Defining Chemical Kinetics

Chemical kinetics helps us to understand the rates of various chemical reactions. A chemical reaction involves three aspects:

1. Feasibility of whether a reaction will occur or not. This is elaborated under Thermodynamics.
2. The extent or degree to which the reaction will occur. The reaction apparently stops when the forward and reverse reactions occur at the same rates. This is studied under equilibrium.
3. The speed or rate at which the reaction occurs until it reaches equilibrium. Chemical Kinetics deals with this study. It helps us to understand not only the rate of a reaction, but also tells us about factors that can enable us to increase or decrease the speed of the reaction.

Rate Of A Reaction

Peter Waage and Cato Guldberg are credited to have pioneered the study of Chemical Kinetics. In 1864, they described the law of mass action. This law states that the speed of a chemical reaction is directly dependent upon the quantity or concentration of the reactants.

The rate of a reaction is defined as the rate at which the products are formed. Conversely, it can also be stated as the rate at which the reacting substances are consumed.

Rate of disappearance of reactant (R) 

= (Decrease in concentration of R) / (Time taken) 

= – ΔR/ Δ T

The -ve sign is added in order to make the rate a positive quantity, since ΔR is negative.

Alternately, 

Rate of appearance of product (P) 

= (Increase in concentration of P) / (Time taken) 

=  ΔP/ Δ T

These are called the average rate of a reaction.

Rate Law

When we show the rate of reaction in terms of concentration of the reactants, then the expression is known as rate law, rate equation, or rate expression.

The above equations show that the rate of a reaction is directly influenced by the concentration of the reactants. Thus, if substances A and B undergo a reaction, then,

Rate = k[A]x[B]y

where, 

k = rate constant,

[A] & [B] = concentration of substances A & B,

x & y = reaction orders

Summation of x and y is called the order of the reaction. It can be a fraction or a whole number (0, 1, 2 etc.). When x+y = 0, it means the reaction is not dependent upon the concentration of the reacting substances.

Half-Life

The half-life of a reaction is defined as the time it takes for the original concentration or pressure of the reactant to reduce by half. For first-order reactions, the half-life remains constant with time and is not dependent upon the concentration of the reactant. A typical example of this type of reaction is radioactive substances.

The rate at which chemical reactions occur varies widely, from several billion years to the order of femtoseconds (1 fs = 10–15 second). Thus, chemical kinetics deals with a very wide range of rates of reactions.

Measuring Slow Reactions

Some processes occur at rates that are too slow to be studied experimentally. Catalysts help to accelerate the speed at which such reactions occur.

Extremely slow reactions can be studied by changing the conditions in which they occur so that there is an observable change in a reasonable time. One such possibility is to increase the temperature.

Measuring Fast Reactions

Very fast reactions are difficult to measure because:

1. The time taken to change the temperature of the system or to mix the reacting substances is very close to the half-life. This makes it difficult to measure the initial time when the reaction started.
2. The time taken to measure the amount of the products is often very similar to the half-life of the reaction.

We measure such reactions using Flow Methods and Pulse & Probe Methods to overcome these difficulties.

Factors Affecting Chemical Reactions

The rate of a reaction can be influenced by several factors such as temperature, the concentration of reactants, pressure, surface area, nature of the reactants, catalysts, presence of light, and even the physical state of the reacting substances.

Applications Of Chemical Kinetics

Chemical kinetics helps us to study both physical processes and chemical reactions. It enables us to understand the actual mechanism of chemical reactions. In several commercial processes involving chemical reactions, there are many ways in which the reactions can be made to occur. Knowledge of chemical kinetics helps us to determine the most favourable path and choose the conditions that will enable the reaction to proceed along that path.

Conclusion

In this article, we learned how the field of chemical kinetics helps us study the rate at which chemical reactions occur, what factors can vary the speed, the rate law, and how we measure various types of reactions.