The rate equation (or rate law) is an equation used to calculate the speed of a chemical reaction (or rate of reaction). For a general reaction aA + bB → C, the rate equation is:
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.