Here, [A] and [B] are the concentrations of A and B. x and y depend on which step is rate-determining. If the reaction mechanism is a very simple one, where A and B hit each other and then go to products through one transition state, then x=a and y=b. k is the rate constant of the reaction. This changes with temperature, pressure and other conditions.
In special cases, it is very easy to solve the equation and find k. For example, in a first-order reaction the equation is:
So, a plot of against time t gives a straight line with a slope of .
Sometimes, experiments can be made so that the reaction looks like a first-order one. If the concentration of one reagent is kept at the same high value then it can be thought of as a constant. The equation becomes where k' is the pseudo-first order rate constant. Then the method above can be used to calculate k'.