Types of Chemical Reactions

Chemical Kinetics

Content

Introduction

Types of Chemical Reactions

3) Reaction Rate

4) Molecularity and Order of Reaction

5) Factors Affecting Rate of Reaction:

6) Collision Theory of Reaction Rate

7) Chemical Equilibrium

8) Catalyst

Chemical kinetics, the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with thermodynamics, which deals with the direction in which a process occurs but in itself tells nothing about its rate. Thermodynamics is time’s arrow, while chemical kinetics is time’s clock. Chemical kinetics relates to many aspects of cosmology, geology, biology, engineering, and even psychology and thus has far-reaching implications. The principles of chemical kinetics apply to purely physical processes as well as to chemical reactions.

One reason for the importance of kinetics is that it provides evidence for the mechanisms of chemical processes. Besides being of intrinsic scientific interest, knowledge of reaction mechanisms is of practical use in deciding what is the most effective way of causing a reaction to occur. Many commercial processes can take place by alternative reaction paths, and knowledge of the mechanisms makes it possible to choose reaction conditions that favor one path over others.

There are two general principal reasons for studying chemical kinetics.

1) To predict the rate of a particular reaction under specified conditions: The conditions can be adjusted to make the reactions to go at a desired rate, either rapidly or slowly or moderately. The field of chemical kinetics is useful in industry as the conditions for maximum yields of industrial products can be ascertained.

2) To predict the mechanism of the reaction: the intelligent guess regarding various elementary processes responsible for the formation of products can be made which should be consistent with experimental data.

Thermodynamics deals with the feasibility of a chemical change. The free energy change, DG, of a reaction helps us to understand whether the reaction will occur or not.

Even though there may be decrease in free energy but reactants do not always form the products instantaneously and actual rate of the reaction may vary from extremely slow to very fast. Thermodynamics is concerned only with initial and final states of reacting systems but offers no explanation about the various stages through which the reactants pass to reach the final state. This leads to following questions concerning chemical changes.

How fast do the chemical reactions go?

How can the speed of the reaction change?

What intermediate steps, the reactants follow to reach the final state of products, i.e., the mechanism of the reaction.

The branch of physical chemistry which deals with the rate at which the chemical reactions take place and the influence of various factors such as concentration, temperature, pressure catalyst, etc., on the reaction rates, is called the chemical kinetics.