Composite Reactions & the Steady State Approximation

BASIC PRINCIPLES

Review

For the reaction NO + NO3 → 2NO2 , the rate of reaction can be expressed as

r = - d[NO]/dt = - d[NO3]/dt = + ½ d[NO2]/dt

Order of Reaction

Returning to the simple reaction, A →B, defining the rate of the reaction doesn't tell the whole story. The order of the reaction must also be known. For the given example the order, n, can be determined such that:

Rate a [A]n

For the general case, aA + bB → Product(s), the rate of the reaction is:

Rate a [A]α[B]β
where the overall order of the reaction is (α + β), the order with respect to (wrt) A is α and the order wrt B is β.
Note : a = α and b = β only if the reaction is elementary.

Review Question

For the experimentally determined rate of reaction, r a [NO]2[O2], it can be stated that

Overall Order = 1, Order wrt NO = 2, Order wrt O2 = 1

Overall Order = 2, Order wrt NO = 1, Order wrt O2 = 1

Overall Order = 2, Order wrt NO = 2, Order wrt O2 = 1

Overall Order = 3, Order wrt NO = 2, Order wrt O2 = 1