Class 12 Chemistry Chapter 3: Chemical Kinetics
Complete NCERT Notes · Textbook-Style Explanation
Introduction
In previous chapters, chemical reactions were discussed mainly in terms of their feasibility and extent. However, it is equally important to understand the rate at which a chemical reaction occurs.
Chemical Kinetics is the branch of chemistry that deals with the study of reaction rates, the factors affecting these rates, and the mechanism by which chemical reactions occur.
Some reactions such as ionic reactions in aqueous solutions occur almost instantaneously, while others like the rusting of iron take place very slowly. Chemical kinetics helps in explaining these differences in reaction rates.
The study of chemical kinetics is important not only from an academic point of view but also for industrial processes, environmental chemistry, biological systems, and pharmaceutical applications.
Rate of Chemical Reaction
The rate of a chemical reaction is defined as the change in concentration of a reactant or a product per unit time. It provides a quantitative measure of how fast or slow a reaction proceeds.
Since the concentration of reactants decreases with time and the concentration of products increases with time, the rate of reaction can be expressed in different ways.
Rate of reaction = Change in concentration of reactant or product / Time taken
For a general reaction:
A → Products
Rate of reaction is expressed as:
Rate = − Δ[A] / Δt
The negative sign indicates that the concentration of the reactant decreases with time.
Average Rate of Reaction
The average rate of reaction is defined as the change in concentration of a reactant or product divided by the time interval during which the change occurs.
Average rate = − Δ[A] / Δt
The average rate gives information about the reaction rate over a finite period of time and does not represent the rate at a specific instant.
Instantaneous Rate of Reaction
The instantaneous rate of reaction is defined as the rate of reaction at a particular moment of time.
Mathematically, it is expressed as the rate of change of concentration with time at that instant.
Instantaneous rate = − d[A] / dt
The instantaneous rate is obtained from the slope of the tangent drawn to the concentration–time curve at a particular point.
Units of Rate of Reaction
Since the rate of reaction is defined as change in concentration per unit time, its units depend on the units of concentration and time.
In SI units, concentration is expressed in mol L−1 and time in seconds.
Unit of rate = mol L−1 s−1
The rate of reaction is always a positive quantity. The negative sign is used only to indicate the decrease in concentration of reactants with time.
Factors Affecting Rate of Reaction
The rate of a chemical reaction is not constant under all conditions. It depends on several factors that influence the frequency and effectiveness of collisions between reacting species.
The important factors affecting the rate of a chemical reaction are discussed below.
1. Effect of Concentration
The rate of a reaction generally increases with an increase in the concentration of reactants. This is because a higher concentration results in a greater number of reacting particles per unit volume.
As the number of reacting particles increases, the frequency of effective collisions also increases, leading to a higher rate of reaction.
The reaction between hydrogen and iodine vapour proceeds faster when the concentration of either hydrogen or iodine is increased.
2. Effect of Temperature
Temperature has a pronounced effect on the rate of chemical reactions. In general, the rate of reaction increases with an increase in temperature.
An increase in temperature raises the kinetic energy of reacting molecules, which increases the number of molecules having energy equal to or greater than the activation energy.
As a result, the number of effective collisions increases and the reaction proceeds at a faster rate.
For many reactions, the rate approximately doubles for every 10°C rise in temperature.
3. Effect of Catalyst
A catalyst is a substance that alters the rate of a chemical reaction without itself undergoing any permanent chemical change.
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
Since the activation energy is lowered, a larger fraction of molecules can participate in effective collisions.
Decomposition of potassium chlorate occurs rapidly in the presence of manganese dioxide as a catalyst.
4. Effect of Surface Area
In reactions involving solids, the rate of reaction depends on the surface area of the solid reactant.
Finely divided solids provide a larger surface area for reaction, resulting in a higher rate of reaction.
Powdered coal burns much faster than a lump of coal due to its larger surface area.
Rate Law and Rate Constant
Experimental studies show that the rate of a chemical reaction depends on the concentration of reactants in a definite manner. This relationship is expressed mathematically by the rate law.
The rate law gives the dependence of the reaction rate on the concentration of reactants raised to some power.
The mathematical expression which relates the rate of reaction with the concentration of reactants is called the rate law.
For a general reaction:
aA + bB → Products
The rate law expression is written as:
Rate = k [A]x[B]y
Where k is the rate constant, and x and y are the experimentally determined powers of concentration.
Rate Constant (k)
The rate constant is a proportionality constant in the rate law expression. For a given reaction at a fixed temperature, it has a definite value.
The value of the rate constant changes with temperature but remains independent of the concentration of reactants.
The numerical value of the rate constant is equal to the rate of reaction when the concentration of each reactant is unity.
Order of Reaction
The order of a reaction is defined as the sum of the powers of the concentration terms of reactants in the rate law expression.
Order of reaction = x + y
The order of a reaction is determined experimentally and may be zero, fractional, or a whole number.
The order of a reaction does not necessarily correspond to the stoichiometric coefficients of reactants in the balanced chemical equation.
Order of Reaction and Molecularity
In chemical kinetics, the terms order of reaction and molecularity are often used to describe reactions. Although they may appear similar, they represent different concepts.
Molecularity of a Reaction
Molecularity is defined as the number of reacting species (atoms, ions, or molecules) that collide simultaneously in an elementary reaction.
Molecularity is a theoretical concept and is applicable only to elementary reactions.
- Unimolecular reaction: Molecularity = 1
- Bimolecular reaction: Molecularity = 2
- Trimolecular reaction: Molecularity = 3
Molecularity is always a positive whole number and can never be zero or fractional.
Difference Between Order of Reaction and Molecularity
| Order of Reaction | Molecularity |
|---|---|
| Determined experimentally from the rate law. | Theoretical concept based on reaction mechanism. |
| Can be zero, fractional, or whole number. | Always a whole number (1, 2, or rarely 3). |
| Applicable to overall reactions. | Applicable only to elementary reactions. |
| May change with conditions such as temperature. | Fixed for a given elementary step. |
Order of reaction and molecularity are equal only for elementary reactions, but they are not necessarily equal for complex reactions.
Integrated Rate Equations
The rate law gives the relationship between rate of reaction and concentration of reactants. To find how the concentration of reactants changes with time, the rate law is mathematically integrated. The resulting expression is known as the integrated rate equation.
Zero Order Reactions
In a zero order reaction, the rate of reaction is independent of the concentration of reactant.
Rate = k
On integrating the rate law, the concentration of reactant at time t is given by:
where [A]0 is the initial concentration and k is the rate constant.
Half-Life of a Zero Order Reaction
Half-life (t1/2) is the time required for the concentration of reactant to become half of its initial value.
For a zero order reaction, half-life depends on the initial concentration.
First Order Reactions
In a first order reaction, the rate of reaction is directly proportional to the concentration of one reactant.
Rate = k[A]
On integrating the rate law, the relationship between concentration and time is:
Half-Life of a First Order Reaction
The half-life of a first order reaction is independent of the initial concentration.
Radioactive decay and many decomposition reactions follow first order kinetics.
Collision Theory of Chemical Reactions
According to the collision theory, chemical reactions occur as a result of collisions between reacting particles such as atoms, molecules, or ions.
However, not all collisions lead to a chemical reaction. Only those collisions which satisfy certain conditions are effective in producing products.
Conditions for an Effective Collision
- Sufficient Energy: The colliding particles must possess energy equal to or greater than the activation energy.
- Proper Orientation: The molecules must collide with suitable orientation so that bonds can break and new bonds can form.
It is the minimum extra energy required by the reactant molecules to form the activated complex and undergo a chemical reaction.
Energy Profile of a Chemical Reaction
The variation of potential energy of reacting species during the course of a reaction is represented by an energy profile diagram.
Fig: Energy profile diagram showing activation energy.
The height of the energy barrier represents the activation energy. Reactions with lower activation energy occur at a faster rate.
Effect of Catalyst on Reaction Rate
A catalyst increases the rate of a reaction by providing an alternative reaction pathway with lower activation energy.
- Catalyst does not change the overall energy of reactants or products.
- It lowers the activation energy.
- Catalyst is regenerated at the end of the reaction.
Worked Numerical Examples
Numerical problems in chemical kinetics are based on integrated rate equations and Arrhenius equation. The following solved examples strictly follow the NCERT method and presentation style.
Numerical Example 1: Zero Order Reaction
Given:
Initial concentration, [A]0 = 0.50 mol L−1
Rate constant, k = 0.02 mol L−1 min−1
Required:
Time required for the concentration to become 0.30 mol L−1
Formula Used (Zero Order):
Substitution:
Calculation:
t = (0.50 − 0.30) / 0.02 = 10 min
Answer: Time required = 10 minutes
Numerical Example 2: Half-Life of a First Order Reaction
Given:
Rate constant, k = 2.31 × 10−3 s−1
Required:
Half-life of the reaction
Formula Used (First Order):
Substitution:
Calculation:
t1/2 = 300 s
Answer: Half-life = 300 seconds
Numerical Example 3: Arrhenius Equation
Given:
k1 = 1.0 × 10−3 s−1 at T1 = 300 K
k2 = 2.0 × 10−3 s−1 at T2 = 310 K
R = 8.314 J mol−1 K−1
Required:
Activation energy (Ea)
Formula Used (Arrhenius):
Substitution:
Calculation:
Ea ≈ 52 kJ mol−1
Answer: Activation energy ≈ 52 kJ mol−1
Formula Summary Table
The following table summarizes the important formulas used in Chemical Kinetics. These formulas are strictly as per the NCERT syllabus and should be used only after understanding the underlying concepts.
| Concept | Formula | Remarks |
|---|---|---|
| Average rate of reaction | −Δ[R] / Δt | Negative sign for reactants |
| Rate law | Rate = k[A]n | n = order of reaction |
| Zero order integrated equation | [A] = [A]0 − kt | Linear plot of [A] vs t |
| Half-life (zero order) | t1/2 = [A]0 / 2k | Depends on initial concentration |
| First order integrated equation | k = (2.303 / t) log ([A]0 / [A]) | Linear plot of log[A] vs t |
| Half-life (first order) | t1/2 = 0.693 / k | Independent of initial concentration |
| Arrhenius equation | k = A e−Ea/RT | Temperature dependence of rate constant |
Practice Problems
The following questions are designed strictly according to the NCERT examination pattern. They help in reinforcing conceptual clarity and numerical application of chemical kinetics.
(A) Very Short Answer Questions
- Define the rate of a chemical reaction.
- What is meant by activation energy?
- Write the unit of rate constant for a zero order reaction.
- Why is molecularity always a whole number?
- State one example of a first order reaction.
- Rate of reaction is the change in concentration of reactant or product per unit time.
- Activation energy is the minimum energy required for a reaction to occur.
- mol L−1 s−1
- Because it represents the number of molecules involved in an elementary reaction.
- Radioactive decay.
(B) Short Answer Questions
- Explain the effect of temperature on the rate of a chemical reaction.
- Differentiate between order of reaction and molecularity.
- Why does a catalyst increase the rate of reaction?
- What is meant by half-life of a reaction?
1. Increase in temperature increases the kinetic energy of molecules. More molecules acquire energy greater than activation energy, resulting in increased rate of reaction.
2. Order of reaction is an experimental quantity and may be fractional, whereas molecularity is a theoretical concept and always a whole number.
3. A catalyst provides an alternative reaction pathway with lower activation energy, thereby increasing the reaction rate.
4. Half-life is the time required for the concentration of a reactant to become half of its initial value.
(C) Numerical Problems
- A first order reaction has a rate constant of 1.15 × 10−3 s−1. Calculate the half-life of the reaction.
- The rate constant of a reaction doubles when the temperature is increased from 298 K to 308 K. Calculate the activation energy.
1. t1/2 = 0.693 / k = 0.693 / (1.15 × 10−3) = 602 s
2. Activation energy ≈ 52 kJ mol−1
Frequently Asked Questions (Academic)
The following questions address common conceptual difficulties faced by students while studying Chemical Kinetics, strictly based on the NCERT syllabus.
Why does thermodynamics not give information about reaction rate?
Why is the half-life of a first order reaction constant?
Why are reactions faster at higher temperatures?
Can molecularity be zero or fractional?
Related Chapters (Internal Links)
For better conceptual continuity in physical chemistry, students are advised to study the following related chapters:
- Chapter 1: Solutions – Concentration terms used in rate expressions.
- Chapter 2: Electrochemistry – Concept of rate constants in electrochemical reactions.
Conclusion
Chemical kinetics provides a quantitative understanding of how fast chemical reactions occur and the factors that control their rates. Concepts such as rate law, order of reaction, integrated rate equations, activation energy, and collision theory together explain the dynamic nature of chemical processes.
A clear understanding of chemical kinetics is essential not only for board examinations but also for advanced studies in chemistry, chemical engineering, biochemistry, and industrial processes.
All explanations, formulas, numericals, and questions in this chapter strictly follow the NCERT Class 12 Chemistry syllabus and terminology.
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Class 12 Chemistry · Chapter 3 · Chemical Kinetics
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