Redox and electrochemistry — Question 5

5a)

Copper ion is reduced and Cu is plated onto the cathode: $\mathrm{Cu}^{2+}\,\mathrm{(aq)}+ 2e^{-}\rightarrow Cu\mathrm{(s)}\,$ Copper anode is oxidised to Cu2+(aq) and is released into solution: $\mathrm{Cu}\mathrm{(s)}\rightarrow Cu^{2+}\,\mathrm{(aq)}+ 2e^{-}\,$ Therefore, for every copper ion that is reduced at the cathode, in principle, another one is oxidised at the anode. Therefore, the concentration of the copper(ll) sulfate solution should stay the same.

• identifies copper ions are reduced to Cu metal at the cathode and reduction half-equation is $\mathrm{Cu}^{2+}\,\mathrm{(aq)}+ 2e^{-}\rightarrow Cu\mathrm{(s)}\text{[1 mark]}\,$

• identifies copper metal is oxidised to Cu ions at the anode and oxidation half-equation is $\mathrm{Cu}\mathrm{(s)}\rightarrow Cu^{2+}\,\mathrm{(aq)}+ 2e^{-}\,\text{[1 mark]}\,$

• predicts no change in concentration of copper(II) sulfate solution [1 mark]

• identifies that copper ions are reduced to copper and copper is oxidised to copper ions at the same rate [1 mark]

5b)

Silver is below copper in the reactivity series and therefore doesn't go into solution, as ions are not oxidised and could be found in the sludge. Zinc impurities are above copper in the electrochemical series and will form ions at the anode and go into solution. However, they won't get discharged at the cathode, provided their concentration doesn't get too high.

• identifies Ag is less reactive than Cu and Zn is more reactive than Cu [1 mark]

• deduces Ag metal is not oxidised (or reduced) and remains as metal [1 mark]

• deduces Zn metal is oxidised to form ions and found in the solution [1 mark]

• explains that Zn²⁺ ions remain in solution at low concentration but are reduced to Zn metal at the cathode if concentration becomes too high [1 mark]