A-LevelChemistryReaction kinetics9701/41 · Oct/Nov 2024 · Q316 Marks

A and B react together to give product AB. A + B → AB When the concentrations of A and B are both 0.0100 moldm¯³, the rate of formation of AB is 7.62 x 10-4 mol dm-3s-1. When the concentrations of A and B are both 0.0200 moldm-3, the rate of formation of AB is 3.05 × 10-3 mol dm-3s-1.

(a)(i)Complete the three possible rate equations that are consistent with these data. rate rate = rate =[2]
(a)(ii)Choose one of the rate equations you have written in (i), and calculate the value of the rate constant, k. Include the units of k. k = units[2]
(a)(iii)Explain why it is not possible to calculate a value for the half-life, t₁, of this reaction 2 using the value of the rate constant k calculated in (ii) and the equation k = 0.693/t1.[1]
(b)Catalysts may be homogeneous or heterogeneous.
(b)(i)Identify two metals that act as heterogeneous catalysts in the removal of NO2 from the exhaust gases of car engines. and[1]
(b)(ii)Iron acts as a heterogeneous catalyst in the Haber process. Describe the mode of action of this iron catalyst.[2]
(b)(iii)Fe2+ ions act as a homogeneous catalyst in the reaction between I-(aq) and S2O82-(aq). Write equations for the two reactions that occur when Fe2+(aq) is added to a mixture of I-(aq) and S2O82-(aq). equation 1 S2O82- + equation 2[2]
(b)(iv)Explain the difference between a homogeneous catalyst and a heterogeneous catalyst.[1]
(c)Fe2+ ions can be oxidised to Fe3+ ions under alkaline conditions by suitable oxidising agents.
(c)(i)Iron is a transition element. Explain why iron forms stable compounds in both the +2 and the +3 oxidation states.[1]
(c)(ii)The half-equation for the reduction of Fe3+ under alkaline conditions, and its Eº value, are shown. Fe(OH)3 + e¯ = Fe(OH)2 + OH- E° = -0.56V Four more half-equations for reactions under alkaline conditions, and their Eº values, are shown. Al(OH)4¯ + 3e¯ 1 Al + 40H- E = -2.35V ClO¯ + H2O + 2e¯ ⇒ Ct + 2OH- E = +0.89V O2 + 2H2O + 4e¯ ⇌ 4OH¯ E = +0.40 V Zn(OH)42- + 2e¯ = Zn + 4OH¯ E = -1.22V Select two oxidising agents that can oxidise Fe2+ ions to Fe3+ ions under alkaline conditions. Write an equation, and give the El value, for each of the two reactions that occur. oxidising agent 1: equation: cell E = V oxidising agent 2: equation: cell E = V[4]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

[1] rate = k[A][B]
[1] rate = k[A]2 / k[A]2[B]° AND rate = k[B]2 / k[A][B]2
[1] k = 7.62 / 7.625 / 7.63
[1] mol-1 dm³ s-1
[1] reaction is 2nd order OR reaction is not 1st order OR reaction does not have constant half-life
[1] Pt, Pd, Rh (any two)
[1] adsorption (of reactants) / bond forms between the surface/catalyst and reactants
[1] bond weakening (in reactants)
[1] desorption (of reactants / products) / bond breaks between the surface/catalyst and products
[1] S2O82- + 2Fe2+ → 2SO42- + 2Fe3+
[1] 2Fe3+ + 2I- → 2Fe2+ + I2
[1] (heterogeneous) catalyst and reactants in different states / phases AND (homogeneous) catalyst and reactants in same states / phases
[1] similar energy of the (3)d and (4)s subshells / orbitals
[1] ClO- chosen as oxidising agent and E°cell value 1.45 stated
[1] O2 chosen as oxidising agent and E°cell value 0.96 stated
[1] Equation 1: 2Fe(OH)2 + ClO¯ + H2O → Cl¯ + 2Fe(OH)3
[1] Equation 2: 4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3