Chemical Kinetics: Zero and First Order Reaction, Half-Life Period (For CBSE, ICSE, IAS, NET, NRA 2022)

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Zero Order Reaction

Zero order reactions are those whose rate is proportional to zeroth power of concentration, that is, the rate is independent of concentration of the reactant.

Examples of Zero Order Reactions

  • Decomposition of on the surface of gold catalyst when the pressure of is high.
  • Photochemical combination of and to form when carried out over water.
  • Enzyme reactions when the substrate concentration is high in comparison to the enzyme concentration.

Consider the reaction:

The order of the reaction is zero, so

Separating the variables, we get

Integrating both side

is the concentration of at time and is the constant of integration.


Substituting in equation

This equation is the equation of straight lene and of the form

It is plotted in graph as shown

Zero Order Reaction

Unit of

Equation 1 can be rearranged as

Unit of for a zero-order reaction is .

Half-Life Period,

The time taken for a reaction to reach the half-way stage i.e.. when half of the starting material has reacted is called half-life period of the reaction.

It is denoted by or .

Equation (1) can be rearrange to


Thus, the half-life period of a zero-order reaction depends upon the initial concentration of the reactant

First Order Reaction

Consider for the reaction

For first order reaction

is the rate constant.

Rearranging the rate expression, we get

Integrating the two sides we get,

Let be concentration of when

Putting constant in equation (1)


Rate Constant, In

Convert it to log to the base 10

This equation is the form of equation of straight line.

If we plot vs , it will give a straight line with a slope equal to

First Order Reaction

Developed by: