Calculate — 5 marks
A fuel company is comparing two hydrocarbon fuels for use in industrial heating systems. Fuel A is pentane (C₅H₁₂) and Fuel B is octane (C₈H₁₈). The company needs to determine which fuel provides better energy efficiency per unit mass.
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(a) Calculate the relative molecular mass (Mr) of pentane, C₅H₁₂. (Ar: C = 12, H = 1)
[1 mark]
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(b) When 1 mole of pentane is completely combusted, 3510 kJ of energy is released. Calculate the energy released per gram of pentane burned. Show your working.
[2 marks]
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(c) Octane (C₈H₁₈) releases 5470 kJ per mole when completely combusted. Calculate the energy released per gram of octane and determine which fuel (pentane or octane) is more efficient in terms of energy per unit mass. Show all your working.
[2 marks]
Show mark scheme
- (a) Mr = (5 × 12) + (12 × 1) = 60 + 12 = 72 g/mol (1 mark)
- (b) Energy per gram = 3510 kJ ÷ 72 g (1 mark)
- (b) = 48.75 kJ/g or 48.8 kJ/g (to 3 s.f.) (1 mark)
- (c) Mr of octane = (8 × 12) + (18 × 1) = 96 + 18 = 114 g/mol; Energy per gram = 5470 ÷ 114 = 47.98 kJ/g or 48.0 kJ/g (to 3 s.f.) (1 mark)
- (c) Pentane is more efficient as it releases 48.8 kJ/g compared to octane at 48.0 kJ/g, or pentane releases approximately 0.8 kJ/g more energy per unit mass than octane (1 mark)
Describe — 3 marks
Crude oil is a fossil fuel that contains many different hydrocarbon compounds. When crude oil is heated, it can be separated into useful products through fractional distillation. These products range from gases used in cooking to thick liquids used for road surfaces.
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(a) Describe what a hydrocarbon is.
[1 mark]
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(b) Describe the difference between alkanes and alkenes in terms of the types of bonds they contain.
[1 mark]
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(c) Describe how the boiling points of alkanes change as the number of carbon atoms in the chain increases.
[1 mark]
Show mark scheme
- (a) A compound made of hydrogen and carbon atoms only / contains only carbon and hydrogen
- (b) Alkanes contain only single bonds (between carbon atoms) / Alkenes contain at least one double bond (between carbon atoms)
- (c) As the number of carbon atoms increases, the boiling point increases / Longer chain alkanes have higher boiling points
Explain — 2 marks
A student is investigating why petrol (a mixture of hydrocarbons) is used as a fuel in car engines rather than crude oil itself. The crude oil contains long-chain hydrocarbons with high boiling points, while petrol contains shorter-chain hydrocarbons with lower boiling points.
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Explain why long-chain hydrocarbons in crude oil are less suitable as a fuel for car engines than the shorter-chain hydrocarbons found in petrol.
[2 marks]
Show mark scheme
- {'mark': 1, 'description': 'Long-chain hydrocarbons have high boiling points so they are liquids/do not vaporise easily at engine temperatures'}
- {'mark': 1, 'description': 'Shorter-chain hydrocarbons have lower boiling points so they vaporise easily/form combustible vapours that burn efficiently in the engine'}
Define — 4 marks
Crude oil is a valuable natural resource that contains many different hydrocarbons. These compounds are separated at an oil refinery using fractional distillation. Understanding what hydrocarbons are is essential for understanding how crude oil is processed and used.
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(a) Define the term 'hydrocarbon'.
[2 marks]
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(b) Define the term 'saturated hydrocarbon' and give one example of a saturated hydrocarbon.
[2 marks]
Show mark scheme
- (a) A compound containing only carbon and hydrogen atoms (1 mark)
- (a) Bonded together by covalent bonds (1 mark)
- (b) A hydrocarbon that contains only single bonds between carbon atoms / has the maximum number of hydrogen atoms bonded to each carbon atom (1 mark)
- (b) One valid example such as: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), hexane (C₆H₁₄), or octane (C₈H₁₈) (1 mark)
Calculate — 2 marks
A camping stove uses propane (C₃H₈) as a fuel. Propane is a hydrocarbon that burns completely in air to produce carbon dioxide and water. A student needs to calculate some values for a chemistry investigation about camping fuels.
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(a) Calculate the relative molecular mass (Mᵣ) of propane (C₃H₈). Relative atomic masses (Aᵣ): C = 12, H = 1
[1 mark]
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(b) Calculate the percentage by mass of carbon in propane.
[1 mark]
Show mark scheme
- (a) (3 × 12) + (8 × 1) = 44
- (b) (36 ÷ 44) × 100 = 81.8% (accept 82%)
GCSE Perfect