A-LevelChemistryElectrochemistry9701/41 · May/June 2020 · Q37 Marks

The overall reaction for photosynthesis is shown. 6CO2 + 6H2O → C6H12O6 + 6O2 Water is oxidised in this process according to the following half-equation. 2H2O → O2 + 4H+ + 4e¯ half-equation 1

(a)(i)Use these equations to deduce the half-equation for the reduction of carbon dioxide in this process.[2]
(a)(ii)Draw a fully labelled diagram of the apparatus that should be used to measure the standard electrode potential, E®, of O2(g) in half-equation 1 under standard conditions. Include all necessary chemicals.[4]
(a)(iii)For the cell drawn in (a)(ii), use the Data Booklet to calculate the Ecell and deduce which electrode is positive. Ecell = .............................................................. V identity of the positive electrode =[1]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[2] 6CO2 + 24H+ + 24e- → C6H12O6 + 6H2O ALLOW 6CO2 + 12H+ + 12e- → C6H12O6 + 3O2 for both marks ALLOW one mark for an unbalanced equation showing the correct species of either equation
[4] salt bridge (indicated) voltmeter / V labelled O2 good delivery system H2 good delivery system Pt electrode H+ / HCI / H2SO4 solution labelled (at least once) 1 atm 1 mol dm-³ quoted Every two correct responses = 1 mark
[1] Ecell = (+) 1.23 V AND positive electrode = O2 half-cell identified

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