Chemical Kinetics: Rate of Chemical Reaction and Average Rate or Instantaneous Rate (For CBSE, ICSE, IAS, NET, NRA 2022)

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Rate of Chemical Reaction

The rate of any chemical reaction can be expressed as the rate of the change in concentration of a reactant (or a product) .

Its unit is mol

Let us consider the following chemical reaction:

represents the molar concentration of

Suppose that is the molar concentration at time and is the molar concentration at time .

Then, change in molar concentration =

Time required for the change =

rate of formation of


rate of the reaction with respect to

rate of the reaction with respect to

For making rate of reaction with respect to different reactants or products equal, the rate expression is divided by the stoichiometric coefficient in the balanced chemical equation.

Average Rate or Instantaneous Rate

  • The rate of a reaction depends on the concentration of reactants.
  • As the reaction proceeds the reactants get consumed and their concentration decreases with time.
  • Therefore, the rate of reaction does not remain constant during the entire reaction.


is the average rate of reaction.

Instantaneous rate is the rate of reaction at any particular instant of time, and express by making very small and given by

Concentration of any of the reactants or products is plotted against time, the graph is given below:

Average Rate or Instantaneous Rate

For the reaction

Average rate of reaction

Instantaneous rate of reaction

Factor Affecting Rate of Reaction

The rate of a chemical reaction is affected by the following factors:

  • The concentration of reactants: Rate of a reaction increases as the concentration of the reactants is increased.
  • Temperature: A reaction is faster when the reaction temperature is increased.
  • Presence of Catalyst: A catalyst alters the reaction rate without being consumed by the reaction.

Rate Law or Dependence of Rate of Reaction on Concentration

Rate is directly proportional to the concentration of the reactants raised to some power.

For example, consider the reaction

a and b are coefficients of A and B in the balanced chemical equation.

  • is the constant of proportionality and called rate constant.
  • Rate law is defined as the mathematical relationship between rate of a reaction and the concentration of the reactants.
  • and are not necessarily equal to the stoichiometric coefficient of and B in the reaction.
  • Each reaction is characterized by its own rate constant, whose value is dependent on the temperature, but independent of concentration of the reactants.

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