Give two reactions in which transition metals or their compounds acts as catalysts.

Explanation

Catalysts, at the solid surface, involve the formation of bonds between reactants molecules and atoms of the surface of the catalyst (I row transition metals utilise 3d and 4s electrons for bonding). This has the effect of increasing the concentration of the reactants at the catalyst surface and also lowering of the activation energy. Transition metal ions sliow variable oxidation states, so they are effective catalysts.
eg. : Reaction `2I^(-)+S_(2)O_(8)^(2-) overset(Fe^(3+))rarr I_(2)+2SO_(4)^(2-)`
Mechanism of catalysing action of `Fe^(3+)` in the above reaction
a) `2Fe^(3+)+2I^(-) rarr 2Fe^(2+) +I_(2)`
b) `2Fe^(+)+S_(2)O_(8)^(2-) rarr 2Fe^(3+) +2SO_(4)^(2-)`