How is a reaction's order related to its rate expression?

A reaction's order is directly related to the exponents of the reactants in its rate expression.

In more detail, the order of a reaction is a term that describes how the rate of reaction is affected by the concentration of one or more of the reactants. It is determined experimentally and is given by the sum of the exponents of the concentrations of the reactants in the rate expression.

For instance, consider a general reaction where A and B react to form products, with the rate expression given as: Rate = k[A]^m[B]^n. Here, 'k' is the rate constant, '[A]' and '[B]' are the concentrations of reactants A and B, and 'm' and 'n' are the orders of reaction with respect to A and B. The overall order of the reaction is the sum of 'm' and 'n'.

If the rate of a reaction is directly proportional to the concentration of a single reactant, the reaction is said to be first order with respect to that reactant. If the rate is proportional to the square of the concentration of a reactant, the reaction is second order with respect to that reactant. Zero order reactions are those where the rate is independent of the concentration of a reactant.

It's important to note that the order of a reaction can't be predicted from the stoichiometric equation of the reaction; it must be determined experimentally. The order of a reaction can provide valuable information about the reaction mechanism, which is the step-by-step sequence of elementary reactions by which overall chemical change occurs.

In summary, the order of a reaction is a crucial concept in chemical kinetics as it helps us understand how the rate of a reaction changes with the concentration of the reactants. It is directly related to the exponents of the reactants in the rate expression, and provides insight into the reaction mechanism.