A-LevelChemistryElectrochemistry9701/41 · May/June 2021 · Q317 Marks

Define the term transition element.

(a)(i)Define the term transition element.[1]
(a)(ii)State how the melting point and density of iron compare to those of calcium.[1]
(b)(i)Define the term standard cell potential, Ecell-[2]
(b)(ii)Draw a fully labelled diagram of the apparatus that can be used to measure the cell potential of a cell composed of a Cu(II)/Cu electrode and an Fe(III)/Fe(II) electrode. Include all necessary reactants.[3]
(c)(i)The reaction between S₂O₈²⁻(aq) and I⁻(aq) is catalysed by adding a few drops of Fe³⁺(aq). Use equations to show the catalytic role of Fe³⁺ in this reaction.[2]
(c)(ii)Fe³⁺(aq) can oxidise I⁻(aq), whereas [Fe(CN)₆]³⁻(aq) cannot oxidise I⁻(aq). Use E° values to explain these observations.[2]
(d)(i)When aqueous solutions of S₂O₈²⁻ and tartrate ions are mixed the reaction proceeds very slowly. However, this reaction proceeds quickly in the presence of an Fe³⁺(aq) catalyst. The overall equation for this reaction is shown. [Figure showing tartrate ion structure] C₄H₄O₆²⁻ + 3S₂O₈²⁻ + 2H₂O → 2CO₂ + 2HCO₂⁻ + 6H⁺ + 6SO₄²⁻ Suggest why this reaction is slow without the Fe³⁺ catalyst.[1]
(d)(ii)Use the overall equation to deduce the half-equation for the oxidation of tartrate ions, C₄H₄O₆²⁻, to carbon dioxide, CO₂, and methanoate ions, HCO₂⁻. C₄H₄O₆²⁻ +[1]
(e)(i)Complete the following table to show the structures of the organic products formed when tartaric acid reacts separately with each reagent. Identify each type of reaction. [Figure showing tartaric acid structure] reagent | structure of organic product | type of reaction --------------------|------------------------------|---------------- an excess of LiAlH₄ | an excess of CH₃COCl |[3]
(e)(ii)Tartaric acid reacts with the amine 1-phenylethylamine, C₆H₅CH(NH₂)CH₃, to form an ionic salt. Draw the structure of the salt formed in this reaction. Include the charges on the ions.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] (an element) forming stable ion / ions / compound(s) / oxidation state(s) AND with partially filled / incomplete AND d orbitals / d subshell / d shell
[1] (melting point) higher AND (density) higher
[1] M1: emf / potential difference / difference in electrode potential between two half-cells / two electrodes (in a cell)
[1] M2: (all solutions being) 1 mol dm-3 AND either 1 atm OR 298 K
[3] salt bridge, voltmeter, Cu(s), Cu2+(aq), Pt(s), Fe2+ and Fe3+(aq) two for one mark, four for two marks, six for three marks
[1] M1: 2I- + 2Fe3+ → I2 + 2Fe2+
[1] M2: S2O82- + 2Fe2+ → 2SO42- + 2Fe3+
[1] M1: I2 / I- +0.54 V AND Fe3+ / Fe2+ + 0.77 V AND [Fe(CN)6]3− / [Fe(CN)6]4− +0.36 V
[1] M2: E® of I2 / I- is more positive / greater than E of [Fe(CN)6]3- / [Fe(CN)6]4- OR Ecell = -0.18 V so no reaction occurs OR E of Fe3+ / Fe2+ is more positive / greater than E of I2 / I- OR Ecell = 0.23 V so reaction occurs [1]
[1] S2O82- and tartrate ions are both negatively charged / both reactants same charge AND so repel each other OR have a high Ea
[1] C4H4O62- + 2H2O = 2CO2 + 2HCO2¯ + 6H+ + 6e-
[1] M1: product with LiA/H4 [Figure showing HO-CH2-CH(OH)-CH(OH)-CH2-OH]
[1] M2: product with CH3COCI [Figure showing CH3CO-O-CH2-CH(OCOCH3)-CH(OCOCH3)-CO-O-CH3 or similar tetraester]
[1] M3: both types of reaction (reduction, condensation)
[1] [Figure showing the ionic salt formed from tartaric acid dianion and 1-phenylethylammonium cations, e.g. HO2C-CH(OH)-CH(OH)-COO- and C6H5CH(NH3+)CH3]