$(a)$ Transition metals can act as catalysts because these can change their oxidation state. How does $Fe(III)$ catalyse the reaction between iodide and persulphate ions ?

$(b)$ Mention any three processes where transition metals act as catalysts.

$(a)$ Reaction between iodide and persulphate ions is given by $2 \mathrm{I}^{-}+\mathrm{S}_{2} \mathrm{O}_{8}^{2-} \stackrel{\mathrm{Fe}(\mathrm{III})}{\longrightarrow} \mathrm{I}_{2}+2 \mathrm{SO}_{4}^{2-}$

Mechanism :

$(i)$ $2 \mathrm{Fe}^{3+}+2 \mathrm{I}^{-} \rightarrow 2 \mathrm{Fe}^{2+}+\mathrm{I}_{2}$

$(ii)$ $2 \mathrm{Fe}^{2+}+2 \mathrm{~S}_{2} \mathrm{O}_{8}^{2-} \rightarrow 2 \mathrm{Fe}^{3+}+2 \mathrm{SO}_{4}^{2-}$

$(b)$ $(i)$ Contact process : $\mathrm{V}_{2} \mathrm{O}_{5}$

$(ii)$ Haber's process : Finely divided : Fe

$(iii)$ Decomposition of $\mathrm{KClO}_{3}: \mathrm{MnO}_{2}$