The order of reaction can be defined as the power dependence of rate on the concentration of all reactants. For example, the rate of a first-order reaction is dependent solely on the concentration of one species in the reaction. Some characteristics of the reaction order for a chemical reaction are listed below.

• Reaction order represents the number of species whose concentration directly affects the rate of reaction.
• It can be obtained by adding all the exponents of the concentration terms in the rate expression.
• The order of reaction does not depend on the stoichiometric coefficients corresponding to each species in the balanced reaction.
• The reaction order of a chemical reaction is always defined with the help of reactant concentrations and not with product concentrations.
• The value of the order of reaction can be in the form of an integer or a fraction. It can even have a value of zero.

There are several different methods which can be followed in order to determine the reaction order. Some of these methods are described in this subsection.

• Initial Rates Method
• Integral Method
• Differential Method
• Different Values of Reaction Order

First-Order Reactions

The rates of these reactions depend on the concentration of only one reactant, i.e. the order of reaction is 1.

In these reactions, there may be multiple reactants present, but only one reactant will be of first-order concentration while the rest of the reactants would be of zero-order concentration.

Example of a first-order reaction: 2H₂O2 → 2H₂O + O₂