O-LevelChemistryElectrolysis5070/21 · Oct/Nov 2023 · Q410 Marks

Concentrated aqueous calcium bromide is electrolysed using graphite electrodes.

(a)Predict the product at each electrode. anode cathode[2]
(b)Molten calcium bromide is electrolysed using graphite electrodes. Construct the ionic half-equation for the reaction at each electrode when molten calcium bromide is electrolysed. anode cathode[2]
(c)Describe a test for aqueous bromide ions. Include the observations for a positive result. test observations[2]
(d)(i)Calcium reduces zinc ions to zinc. Ca + Zn²⁺ → Zn + Ca²⁺ Explain, in terms of the movement of electrons, how calcium acts as a reducing agent in this equation.[1]
(d)(ii)State the oxidation number of the Zn²⁺ ion.[1]
(e)Sulfur dibromide is produced when sulfur reacts with bromine. Complete Fig. 4.1 to show the dot-and-cross diagram for a molecule of sulfur dibromide. Show only the outer shell electrons. [Figure 4.1][2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[1] (a) anode: bromine
[1] (a) cathode: hydrogen
[1] (b) anode: 2Br⁻ → Br₂ + 2e⁻
[1] (b) cathode: Ca²⁺ + 2e⁻ → Ca
[1] (c) add (acidified) silver nitrate
[1] (c) cream precipitate
[1] (d)(i) calcium gives electrons to zinc ions / electrons transferred from calcium to zinc ions
[1] (d)(ii) +2
[1] (e) pair of electrons between each S and Br
[1] (e) 6 non-bonding e¯ on each Br and 4 non-bonding e on S

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