The average rate of a chemical reaction is the change in concentration of a reactant or product over a specific time interval. It is calculated by dividing the change in concentration by the corresponding change in time.
On the other hand, the initial rate of reaction specifically refers to the rate at the beginning of the reaction, precisely at the initial moment when the reactants are mixed. It is determined by measuring the concentration change over a very short time interval right after the reaction starts. The initial rate is crucial in studying the early stages of a reaction and understanding the factors affecting the reaction kinetics.