A-LevelChemistryHydrocarbons9701/23 · Oct/Nov 2019 · Q313 Marks

Crude oil is a natural source of hydrocarbons that are used as fuels.

(a)(i)Hydrocarbons with low relative molecular mass, Mᵣ, are used as fuels in industry, in the home and for transport. There is a high demand for the hydrocarbons with low Mᵣ. Name the process by which long-chain hydrocarbons are broken down into shorter-chain hydrocarbons.[1]
(a)(ii)Give one reason why hydrocarbons with low Mᵣ are suitable for use as fuels.[1]
(a)(iii)Incomplete combustion of hydrocarbons can release carbon monoxide, CO, into the atmosphere. Write an equation for the formation of CO from the incomplete combustion of butene, C₄H₈.[1]
(a)(iv)Identify an analytical technique that can be used to monitor the levels of CO in the atmosphere. Outline how this analytical technique may be used to monitor the levels of CO.[2]
(b)(i)Thiophene, C₄H₄S(l), is an organic compound that is found as a contaminant in crude oil. Construct the equation for the complete combustion of thiophene, C₄H₄S(l). Include state symbols in your answer.[2]
(b)(ii)A student carries out an experiment to determine the enthalpy change of combustion of C₄H₄S(l). Explain the meaning of the term enthalpy change of combustion.[2]
(b)(iii)The student uses the following apparatus in the experiment. [Figure showing apparatus with thermometer, water, copper container, fuel burner containing C₄H₄S(l)] mass of water in copper container/g 200 initial temperature of water/°C 18.5 highest temperature of water/°C 37.5 Calculate the heat energy released, in J, by the reaction. Assume that 4.18 J of heat energy changes the temperature of 1.0 cm³ of water by 1.0°C. Assume no heat is lost to the surroundings.[2]
(b)(iv)The student used 0.63 g of C₄H₄S(l) in the experiment. Calculate the enthalpy change of combustion of thiophene, ∆Hᶜ(C₄H₄S(l)). Include a sign in your answer.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] cracking
[1] enthalpy change of combustion / ∆Hᶜ is high / large energy release (per mole / per unit mass) OR combust / burn easily
[1] C₄H₈ + 4O₂ → 4CO + 4H₂O
[2] M1: infrared spectroscopy M2: Compare / measure (characteristic) wavelengths
[2] • correct species • balancing • state symbols Award one mark for two correct bullet points, award two marks for all three correct. C₄H₄S(l) + 6O₂(g) → 4CO₂(g) + 2H₂O(l) + SO₂(g)
[2] M1 (enthalpy change when) 1 mol of a substance M2 EITHER burns / combusts / reacts in excess air / oxygen OR completely burns / combusts / reacts in air / oxygen
[2] M1 m = 200 and ∆T = 37.5−18.5 M2 Q = mc∆T = 200 × 4.18 × (37.5 – 18.5) = 15 884 (J)
[2] M1 mol of thiophene used = 0.63/84.1 OR 7.49(1082 045) × 10⁻³ M2 calculation ÷ 1000 AND negative sign ∆Hᶜ = −(iii)/1000 ÷ n = −(iii)/(21000/(0.63/84.1)) = −2120 (−2120.39) (kJ mol⁻¹)