Effect of Catalysts

A catalyst is a substance that increases the rate of a reaction without being used up. It is chemically unchanged at the end of the reaction.A catalyst works by providing an alternative reaction pathway with a lower activation energy. It often does this by forming an intermediate with a reactant, or by holding reactant molecules on its surface where bonds are weakened.The effect is shown on a Maxwell-Boltzmann distribution. The curve itself does not change, because the temperature is the same. But the catalysed activation energy, E_cat, is to the left of the uncatalysed E_a, so a much larger area lies to the right of it.A greater proportion of molecules now have energy greater than or equal to the activation energy, so a greater proportion of collisions are successful and there are more successful collisions per unit time.On a reaction profile, the catalysed pathway has a lower peak, but the energies of the reactants and products are unchanged, so the enthalpy change, ΔH, is the same.For example, manganese(IV) oxide catalyses the decomposition of hydrogen peroxide:2H₂O₂(aq) → 2H₂O(l) + O₂(g)A homogeneous catalyst is in the same phase as the reactants. A heterogeneous catalyst is in a different phase, usually a solid with gaseous or liquid reactants, as with MnO₂ here.