Chemical Kinetics – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Rate of a Chemical Reaction

The rate of a chemical reaction refers to the change in the concentration of reactants or products over time. It is an essential concept in understanding how fast a reaction occurs and the factors that influence this speed.

2. Factors Affecting the Rate of Reactions

• Concentration: Higher concentration of reactants typically increases the rate of reaction.
• Temperature: Higher temperatures increase the energy of particles, thus increasing the reaction rate.
• Pressure: Increased pressure increases the rate of reactions involving gases by reducing volume and increasing concentration.
• Catalyst: Catalysts increase the rate of reaction without being consumed, by lowering the activation energy.

3. Elementary and Complex Reactions

Elementary reactions involve a single step, while complex reactions occur in multiple steps. The rate law for elementary reactions can be determined from the molecularity of the reaction.

4. Order and Molecularity of Reactions

• Order of Reaction: The sum of the powers of the concentration terms in the rate law equation.
• Molecularity: The number of reactant molecules involved in the elementary reaction.

5. Rate Law and Rate Constant

The rate law expresses the relationship between the rate of a reaction and the concentrations of the reactants. The rate constant (k) is a proportionality constant that depends on the reaction temperature.

6. Differential and Integral Forms of Zero and First-Order Reactions

Zero-Order Reactions

• Rate Law: Rate = k [A]⁰
• Integrated Rate Law: [A]ₜ = [A]₀ - kt
• Half-Life: t₁/₂ = [A]₀ / 2k

First-Order Reactions

• Rate Law: Rate = k [A]
• Integrated Rate Law: ln [A]ₜ = ln [A]₀ - kt
• Half-Life: t₁/₂ = ln 2 / k

7. Characteristics of Zero and First-Order Reactions

• Zero-Order: The rate is independent of the concentration of reactants.
• First-Order: The rate is directly proportional to the concentration of a single reactant.

8. Effect of Temperature on the Rate of Reactions

The rate of reactions generally increases with temperature. The relationship between temperature and reaction rate can be described by the Arrhenius equation.

9. Arrhenius Theory

According to Arrhenius theory, the rate constant (k) of a reaction increases exponentially with temperature. The equation is given by:k = Ae^(-Ea/RT)where A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin.

10. Activation Energy and Its Calculation

Activation energy is the minimum energy required for a reaction to occur. It can be calculated from the Arrhenius equation by plotting ln k against 1/T and determining the slope.

11. Collision Theory of Bi-molecular Gaseous Reactions

According to the collision theory, for a reaction to occur, reacting molecules must collide with sufficient energy and proper orientation. The frequency and energy of collisions determine the rate of the reaction. This theory is particularly applicable to bimolecular reactions, where two molecules collide.