A-LevelChemistryReaction kinetics9701/43 · Oct/Nov 2022 · Q210 Marks

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

(a)(i)Explain what is meant by the following terms: homogeneous catalyst heterogeneous catalyst[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]
(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]
(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 dm⁶ s⁻¹. Construct the rate equation for this reaction. rate =[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 × 10⁻⁵ s⁻¹ under certain conditions. This reaction has a constant half-life. Explain why this is the case.[1]
(c)(ii)Use the information in this question to show that the half-life of this reaction is 2.77 × 10⁴ s.[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 rate = 8.47 × 10⁻³ (mol dm⁻³ s⁻¹) min 2sf
[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 × 10⁵ s [1] min 2 sf