Explain the catalytic properties of transition metals.

(N/A) Transition metals and their compounds are known for their catalytic activity. This is primarily due to their ability to adopt multiple oxidation states and to form complexes.
Examples:
$(i)$ Vanadium$(V)$ oxide $(V_2O_5)$ in the Contact process.
$(ii)$ Finely divided iron $(Fe)$ in the Haber process.
$(iii)$ Finely divided nickel $(Ni)$ in catalytic hydrogenation.
Explanation of catalytic action:
$(i)$ Adsorption: The reactant molecules get adsorbed on the surface of the catalyst. The transition metal atoms use their $3d$ and $4s$ electrons to form bonds with reactant molecules. This increases the concentration of reactants on the surface and weakens the bonds in the reactant molecules,thereby lowering the activation energy.
$(ii)$ Variable oxidation states: Transition metals can change their oxidation states,which helps in providing an alternative pathway with lower activation energy.
Example: $Fe^{3+}$ ions catalyze the reaction between iodide $(I^-)$ and persulphate $(S_2O_8^{2-})$ ions:
$(i)$ $2I^- + S_2O_8^{2-} \xrightarrow{Fe^{3+}} I_2 + 2SO_4^{2-}$
$(ii)$ $2Fe^{2+} + S_2O_8^{2-} \rightarrow 2Fe^{3+} + 2SO_4^{2-}$