● CBSE · Class 12 · Chemistry · Chapter 4

Chemical
Kinetics

Complete chapter resources for CBSE Class 12 Chemistry — rate laws, integrated rate equations, Arrhenius equation, activation energy, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
5–7Board marks
8Sample questions
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Key Formulas — Chapter 4
  • Rate law: rate = k[A]^m [B]^n
  • 1st-order integrated: ln[A]t = ln[A]0 − kt
  • Half-life (1st order): t½ = 0.693 / k
  • Arrhenius equation: k = A · e^(−Ea/RT)
  • log form: log(k2/k1) = Ea/2.303R · (1/T1 − 1/T2)
  • 2nd-order integrated: 1/[A]t = 1/[A]0 + kt
Chapter Overview

What this chapter covers

Chemical Kinetics is the branch of chemistry that studies the speed (rate) at which chemical reactions proceed and the factors that affect those rates. The rate of reaction is expressed as the change in concentration of a reactant or product per unit time. The rate law relates the reaction rate to the molar concentrations of reactants: rate = k[A]m[B]n, where k is the rate constant and m, n are the orders with respect to each reactant — determined experimentally, not from stoichiometry.

A central topic is integrated rate equations, which express concentration as a function of time for zero-, first-, and second-order reactions. For first-order reactions — the most commonly tested — the integrated form ln[A] = ln[A]0 − kt gives the half-life t½ = 0.693/k, which is independent of initial concentration. Board questions frequently ask students to distinguish reaction orders using graphical plots (ln[A] vs. t for first-order; 1/[A] vs. t for second-order).

The Arrhenius equation k = Ae−Ea/RT quantifies the dependence of rate constant on temperature and activation energy (Ea). It explains why a 10 K rise approximately doubles the rate for many reactions. This section also introduces the concept of activation energy and the role of catalysts in lowering Ea, providing an alternative pathway with a lower energy barrier — a concept that connects this chapter to surface chemistry and enzyme catalysis in Class 12.

Topics

What's inside Chapter 4

As per NCERT Class 12 Chemistry Part I (CBSE syllabus)

Topic 1
Rate of a Chemical Reaction
Average rate vs. instantaneous rate. Expressing rate in terms of reactants and products using stoichiometric coefficients. Units of rate (mol L⁻¹ s⁻¹). Factors affecting reaction rate: concentration, temperature, catalyst, surface area.
Topic 2
Rate Law and Order of Reaction
Rate law expression. Rate constant k and its units. Order vs. molecularity. Determining order experimentally by the initial rate method. Zero-, first-, and second-order reactions. Pseudo-first-order reactions.
Topic 3
Integrated Rate Equations & Half-Life
Derivation and application of integrated rate laws for zero, first, and second order. Half-life formula and its independence from initial concentration for first-order reactions. Graphical methods to identify reaction order.
Topic 4
Arrhenius Equation & Activation Energy
Temperature dependence of reaction rates. Arrhenius equation k = Ae^(−Ea/RT). Calculating Ea from two temperatures. Energy profile diagrams, transition state, and role of catalysts in reducing activation energy.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Class 11 · Thermodynamics
Gibbs energy, spontaneity, energy profile diagrams
Class 11 · States of Matter
Collision theory, Maxwell-Boltzmann distribution
Chapter 4 Chemical
Kinetics
Leads to
Ch 5 · Surface Chemistry
Catalysis, adsorption, enzyme kinetics
Electrochemistry (Ch 3)
Rate concepts applied to electrode kinetics and corrosion
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 12 Chemistry board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion–Reason 1 1–2 Order identification, half-life facts, or rate constant units
Very Short Answer 2 1 Integrated rate law calculation or half-life for first-order
Short Answer 3 1 Arrhenius equation — calculate Ea or k at a new temperature
Long Answer / Numerical 5 0–1 Multi-step problem involving order determination and Arrhenius calculation
Total (approximate) 5–7 3–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 12 Chemistry Chapter 4. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
The half-life of a first-order reaction is 30 minutes. The rate constant k for this reaction is: (a) 0.693 min⁻¹ (b) 0.0231 min⁻¹ (c) 0.0462 min⁻¹ (d) 30 min⁻¹
Q2 Easy 2 marks Short Answer
The rate of reaction 2A + B → products is expressed as rate = k[A]²[B]. State the overall order of the reaction and write the units of rate constant k.
Q3 Medium 2 marks Short Answer
For a first-order reaction, the initial concentration of reactant A is 0.5 mol L⁻¹ and the rate constant k = 6.93 × 10⁻² min⁻¹. Calculate the concentration of A after 20 minutes.
Q4 Medium 3 marks Short Answer
The rate constant of a reaction increases from 4.5 × 10⁻³ s⁻¹ at 298 K to 9.0 × 10⁻³ s⁻¹ at 308 K. Calculate the activation energy (Ea) for the reaction. (R = 8.314 J mol⁻¹ K⁻¹)
Q5 Medium 3 marks Short Answer
Distinguish between order of reaction and molecularity of reaction. Give one example each of a first-order reaction and a pseudo-first-order reaction.
Q6 Hard 4 marks Word Problem
In a first-order reaction A → products, 75% of the reactant decomposes in 60 minutes. (i) Calculate the rate constant k. (ii) How long will it take for 90% of the reactant to decompose? (Given: log 2 = 0.3010, log 10 = 1)
Q7 Hard 5 marks Word Problem
The following data were obtained for the reaction A + B → products at 300 K: Exp. [A] (mol L⁻¹) [B] (mol L⁻¹) Rate (mol L⁻¹ s⁻¹) 1 0.10 0.10 2.5 × 10⁻⁴ 2 0.20 0.10 5.0 × 10⁻⁴ 3 0.10 0.20 2.5 × 10⁻⁴ (i) Determine the order of reaction with respect to A and B. (ii) Write the rate law and calculate the value of k with units.
Q8 Hard 5 marks Case-Based
The Arrhenius equation for a reaction is given as k = (3.0 × 10¹⁴ s⁻¹) × e^(−Ea/RT), where Ea = 97.4 kJ mol⁻¹. (i) Calculate the rate constant at 400 K. (R = 8.314 J mol⁻¹ K⁻¹) (ii) How will the rate constant change if a catalyst lowers Ea by 20 kJ mol⁻¹? (iii) Draw and label the energy profile diagram for an exothermic catalysed reaction, showing ΔH, Ea (uncatalysed), and Ea (catalysed).
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 12 Chemistry board papers — Chemical Kinetics chapter.

Board 20222 marks
The rate constant of a first-order reaction is 1.386 × 10⁻² min⁻¹. Calculate the half-life period and the time required to reduce the initial concentration to 25% of its original value. (CBSE All India 2022)
Board 20233 marks
The activation energy for a reaction is 100 kJ mol⁻¹. If the rate constant at 500 K is 4.0 × 10⁻⁵ s⁻¹, calculate the rate constant at 600 K using the Arrhenius equation. (R = 8.314 J mol⁻¹ K⁻¹) (CBSE Delhi 2023)
Board 20203 marks
For a reaction A → products, the following data were collected: t (s): 0 300 600 [A] (mol L⁻¹): 0.80 0.40 0.20 Prove that the reaction is first-order and calculate the rate constant. (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Chemical Kinetics carry in the CBSE Class 12 Chemistry board exam? +
Chemical Kinetics typically carries 5–7 marks in the CBSE Class 12 Chemistry board exam. Questions appear as a 1-mark MCQ or assertion-reason, one 2-mark short answer (often on integrated rate laws or half-life), and one 3-mark question on the Arrhenius equation or graphical interpretation. The chapter has been part of every board paper in the last several years.
What is the difference between rate of reaction and rate constant? +
The rate of reaction is the change in concentration of a reactant or product per unit time — it changes as the reaction proceeds. The rate constant k is the proportionality constant in the rate law (rate = k[A]^m[B]^n); it depends only on temperature and the nature of the reaction, not on concentration. Board questions frequently test this distinction in 1-mark or 2-mark questions.
How do I derive the half-life of a first-order reaction for board exams? +
For a first-order reaction, the integrated rate law is ln[A] = ln[A]₀ − kt. At t = t½, [A] = [A]₀/2. Substituting: ln(1/2) = −kt½, giving t½ = 0.693/k. The key result to remember is that the half-life of a first-order reaction is independent of initial concentration — this fact is asked directly in board exams as a 1–2 mark question almost every year.
What is the Arrhenius equation and how is it used in board exam questions? +
The Arrhenius equation is k = Ae^(−Ea/RT), where A is the frequency (pre-exponential) factor, Ea is the activation energy, R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T is temperature in Kelvin. Board questions use the logarithmic form log(k₂/k₁) = (Ea/2.303R)(1/T₁ − 1/T₂) to calculate Ea or find k at a new temperature. This is consistently a 3-mark question.
How do I generate a custom question paper for Chemical Kinetics using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 12 Chemistry, select Chapter 4 (Chemical Kinetics), set your preferred question-type mix (MCQ, short answer, numerical) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.