(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.

Explanation

(a) Reaction between iodide and persulphate ions is
`2I^(-)+S_(2)O_(8)^(2-)overset(Fe(III))rarrI_(2)+2SO_(4)^(2-)`
Role of Fe(III) ions `2Fe^(3+)+2I^(-)rarr2Fe^(2+)+I_(2)`
`2Fe^(2+)+S_(2)O_(8)^(2-)rarr2Fe^(3+)+2SO_(4)^(2-)`
(i) Vanatium (V) oxide used in contact process for oxidation of `SO_(2)` to `SO_(3)`.
Finely divided iron in Haber's process in conversion of `N_(2)` and `H_(2)` to `NH_(3)`.
`MnO_(2)` in preparation of oxygen from `KClO_(3)`.