A-LevelChemistryChemical energetics9701/41 · May/June 2023 · Q513 Marks

The exhaust systems of most modern gasoline-fuelled cars contain a catalytic converter with three metal catalysts. These metals act as heterogeneous catalysts. Name three metal catalysts used in catalytic converters. 1 2 3

(a)(i)The exhaust systems of most modern gasoline-fuelled cars contain a catalytic converter with three metal catalysts. These metals act as heterogeneous catalysts. Name three metal catalysts used in catalytic converters. 1 2 3[1]
(a)(ii)Explain what is meant by a heterogeneous catalyst.[1]
(b)(i)The exhaust systems of many diesel-fuelled cars contain an additional system to reduce vehicle emissions. This uses a liquid that is added to the exhaust system. This liquid contains urea, (NH2)2CO, which decomposes on heating to isocyanic acid, HNCO, and ammonia. reaction 1 (NH2)2CO → HNCO + NH3 Isocyanic acid reacts with water vapour to form ammonia and carbon dioxide. reaction 2 HNCO(g) + H2O(g) → NH3(g) + CO2(g) Some values for standard enthalpy changes of formation, ΔHºₒ, and standard entropies, Sº, are given in Table 5.1. [Table 5.1] Explain what is meant by the term entropy of a system.[1]
(b)(ii)Use the data in Table 5.1 to calculate ΔGº for reaction 2 at 25°C. Show your working.[4]
(c)The ammonia formed in reactions 1 and 2 can be used to remove nitrogen dioxide from exhaust emissions, as shown. reaction 3 8NH3 + 6NO2 → 7N2 + 12H₂O Use the equations for reactions 1, 2 and 3 to construct an overall equation for the reduction of NO2 by (NH2)2CO.[1]
(d)Isocyanic acid, HNCO, can form cyanuric acid, C3H3N3O3, under certain conditions. C3H3N3O3 has a cyclic structure containing alternating carbon and nitrogen atoms in the ring system. Suggest a structure for cyanuric acid.[1]
(e)(i)Isocyanic acid, HNCO, is a weak acid. HNCO + H2O = H3O+ + NCO- pK = 3.70 at 25 °C Write the mathematical expressions for pK and pH.[1]
(e)(ii)Calculate the pH of 0.120 moldm⁻³ HNCO(aq). Give your answer to three significant figures.[2]
(e)(iii)Calculate the percentage of HNCO molecules that are ionised in 0.120 moldm⁻³ HNCO.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[1] palladium, platinum and rhodium / Pt, Pd, Rh
[1] a catalyst in a different state / phase to the reactants / substrate
[1] measure / degree of disorder/randomness of a system OR the number of possible arrangements of the particles and the energy in a system
[4] M1 ΔS° = (192.8) + 213.8 – 238.2 – 188.8 ΔS° = -20.4 (J K⁻¹ mol⁻¹)
[4] M2 ΔH° = (−45.9) + (−393.5) – (−101.7) – (−241.8) ΔH° = -95.9 (kJ mol⁻¹)
[4] M3 ΔG° = ΔH° – TΔS°
[4] M4 ΔG° = −95.9 – (298 × −0.0204) = −89.8 (kJ mol⁻¹)
[1] 4(NH2)2CO + 6NO2 → 7N2 + 8H2O + 4CO2
[1] (structure of cyanuric acid)
[1] pKa = -log Ka AND pH = -log [H+]
[2] M1 [H+] = √0.120 × 2.00 × 10⁻⁴ = 4.89(317) × 10⁻³
[2] M2 pH = 2.31
[1] % ionisation = 4.89 × 10⁻³/0.12 × 100 = 4.1%