A-LevelChemistryHydrocarbons9701/21 · Oct/Nov 2019 · Q313 Marks

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

(a)(i)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 3.1] 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. heat energy released =[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. ∆Hc(C4H4S(l)) =[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] 3(a)(i) cracking
[1] 3(a)(ii) enthalpy change of combustion / ∆Hc is high / large energy release (per mole / per unit mass) OR combust / burn easily
[1] 3(a)(iii) C4H8 + 4O2 → 4CO + 4H2O
[2] 3(a)(iv) M1: infrared spectroscopy M2: Compare / measure (characteristic) wavelengths
[2] 3(b)(i) C4H4S(l) + 6O2(g) → 4CO2(g) + 2H2O(l) + SO2(g) • correct species • balancing • state symbols Award one mark for two correct bullet points, award two marks for all three correct.
[2] 3(b)(ii) 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] 3(b)(iii) M1 m = 200 and ∆T = 37.5−18.5 M2 Q = mc∆T = 200 × 4.18 × (37.5 – 18.5) = 15 884 (J)
[2] 3(b)(iv) 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) ÷ (0.63/84.1) = -2120 (-2120.39) (kJ mol-1)

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