Chemical Kinetics

The branch of physical chemistry which deals with rate and mechanism of a chemical reaction is known as chemical kinetics.

Rate of reaction:

The Change in concentration of the reactant or product per unit time is known as rate of reaction.

In terms of reactant, it is defined as the decrease in concentration of reactant per unit time. But in terms of product, it is defined as increase in concentration of product per unit time.

Let us consider a general reaction where reactant “A” gives a product “B”.

Chapter -3 Chemical Kinetics.pdf - 812 KB

Types of rate of reaction:

There are three types of rate of reaction. They are

1. Initial rate

If the rate of the reaction is measured at initial time of a reaction, then it is known as initial rate.

(where t ⇾ o)

2. Average rate

If the rate of the reaction is measured at a large time interval, then it is called average rate.

3. Instantaneous rate

If the rate of the reaction is measured in very small-time interval, then it is called instantaneous rate. It is denoted by the symbol

Factors affecting the rate of reaction:

There are numerous factors that affect the rate of chemical reaction. They are:

1. Nature of reactant:

In a chemical reaction, the bond between the reactant molecules are broken or dissociated and new bonds are formed to give the product. If the bonds between the reactant molecules are weak, small amount of energy is sufficient for their dissociation i.e. rate of reaction is fast. If the bond between the reactant molecules are strong, large amount of energy is required for its dissociation i.e. rate of the reaction is slow.

2. Temperature:

Temperature has a significant effect on the rate of chemical reaction.. A rise of temperature not only increases the speed of the reaction but in many cases it actually initiate the reaction. The effect of temperature on the rates of the chemical reaction can be explained by considering the following facts as:

For most of the reactions, the rate of reactions increases with rise of temperature. The rate of reaction becomes nearly double or triple for every 10ºC rise in temperature. Large increase in the rate of the reaction is due to the increase in collision frequency & increase in effective number of collision.

3. Concentration:

When a reaction precedes, the concentration of the reactant decreases with the passage of time and that of product increases with time. It is observed experimentally that the rate of reaction in terms of reactant also decreases with time and in terms of product increases with time. Which is concluded from the following relation.

Chapter -3 Chemical Kinetics.pdf - 812 KB

Rate =𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛

𝑡𝑖𝑚𝑒 which indicate that rate of reaction is directly proportional to the concentration of reactant.

4. Surface area

Surface area of the reactant also alters the rate of reaction. Larger the surface area of the reactant, higher will be the rate of reaction and vice versa.

5. Catalyst

Catalyst also alters the rate of chemical reaction. Positive catalyst increase the rate of reaction but negative catalyst decreases the rate of reaction.

Order of reaction

The sum of power of concentration term in the rate equation, on which the rate of reaction actually depends is called the order of the reaction.

NUMERICAL

Acid-Base catalysis:

Homogeneous catalytic reaction which are catalysed by acids or base are known as Acid-Base catalysis.
The substance which has a tendency to lose or gain proton can act as a catalyst and known as general
acid bas e catalysis.
Enzyme catalysis:
Enzyme are the protein molecule present in the living system, which catalyze the reactions taking place
in the living cell and also help in controlling their reaction rates. The reaction in enzyme catalysis occurs
at a specific site on the protein molecule. Thus is called active site. The reactants in an enzymes reaction
are referred to as substrate.
On the other hand, enzyme catalysts are highly specific biological catalysts.
An examples of enzyme catalyzed laboratory reaction is

Diastage Maltase Zymase
(i) Starch → Maltose → Glucose → Ethanol + CO2.