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It may be defined as the relationship between the concentrations of the reactants and the observed reaction rate. Each reaction has its own rate equation which may be determined experimentally by altering the concentration of the reactants and then measuring the change in rate.

Consider a general reaction of the type: aA + bB→products

The reaction rate is usually proportional to the concentrations of the reactants [A] and [B] which are raised to powers and respectively. The rate expression can, then be written as:

rate =k_r[A]^a[B]^b, where k_r= rate constant.

The order of a reaction may be defined as the sum of the exponents to which the concentration terms in the rate law are raised to express the observed rate of reaction.

From the general equation of the type: aA + bB → products, the rate expression can be written as, rate =k_r [A]^a[B]^b .

a and b are called the orders of the reaction with respect to A and B. Depending on whether (a+b) is equal to 0,1,2 or 3, the reactions are said to be of zero order, first order, second order, and third order respectively.

(a) From the rate equation, it is evident that the order with respect to cyclohexene is first order.

(b) From the rate equation , it is evident that the order with respect to bromine is second order.

(c) Order with respect to cyclohexene is first order and the order with respect to bromine is second order. Therefore, overall order is which is equal to 3 (third order).