fractions, or zero, and must be determined experimentally. It is most important to realize that the
exponents of the rate law are not necessarily equal to the stoichiometric coefficients in the overall
reaction equation. It is generally not the case that x = a and y = b, for example, unless the reaction is
a one-step process in which the stoichiometric equation is actually a microscopic description of how
the molecules collide.
The overall order of a reaction (or the reaction order) is defined as the sum of the exponents, here
equal to x + y.
Example: Given the data below, find the rate law for the following reaction at 300 K.
A + B → C + D
Trial [A]initial(M) [B]initial(M) rinitial(M/s)
1 1.00 1.00 2.0
2 1.00 2.00 8.1
3 2.00 2.00 15.9Solution: a) In trials 1 and 2, the concentration of A is kept constant while the concentration of B
is doubled. The rate increases by a factor of approximately 4. Write down the rate
expressions of the two trials.
Dividing the second equation by the first,b) In trials 2 and 3, the concentration of B is kept constant while the concentration of A