A-LevelChemistryReaction kinetics9701/41 · Oct/Nov 2022 · Q210 Marks

Explain what is meant by the following terms: homogeneous catalyst heterogeneous catalyst [1]

(a)(i)Explain what is meant by the following terms: homogeneous catalyst heterogeneous catalyst [1][1]
(a)(ii)Iodide ions react with peroxydisulfate ions. 2I- + S2O82- → I2 + 2SO42- This reaction is slow, but it is catalysed by Fe2+ ions. Write two equations to explain how this reaction is catalysed by Fe2+ ions. 1 2 [2][2]
(a)(iii)Suggest why the alternative route in the presence of Fe2+ ions has a lower activation energy than the route in the absence of a catalyst. [1][1]
(b)(i)Nitrogen monoxide reacts with oxygen. 2NO(g) + O2(g) → 2NO2(g) This reaction is second order with respect to nitrogen monoxide and first order with respect to oxygen. Under certain conditions the value of the rate constant, k, is 8.60 × 106 mol-2 dm6s-1. Construct the rate equation for this reaction. rate = [1][1]
(b)(ii)Calculate the initial rate of the reaction under these conditions when the initial concentration of nitrogen monoxide is 7.20 × 10-4 mol dm-3 and the initial concentration of oxygen is 1.90 × 10-3 mol dm-3. rate of reaction = ...................................... mol dm-3 s-1 [1][1]
(c)(i)The drug cisplatin, Pt(NH3)2Cl2, hydrolyses in water. Pt(NH3)2Cl2 + H2O → [Pt(NH3)2Cl(H2O)]+ + Cl- The rate equation is shown. rate = k[Pt(NH3)2Cl2] The value of k is 2.50 × 10-5s-1 under certain conditions. This reaction has a constant half-life. Explain why this is the case. [1][1]
(c)(ii)Use the information in this question to show that the half-life of this reaction is 2.77 × 104 s. [1][1]
(c)(iii)8.00 × 10–6 moles of Pt(NH3)2Cl2 are added to 100 cm³ of water. Calculate the time taken for the concentration of Pt(NH3)2Cl2 to fall to 2.50 × 10-6 mol dm-3. time taken = ...................................... s [2][2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[1] (homogeneous is in the) same phase / state as reactants AND (heterogeneous is in a) different phase / state to reactants
[2] 1 S2O82- + 2Fe2+ → 2Fe3+ + 2SO42- [1] 2 2I- + 2Fe3+ → 2Fe2+ + I2 [1]
[1] reactants are both anions / negatively charged AND so they repel each other OWTTE
[1] rate = k[NO]2[O2] OR rate = 8.6 × 106 [NO]2[O2]
[1] rate = 8.6 × 106 × (7.2 × 10–4)2 x 1.9 × 10-3 [1] rate = 8.47 × 10-3 (mol dm³ s¯¹) min 2sf [1]
[1] (reaction is) first order wrt cisplatin / overall OR rate is directly proportional to concentration of cisplatin
[1] 0.693/2.50 × 10–5 = (2.77 × 104 s) OR In 2/2.50 × 10–5 = (2.77 × 104 s)
[2] initial concentration is 8.0 × 10-5 mol dm-3 five half-life periods have elapsed [1] time = 5 × 27720 = 1.39 × 105s [1] min 2 sf