The standard electrode potentials of some half-reactions are given below.

     \({\text{S}}{{\text{n}}^{4 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.15{\text{ V}}\)

    \(\frac{1}{2}{{\text{I}}_2}{\text{(s)}} + {{\text{e}}^ - } \rightleftharpoons {{\text{I}}^ - }{\text{(aq)}}\)     \({E^\Theta } =  + 0.54{\text{ V}}\)

     \({\text{F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{F}}{{\text{e}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.77{\text{ V}}\)

Which of the following reactions will occur spontaneously?

A.     Iodine reduces \({\text{F}}{{\text{e}}^{3 + }}\) to \({\text{F}}{{\text{e}}^{2 + }}\)

B.     Iodine reduces \({\text{S}}{{\text{n}}^{4 + }}\)to \({\text{S}}{{\text{n}}^{2 + }}\)

C.     Iodine oxidizes \({\text{F}}{{\text{e}}^{2 + }}\)to \({\text{F}}{{\text{e}}^{3 + }}\)

D.     Iodine oxidizes \({\text{S}}{{\text{n}}^{2 + }}\) to \({\text{S}}{{\text{n}}^{4 + }}\)

D

Two respondents stated in their G2 forms that the question was tricky and may have confused the candidates as they should apply the difference between Iodine and Iodide. If at Higher level candidates are not able to recognize that the symbol of Iodine is I2 and that of the Iodide ion is I- there is a severe problem in the candidates’ basic preparation in Chemistry. Indeed, 33.08% of the candidates chose A as the correct answer thinking Iodine is I-, with only 32.05% choosing the correct answer D. This proved to be the third most difficult question in the paper with a discrimination index of 0.38.