A-LevelChemistryElectrochemistry9701/42 · Feb/Mar 2018 · Q210 Marks

Describe the trend in the reactivity of the halogens Cl2, Br2 and I₂ as oxidising agents. Explain this trend using values of E° (X2/X¯) from the Data Booklet.

(a)Describe the trend in the reactivity of the halogens Cl2, Br2 and I₂ as oxidising agents. Explain this trend using values of E° (X2/X¯) from the Data Booklet.[2]
(b)(i)Write an equation for the reaction between chlorine and water.[1]
(b)(ii)Use standard electrode potential, E°, data from the Data Booklet to calculate the Ecell for the following reaction. Cl₂ + 2OH⁻ ⇌ Cl⁻ + ClO⁻ + H₂O Ecell = ______ V[2]
(b)(iii)The [OH⁻] was increased and the Ecell was measured. Indicate how the value of the Ecell measured would compare to the Ecell calculated in (ii) by placing one tick (√) in the table. Ecell becomes less positive than E°cell Ecell stays the same as E°cell Ecell becomes more positive than E°cell Explain your answer.[2]
(c)(i)A half-equation involving bromate(V) ions, BrO3¯, and bromide ions is shown. BrO₃⁻(aq) + 3H₂O(l) + 6e⁻ ⇌ Br⁻(aq) + 6OH⁻(aq) E° = +0.58 V An alkaline solution of chlorate(I), ClO⁻, can be used to oxidise bromide ions to bromate(V) ions. Use the Data Booklet and the half-equation shown to write an equation for this reaction.[1]
(c)(ii)Calculate the Ecell for the reaction in (i). Ecell = ______ V[1]
(c)(iii)When a concentrated solution of bromic(V) acid, HBrO₃, is warmed, it decomposes to form bromine, oxygen and water only. Write an equation for this reaction. The use of oxidation numbers may be helpful.[1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

[2] the E° for X2/X¯ becomes less positive / decrease down the group so the halogens are less reactive (as oxidants) down the group
[1] Cl2 + H2O → HCl + HClO
[2] Cl2/Cl¯ = +1.36 V and ClO¯/ (Cl¯ + OH¯) = +0.89 V so Ecell = 1.36 – 0.89 = (+) 0.47 V
[2] box three ticked Le Chatelier argument, more OH¯/increase reactant concentration so equilibrium shifts right or an argument based on the half cell with OH-
[1] Br¯ + 3ClO⁻ → BrO₃⁻ + 3Cl⁻
[1] Ecell = 0.89 - 0.58 = + 0.31 V
[1] 4HBrO3 → 2Br2 + 5O2 + 2H2O

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