A-LevelChemistryElectrochemistry9701/42 · May/June 2022 · Q58 Marks

Complete Table 5.1 to predict the substance liberated at each electrode during electrolysis of the indicated electrolyte with inert electrodes. [Figure Table 5.1]

(a)Complete Table 5.1 to predict the substance liberated at each electrode during electrolysis of the indicated electrolyte with inert electrodes. [Figure Table 5.1][3]
(b)(i)An electrolytic cell is set up to determine a value for the Avogadro constant, L. The electrolyte is dilute sulfuric acid and both electrodes are copper. When a current of 0.600A is passed through the acid for 30.0 minutes, the anode decreases in mass by 0.350 g. State the relationship between the Faraday constant, F, and the Avogadro constant, L.[1]
(b)(ii)Use the experimental information in (b) and data from the table on page 23 to calculate a value for the Avogadro constant, L. Show all working.[4]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[3] [Table values for electrolyte: PbBr2(I) anode: Br2/bromine cathode: Pb/lead; concentrated NaCl(aq) anode: Cl2/chlorine cathode: H2/hydrogen; Cu(NO3)2(aq) anode: O2/oxygen (+ H2O) cathode: Cu/copper] two for one mark, four for two marks, six for three marks
[1] F = Le OR F is directly proportional to L
[4] M1 number of Cu2+ formed = 0.35/63.5 = 5.51 × 10-3
[4] M1 Q = I x t = 0.60 × 30 × 60 = 1080 C
[4] M1 number of electrons = 1080/1.6×10-19 = 6.75 × 1021 ecf
[4] M1 number of Cu2+ ions = 6.75 × 1021 / 2 = 3.375 × 1021 ecf
[4] M1 number of Cu2+ ions per mole (L) = 3.375 × 1021 / 5.51 × 10-3 = 6.12 × 1023 ecf min 2sf

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