Science Class 12 - Chemical Kinetics Notes

Chemical Kinetics

Welcome to the chapter on Chemical Kinetics for Class 12. In this chapter, you will learn about the rates of chemical reactions, factors affecting these rates, and how to calculate and interpret rate laws. By the end of this chapter, you will be able to analyze reaction mechanisms and solve numerical problems related to chemical kinetics.

Introduction

Chemical kinetics is the study of the speed or rate at which chemical reactions occur and the factors that affect these rates. Understanding kinetics helps us control reactions in laboratories and industries.

Key Concepts

• Rate of Reaction: The change in concentration of a reactant or product per unit time.
• Rate Law: An equation that relates the rate of a reaction to the concentration of reactants.
• Order of Reaction: The sum of the powers of concentration terms in the rate law.
• Molecularity: The number of molecules colliding in an elementary step.
• Activation Energy (Ea): The minimum energy required for a reaction to occur.

Rate of Reaction

The rate of a reaction can be expressed as:

Rate = - (Δ[R]) / Δt = + (Δ[P]) / Δt

• [R] = concentration of reactant
• [P] = concentration of product
• Δt = change in time

Factors Affecting Rate of Reaction

• Concentration of reactants
• Temperature
• Presence of a catalyst
• Surface area of reactants
• Nature of reactants

Rate Law and Order of Reaction

For a reaction: aA + bB → Products

The rate law is: Rate = k [A]^m [B]ⁿ

• k = rate constant
• m, n = order with respect to A and B
• Overall order = m + n

Integrated Rate Laws

• Zero Order: [A]_t = [A]₀ - kt
• First Order: ln([A]₀/[A]_t) = kt
• Second Order: 1/[A]_t = 1/[A]₀ + kt

Half-Life (t_1/2)

• First Order: t_1/2 = 0.693 / k
• Zero Order: t_1/2 = [A]₀ / 2k

Arrhenius Equation

The Arrhenius equation shows how the rate constant (k) changes with temperature:

k = A e^-Ea/RT

• A = frequency factor
• Ea = activation energy
• R = gas constant
• T = temperature (in Kelvin)

Practice Questions

1. Define rate of reaction and write its unit.
2. Write the rate law for the reaction: 2A + B → C.
3. How does temperature affect the rate of a reaction?
4. Calculate the half-life of a first-order reaction with k = 0.231 min^-1.
5. What is activation energy?

Challenge Yourself

• A reaction is first order in A and second order in B. Write the rate law and overall order.
• If the rate constant doubles when temperature increases by 10°C, what does this suggest about the activation energy?

Did You Know?

• Catalysts speed up reactions without being consumed.
• Enzymes are biological catalysts that control reactions in our bodies.

Glossary

• Rate Constant (k): A proportionality constant in the rate law.
• Activation Energy (Ea): Minimum energy needed for a reaction to occur.
• Half-Life (t_1/2): Time taken for half of the reactant to be consumed.
• Order of Reaction: The sum of the powers of concentration terms in the rate law.

Answers to Practice Questions

1. The rate of reaction is the change in concentration of a reactant or product per unit time. Unit: mol L^-1 s^-1
2. Rate = k [A]^m [B]ⁿ (where m and n are determined experimentally)
3. Increasing temperature increases the rate of reaction by providing more energy to the molecules, leading to more effective collisions.
4. t_1/2 = 0.693 / 0.231 = 3 min
5. Activation energy is the minimum energy required for a reaction to take place.

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Understanding chemical kinetics helps you control and optimize chemical reactions in real life and industry!