The standard electrode potentials for three reactions involving copper and copper ions are:

     \({\text{C}}{{\text{u}}^{2 + }}{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{C}}{{\text{u}}^ + }{\text{(aq)     }}{E^\Theta } =  + 0.15{\text{ V}}\)

     \({\text{C}}{{\text{u}}^{{\text{2}} + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Cu(s)     }}{E^\Theta } =  + {\text{0.34 V}}\)

     \({\text{C}}{{\text{u}}^ + }{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{Cu(s)     }}{E^\Theta } =  + {\text{0.52 V}}\)

Which statement is correct?

A.     \({\text{C}}{{\text{u}}^{{\text{2}} + }}\) ions are a better oxidizing agent than \({\text{C}}{{\text{u}}^ + }\) ions.

B.     Copper metal is a better reducing agent than \({\text{C}}{{\text{u}}^ + }\) ions.

C.     \({\text{C}}{{\text{u}}^ + }\) ions will spontaneously form copper metal and \({\text{C}}{{\text{u}}^{{\text{2}} + }}\) ions in solution.

D.     Copper metal can be spontaneously oxidized by \({\text{C}}{{\text{u}}^{{\text{2}} + }}\) ions to form \({\text{C}}{{\text{u}}^ + }\) ions.

C

Candidates found this question tough – but it was fair, the data was set out in the conventional way and all candidates had to do was to apply the rules they had learnt and the understanding gained. Only 24% gave the correct answer whilst most (36%) chose B.