$(a)$ Transition metals can act as catalysts because they can change their oxidation state. How does $Fe(III)$ catalyze the reaction between iodide and persulphate ions?
$(b)$ Mention any three processes where transition metals act as catalysts.

(N/A) The reaction between iodide and persulphate ions is given by: $2 I^{-} + S_{2}O_{8}^{2-} \xrightarrow{Fe^{3+}} I_{2} + 2 SO_{4}^{2-}$
Mechanism:
$(i)$ $2 Fe^{3+} + 2 I^{-} \rightarrow 2 Fe^{2+} + I_{2}$
$(ii)$ $2 Fe^{2+} + S_{2}O_{8}^{2-} \rightarrow 2 Fe^{3+} + 2 SO_{4}^{2-}$
$(b)$ Three processes where transition metals act as catalysts are:
$(i)$ Contact process: $V_{2}O_{5}$ is used as a catalyst.
$(ii)$ Haber's process: Finely divided $Fe$ is used as a catalyst.
$(iii)$ Decomposition of $KClO_{3}$: $MnO_{2}$ is used as a catalyst.