Suppose that an electric current were passed through an electrolytic cell contai

Question

Suppose that an electric current were passed through an electrolytic cell containing a solution of 1.0-M NiCl2(aq). What would you predict the half-reaction equations observed at the cathode and anode to be based on the table of standard reduction potentials? List all possible half-reaction equations that could occur at each electrode in this solution and then circle the one that you believe would actually occur at that electrode. Justify your selections of half-reaction equations below.

Cathode: Possible half-reaction equations occurring at this electrode (circle one):

Justification for choice circled:

Anode: Possible half-reaction equations occurring at this electrode (circle one):

Justification for choice circled:

Explanation / Answer

An aqueous solution of NiCl2 undergoes dissociation as

NiCl2 (aq) ------> Ni2+ (aq) + 2 Cl- (aq)

The reactions taking place at the cathode and anode are

Ni2+ (aq) + 2 e- -------> Ni (s) ; E0 = - 0.24V (reduction at cathode)

2 Cl- (aq) -------> Cl2 (g) + 2 e- ; E0 = - 1.36V (oxidation at anode)

Net Reaction: Ni2+ (aq) + 2 Cl- (aq) -------> Ni (s) + Cl2 (g) ; E0 = -1.60V

Since, we have an aqueous solution; water may undergo reduction at the cathode or oxidation at the anode. The two reactions are:

2 H2O + 2 e- ------> H2 (g) + 2 OH- ; E0 = - 0.83V (reduction of water at cathode)

H2O ------> ½ O2 (g) + 2 H+ + 2 e- ; E0 = -1.23 V (oxidation of water at anode)

Now there are two possible reactions which may occur at the cathode or the anode. However, the reaction which occurs at the cathode is:

Ni2+ (aq) + 2 e- ------> Ni (s)

This reaction occurs at the cathode because it requires less energy (E0 = -0.24V) whilst the other reaction, reduction of water, requires more energy (E0= -0.83V).

The predominant anode reaction is oxidation of water (this reaction requires less energy than the other competing reaction, i.e, oxidation of chloride for which the energy required is -1.36V) (ans)

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