O-LevelChemistryElectrolysis5070/21 · Oct/Nov 2017 · Q213 Marks

A2 Copper(II) sulfate is an ionic compound.

(a)Describe the arrangement of the ions and the type of attractive forces between the ions in solid copper(II) sulfate. arrangement type of attractive forces[2]
(b)Explain why solid copper(II) sulfate does not conduct electricity but aqueous copper(II) sulfate does conduct.[2]
(c)(i)State the source of the hydroxide ions.[1]
(c)(ii)Complete the equation for the reaction at the positive electrode. OH- O2 + H2O +[1]
(c)(iii)Suggest why hydroxide ions and not sulfate ions are discharged at the positive electrode.[1]
(d)(i)Give the formulae of the four ions present in aqueous copper(II) sulfate.[1]
(d)(ii)Suggest why the solution becomes acidic as the electrolysis proceeds.[2]
(d)(iii)Suggest why the blue colour of the aqueous copper(II) sulfate fades as the electrolysis proceeds.[1]
(e)Draw a 'dot-and-cross' diagram of an oxygen molecule. Show only the outer shell electrons.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] A2(a) arrangement: ordered / lattice / regular / layers / uniformly arranged / repeated pattern
[1] A2(a) type of force: electrostatic
[1] A2(b) solid: ions cannot move
[1] A2(b) aqueous: ions can move
[1] A2(c)(i) (ionisation of) water
[1] A2(c)(ii) 4OH¯ → O2 + 2H2O + 4e¯
[1] A2(c)(iii) hydroxide (ions) lower in reactivity (than sulfate (ions)) / sulfate (ions) less easily oxidised (than hydroxide (ions))
[1] A2(d)(i) H⁺, OH⁻, SO4²⁻, Cu²⁺
[1] A2(d)(ii) hydroxide removed
[1] A2(d)(ii) idea of H⁺ remaining (causing acidity)
[1] A2(d)(iii) Cu²⁺ ions removed (at negative electrode)
[1] A2(e) two pairs of bonding electrons
[1] A2(e) rest of structure correct

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