Chemical Kinetics

part 1

Rate of Reaction

• Rate of reaction — average and instantaneous
• Factors affecting rate of reaction
• Rate law and rate constant
• Order of reaction and molecularity

part 2

Integrated Rate Equations

• Zero order reactions
• First order reactions
• Half-life of zero and first order reactions
• Pseudo first order reactions

part 3

Activation Energy and Catalysis

• Temperature dependence of rate — Arrhenius equation
• Activation energy and activated complex
• Effect of catalyst
• Collision theory of reaction rates

Rate law

$\text{Rate} = k[A]^m[B]^n$ (m, n = order w.r.t. A, B)

★ board favourite

First order: integrated rate law

$\ln[A] = \ln[A]_0 - kt$; $t_{1/2} = \frac{0.693}{k}$

★ board favourite

First order k from concentration

$k = \frac{2.303}{t}\log\frac{[A]_0}{[A]}$

★ board favourite

Arrhenius equation

$k = Ae^{-E_a/RT}$ or $\ln k = \ln A - \frac{E_a}{RT}$

★ board favourite

Arrhenius: k from two temperatures

$\log\frac{k_2}{k_1} = \frac{E_a}{2.303R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)$

★ board favourite

Rate of reaction

$\text{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = +\frac{1}{b}\frac{d[B]}{dt}$

★ frequently asked

Zero order: integrated rate law

$[A] = [A]_0 - kt$; $t_{1/2} = \frac{[A]_0}{2k}$

★ frequently asked

Second order: integrated rate law

$\frac{1}{[A]} = \frac{1}{[A]_0} + kt$; $t_{1/2} = \frac{1}{k[A]_0}$

★ frequently asked

A first-order reaction takes 40 minutes for 30% decomposition. Calculate its half-life. (log 0.7 = −0.155)

For a reaction A + B → products, the rate law is Rate = k[A][B]². What is the overall order of the reaction, and by what factor does the rate change if [B] is doubled?

Write the Arrhenius equation. The rate constant for a reaction doubles when the temperature increases from 300K to 310K. Calculate the activation energy. (R = 8.314 J/mol·K)

Explain the collision theory of chemical reactions. Define activation energy and threshold energy. What is the role of the orientation factor?

Define order and molecularity of a reaction. Derive the integrated rate equation for a first-order reaction and show that its half-life is independent of initial concentration.

(a) Derive the integrated rate equation for a zero-order reaction. (b) The rate constant of a first-order reaction is 60/s. How much time will it take to reduce the initial concentration to 1/16th of its value?

For the reaction 2A → B, the rate of disappearance of A is 0.02 mol/L/s. What is the rate of formation of B?

The activation energy of a reaction is 75.2 kJ/mol in the absence of a catalyst and 50.14 kJ/mol in its presence. How many times will the rate of the reaction change at 25 °C if it is carried out in the presence of the catalyst? (R = 8.314 J/mol·K)

What is a pseudo first-order reaction? Explain with a suitable example why it behaves as first order though it is bimolecular.

Can the molecularity of a reaction be zero? Give a reason for your answer.

Show that for a first-order reaction the time required for 99% completion is twice the time required for 90% completion.

How many marks is Chemical Kinetics worth in the Class 12 board exam?

Around 7 marks of the 70-mark CBSE Chemistry theory paper, based on the official unit-wise weightage. Order of reactions and Arrhenius equation numerics.

Is Chemical Kinetics easy or hard?

It's rated hard — one of the tougher chapters in Class 12 Chemistry. Most students need about 4 hours to cover it well.

What are the important topics in Chemical Kinetics?

The chapter covers 11 NCERT topics in 3 areas: Rate of Reaction; Integrated Rate Equations; Activation Energy and Catalysis.

What questions come from Chemical Kinetics in board exams?

Between 2018–2024, CBSE board papers asked questions from this chapter on First-Order Kinetics, Rate Law, Arrhenius Equation, Collision Theory — 1- to 5-mark questions.