Order of a Reaction

The powers to which the concentration terms are raised in the rate expression describes the order of the reaction with respect to that particular reactant.

Rate = k[A]^x[B]^y

The values of x and y are order of the reaction with respect to the reactants A and B respectively. The sum of the powers x + y represents the overall order of the reaction.

Order of a reaction must be determined experimentally and cannot be deduced from the coefficients in the balanced equation. The order of reaction can be 0, 1, 2, 3 called as zero order, first order, second order and third order respectively. The order of a reaction can be a fraction as well.

Zero Order Reaction

Zero order reactions are those, in which n = 0. Here rate is independent of reactant concentration. Such reactions are uncommon. One such example is the decomposition of ammonia on a platinum or tungsten metal surface. Under high pressure of ammonia the rate at which ammonia decomposes is always the same regardless of its concentration.

For zero order:

Rate = k[A]º

Rate = k

The unit for rate of a reaction is mol L^–1 s^–1. The unit of k for zero order is same as that for rate.