Define oxidation in terms of electron transfer.

State the oxidation number of manganese in \({\text{KMn}}{{\text{O}}_{\text{4}}}\) and in \({\text{MnC}}{{\text{l}}_{\text{2}}}\).

\({\text{KMn}}{{\text{O}}_{\text{4}}}\)

\({\text{MnC}}{{\text{l}}_{\text{2}}}\)

Deduce which species has been oxidized in this reaction and state the change in oxidation number that it has undergone.

Loss of (one or more) electrons;

\({\text{(KMn}}{{\text{O}}_{\text{4}}}{\text{)}} + 7\);

\({\text{(MnC}}{{\text{l}}_{\text{2}}}{\text{)}} + 2\);

Must have + sign for mark.

[1 max] if roman numerals or 7+ or 2+ used or if + signs are missing.

\({\text{C}}{{\text{l}}^ - }\) / chloride / chlorine / Cl (has been oxidized) / HCl;

oxidation number from –1 to 0 / has increased by one;

If HCl is given for first mark, it must be clear that it is the Cl that has the change of oxidation number.

This question was probably the best answered on the paper. Most candidates were able to score the mark for the definition of oxidation.

In Part (b)(i) many candidates scored both marks, though a significant number received a single mark penalty for not including the + sign.

In (b)(ii) most candidates were able to identify chlorine as the substance oxidised but many suggested rather odd values for its oxidation states.