Introduction to Chemical Kinetics

Chemical kinetics deals with the study of the rate of chemical reactions, the factors affecting the rate of the reactions and the mechanism by which the reactions proceed.

Reactions can be classified on the basis of their rates as instantaneous reactions (ionic reactions), very slow and moderate reactions and on the basis of their mechanism as simple reactions and complex reactions.

During a chemical reaction, breaking of old bonds and formation of new bonds takes place. Breaking of different bonds requires different amount of energy. Also, the formation of different kinds of new bonds liberates different amount of energy. So, there is a difference in the rates of chemical reactions.

Ionic reactions are very fast while molecular reactions are very slow.

There are different steps in a complex reaction and each of these steps of the reaction is called an elementary reaction. The rate of a complex reaction depends upon the slowest step called the rate determining step.

Rate of a reaction is the change in the concentration of any one of the reactants or products per unit time.

Rate of reaction changes with time, therefore, it must be expressed with reference to a particular instant of time, which is called instantaneous rate of reaction.

The unit of rate of reaction is mole per litre per second while that for gaseous reactions, it is atmosphere per second.

When a reaction proceeds, the concentration of the reactants decreases and that of the products increases.

The relationship between concentration and rate of reaction is known as law of mass action.

If all concentrations are taken as unity, then rate is equal to rate constant.

According to rate law, the rate of reaction depends upon the concentration of terms on which the rate of reaction actually depends, as observed experimentally.

Order of a reaction is the sum of the powers to which the molar concentrations in the rate law equation are raised.

For zero order reaction, rate does not depend on the concentration of the reactants.

Fractional order means the sum of the exponents to which the concentration terms are raised in the rate law expression is a non-integer.

Molecularity is the number of atoms, ions or molecules that must collide with one another simultaneously so as to result into a chemical reaction.

In case of complex reactions, each step has its own molecularity.

The reaction may be unimolecular or bimolecular.

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