Electrochemistry: compare question

In acidic solution, sulfite ions, $\ce{SO3^{2-}}$, are oxidised to sulfate ions, $\ce{SO4^{2-}}$, by dichromate ions, $\ce{Cr2O7^{2-}}$, which are reduced to chromium(III) ions, $\ce{Cr^{3+}}$.

Compare oxidation and reduction in this reaction in terms of electron transfer and changes in oxidation number. In your answer you should:

• state the oxidation number of sulfur in $\ce{SO3^{2-}}$ and in $\ce{SO4^{2-}}$,
• state the oxidation number of chromium in $\ce{Cr2O7^{2-}}$ and in $\ce{Cr^{3+}}$,
• identify which species is the oxidising agent and which is the reducing agent,
• use the oxidation number changes to help balance the equation for the reaction.