Discuss the catalytic properties of transition elements.

The transition elements and their compounds (mainly oxides) are known for their catalytic properties. This is because of their ability to adopt multiple oxidation states.

In a first transition series, elements such as $\mathrm{Fe}, \mathrm{Ni}, \mathrm{Mn}$, Co etc. utilizes their $3 d$ and $4 s$ electrons for bonding with the reactant molecules on their surface. As a result, the concentration of the reactants increases on the surface on these elements and bonds in the reactant molecule gets weak, there by decreasing activation energy of the reaction.

As, transition elements can change their oxidation states, they become more effective as catalyst.

Ex. : Fe(III) catalyses the reaction between iodide and persulphate ions.

$2 \mathrm{I}^{-}+\mathrm{S}_{2} \mathrm{O}_{8}^{2-} \rightarrow \mathrm{I}_{2}+2 \mathrm{SO}_{4}^{2-}$

The catalytic action of $\mathrm{Fe}^{3+}$ can be shown as : $2 \mathrm{Fe}^{3+}+2 \mathrm{I}^{-} \rightarrow \mathrm{I}_{2}+2 \mathrm{Fe}^{2+} 2 \mathrm{Fe}^{2+}+\mathrm{S}_{2} \mathrm{O}_{8}^{2-} \rightarrow 2 \mathrm{SO}_{4}^{2-}$ $+2 \mathrm{Fe}^{3+}$

Other examples include $\mathrm{V}_{2} \mathrm{O}_{5}$ in contact process, $\mathrm{Fe}$ in Haber's process, $\mathrm{Ni}$ in Hydrogenation of fats, $\mathrm{TiCl}_{4}$ in polymerisation etc.

Thus, the catalytic properties of transition element is because of presence of vacant $d$-orbitals, tendency to exists in variable oxidation states and ability to form a complexes.