Which changes could take place at the positive electrode (cathode) in a voltaic cell?

I.     \({\text{Z}}{{\text{n}}^{2 + }}{\text{(aq)}}\) to Zn(s)

II.     \({\text{C}}{{\text{l}}_{\text{2}}}{\text{(g)}}\) to \({\text{C}}{{\text{l}}^ - }{\text{(aq)}}\)

III.     Mg(s) to \({\text{M}}{{\text{g}}^{2 + }}{\text{(aq)}}\)

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

A

One respondent commented that a full cell representation should have been given in the question so that candidates could ascertain the direction of electron flow. This question is based on AS 9.4.2 which states that candidates should know that reduction takes place at the cathode and oxidation takes place at the anode. In this question, the correct answer is A, corresponding to I. and II., as in I. zinc changes from a +2 oxidation state in \({\text{Z}}{{\text{n}}^{2 + }}{\text{(aq)}}\) to 0 in Zn(s), which is reduction and in II. Chlorine changes from a 0 oxidation state in \({\text{C}}{{\text{l}}_{\text{2}}}{\text{(g)}}\) to a –1 oxidation state in \({\text{C}}{{\text{l}}^ - }{\text{(aq)}}\). III. is ruled out as magnesium is in a 0 oxidation state in Mg(s) and changes to a +2 oxidation state in \({\text{M}}{{\text{g}}^{2 + }}{\text{(aq)}}\) which is oxidation. Hence, it was not necessary to know the direction of electron flow from a representation of the voltaic cell to answer this question. The question was also well answered by candidates with 70% giving A as the correct answer.