The study of chemical reactions has been of great interest to the chemists. They wish to investigate the manner in which the reaction occurs and the speed at which it takes place. They are also interested in studying the effect of various parameters like temperature, pressure and concentration on the rate of the reaction. Chemical kinetics deals with all the above and can be defined as the branch of chemistry which deals with
• Rate of chemical reaction
• Mechanism by which the reactants are converted into products and
• Factors affecting the rate of a reaction
It is observed that different reactions occur at different rates. Some reactions are fast and some reactions occur very slowly and take months or even years for their completion, while some reactions occur at a moderate rate. It is these reactions occurring at moderate speed which are of great interest to the chemists as their rates can easily be measured in the laboratory.
Depending upon their speed the chemical reactions are categorized as follows
1. Fast Reactions Reactions that occur instantaneously as soon as the reactants are mixed together are termed as fast reactions. For example, the precipitation of the solution of silver nitrate and sodium chloride.
AgNO3 + NaCl → AgCl+ NaNO3
Similarly, neutralization of acid and base also occurs as soon as the two substances are mixed. Generally, ionic reactions are fast reactions because they involve ions that are held together by electrostatic forces and no bonds are broken in them. The rates of these reactions are too fast to be determined by conventional methods. These methods cannot deal with reactions whose half lives are less than a second or so. Special techniques are employed to measure the rate of such reactions.
2. Very slow reactions Some reactions like rusting of iron occur so slowly that one can be misled into thinking that no reaction is occurring at all. These reactions proceed very slowly and it becomes difficult to determine the speed of the reactions.