NCERT Class 12 Chemistry Chemistry-I: Chapter 3 — Chemical Kinetics
This chapter, Chemical Kinetics, delves into the study of reaction rates and their mechanisms. It explains that while thermodynamics predicts the feasibility of a reaction, chemical kinetics determines its speed. The chapter defines the rate of a chemical reaction as the change in concentration of reactants or products per unit time. It introduces the concepts of average and instantaneous rates, and how they can be expressed in terms of the disappearance of reactants or the appearance of products. The text highlights that factors like concentration, temperature, pressure, and catalysts influence reaction rates. It also touches upon elementary and complex reactions, molecularity, order of reaction, rate constant, and integrated rate equations for zero and first-order reactions. Finally, it introduces collision theory as a way to understand reaction mechanisms at a molecular level. Understanding these concepts is crucial for predicting and controlling chemical processes in various applications.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Chemistry |
| Book | Chemistry-I |
| Chapter | Chapter 3 — Chemical Kinetics |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 676 |
Learning outcomes
- Define average and instantaneous rates of reaction.
- Express reaction rates in terms of concentration changes.
- Differentiate between elementary and complex reactions.
- Distinguish between molecularity and order of reaction.
- Define and discuss the rate constant.
- Describe factors affecting reaction rates (concentration, temperature, catalyst).
- Derive and use integrated rate equations for zero and first-order reactions.
- Describe collision theory.
Vocabulary
| Word | Meaning |
|---|---|
| Chemical Kinetics | Branch of chemistry dealing with the study of reaction rates and their mechanisms. |
| Rate of reaction | Change in concentration of a reactant or product per unit time. |
| Concentration | Amount of a substance in a given volume, usually expressed in moles per litre (M). |
| Reactant | A substance that is consumed during a chemical reaction. |
| Product | A substance that is formed during a chemical reaction. |
| Thermodynamics | Branch of physics concerned with heat and its relation to other forms of energy and work. |
| Feasible reaction | A reaction that can occur spontaneously under given conditions (DG < 0). |
| Chemical equilibrium | State where forward and reverse reaction rates are equal, and net concentrations do not change. |
| Catalyst | A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. |
| Mechanism | The sequence of elementary steps by which an overall chemical reaction occurs. |
| Elementary reaction | A reaction that occurs in a single step. |
| Complex reaction | A reaction that proceeds through more than one step. |
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Practice questions
- Define the rate of a chemical reaction. Answer: The rate of a chemical reaction is defined as the change in concentration of a reactant or product in unit time.
- Give an example of a very fast reaction. Answer: Precipitation of silver chloride by mixing aqueous solutions of silver nitrate and sodium chloride.
- Give an example of a very slow reaction. Answer: Rusting of iron in the presence of air and moisture.
- What is the difference between thermodynamics and chemical kinetics? Answer: Thermodynamics predicts the feasibility of a reaction, while chemical kinetics determines the rate of a reaction.
- What are the units for the rate of reaction? Answer: The units for the rate of reaction are typically M/s (molarity per second) or mol L⁻¹ s⁻¹.
Practice MCQs
Q1. Which branch of chemistry deals with the study of reaction rates and their mechanisms?
Explanation: Chemical Kinetics is specifically defined as the branch of chemistry that studies the rates of chemical reactions and the factors that affect them.
Q2. Which of the following is an example of a very fast reaction?
Explanation: The precipitation of silver chloride (AgCl) occurs almost instantaneously when solutions of silver nitrate and sodium chloride are mixed, making it a very fast reaction.
Q3. The rate of a reaction can be expressed in terms of:
Explanation: The rate of reaction can be measured by how quickly reactants are consumed (rate of disappearance) or how quickly products are formed (rate of appearance).
Q4. What does thermodynamics predict about a reaction?
Explanation: Thermodynamics helps determine if a reaction is possible or spontaneous under given conditions, indicated by a negative change in Gibbs free energy (ΔG < 0).
Q5. The word 'kinetics' is derived from the Greek word 'kinesis' which means:
Explanation: The term 'kinetics' originates from the Greek word 'kinesis', signifying movement, which relates to the study of how reactions proceed over time.
Frequently asked questions
What is the primary focus of Chemical Kinetics?
Chemical Kinetics focuses on studying the rates of chemical reactions and the mechanisms by which they occur.
How is the rate of a chemical reaction defined?
The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time.
What is the difference between thermodynamics and chemical kinetics?
Thermodynamics predicts the feasibility of a reaction, while chemical kinetics determines its speed and mechanism.
What are some factors that affect the rate of a chemical reaction?
Factors such as concentration of reactants, temperature, pressure, and the presence of a catalyst affect the rate of a reaction.
What is an example of a very fast reaction?
The precipitation of silver chloride by mixing aqueous solutions of silver nitrate and sodium chloride is an example of a very fast reaction.
What is an example of a very slow reaction?
The rusting of iron in the presence of air and moisture is an example of a very slow reaction.
What is the significance of studying chemical kinetics?
It helps understand how reactions occur, control reaction speeds, and design processes for various applications like fuel burning or material setting.
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NCERT Class 12 Chemistry — Chemistry-I — Chapter 3 — Chemical Kinetics. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.