Q.Why is finely divided nickel used in hydrogenation of oil ?

A catalyst works purely by lowering the activation energy of a reaction (via an alternate, lower-energy pathway); it leaves the thermodynamic quantities (enthalpy, internal energy, entropy) of the overall reaction unchanged.

A catalyst increases the rate of a reaction by providing an alternative mechanism/pathway that requires a lower activation energy ($E_a$) than the uncatalysed reaction, so a much larger fraction of colliding molecules now possess enough energy to cross the (now-lower) energy barrier. Because a catalyst is regenerated at the end of the reaction and does not appear in the overall balanced equation, the initial and final states of the reactants/products are unchanged — so $\Delta H$ (enthalpy change), $\Delta U$ (internal energy change), and $\Delta S$ (entropy change) of the overall reaction, all of which are state functions d …