Concept of reaction rate

Processes by which substances are transformed to one or several products are called chemical reactions. Chemical processes that occur in the presence of light, like sunlight, are called photochemical reactions. These processes can occur naturally or under controlled conditions in the laboratory.

To a chemist, there are different features of a chemical reaction that are of interest of study. Investigating the feasibility of a chemical reaction under certain conditions forms one area of study, while trying to find out to what extent the reaction will proceed forms another area of study. These two aspects of a chemical reaction can be explored with the knowledge of thermodynamics. The third feature of a chemical reaction that is of interest here is the kinetics of a chemical reaction.

Chemical kinetics is the study of rates of reactions and their mechanisms. Study of chemical kinetics is important as it has wide ranging applications - from economical industrial production of chemicals to mechanism of transformation of drugs in the human body. In the modern day world, with so many chemical pollutants in the environment, it becomes very essential to study the transformation of these pollutants in the environment. To do this, one needs to have a knowledge of chemical kinetics.

The first chemical reaction whose rate was quantitatively studied was the hydrolysis of sucrose. It was studied by L. Wilhemly in 1850. Since then many advances have been made in this field, which enable chemists to study very fast reactions (reactions that occur within less than a second). This has enabled the chemists to discover intermediates in many elementary reactions and thereby propose mechanisms of complex reactions. Chemical kinetics is vast and complex. Here, in this chapter basic aspects of chemical kinetics relating to rates of reactions and their mechanisms are discussed.

Reaction rate

Rates of chemical reaction depends on the inherent characteristics or nature of the reactants. That is the reason, why some reactions are fast while others are slow. For E.g., oxidation of ferrous ion by KMNO₄ (potassium permanganate) in acidic medium is fast, while the oxidation of oxalate ion by the same reagent is slow. For E.g., oxidation of ferrous ion by KMNO₄ (potassium permanganate) in acidic medium is fast, while the oxidation of oxalate ion by the same reagent is slow.

Apart from the nature of the reactants, the factors that may affect the rate of a reaction are as follows:

• concentration of the reactants.

• temperature at which the reaction occurs.

• concentration of catalyst (in homogenous reactions).

• surface area of a solid reactant or catalyst (in heterogenous reactions).