C₂H₄) = 28(1)
(c) moles of decane = 284 000 / 142 = 2000 mol (= moles of ethene, 1 : 1 ratio); allow 284 / 142 = 2 (kmol) or mass ratio 284 × 28 / 142(1)
(c) 56 (kg); correct answer with no working scores 3 marks; allow ecf from incorrect M_r values(1)
Answer: (a) C₅H₁₂, an alkane (b) two alkanes need 2n + 4 H atoms but only 2n + 2 are available, so one product must be an alkene (c) 56 kg
Examiner tip: In (b) a general statement like 'cracking always makes alkenes' scores nothing – you must count the hydrogen atoms using C_nH_2n+2.

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