Class 12 Chemistry Chapter 4 Notes: Chemical Kinetics | Important Questions & PYQs

 Class 12 Chemistry – Chapter 4: Chemical Kinetics (NCERT Notes)

1. Chemical Kinetics

Chemical kinetics deals with the rate of chemical reactions, the factors affecting the rate, and the mechanism of reactions.

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2. Rate of a Chemical Reaction

Definition

Rate of reaction = change in concentration per unit time.

$$\text{Rate}=-\frac{\mathrm{\Delta }[\text{Reactant}]}{\mathrm{\Delta }t}=\frac{\mathrm{\Delta }[\text{Product}]}{\mathrm{\Delta }t}$$

• Unit: mol L⁻¹ s⁻¹

• Rate decreases with time as reactants are consumed

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3. Factors Affecting Rate of Reaction

1. Concentration of reactants
   Higher concentration → higher rate

2. Temperature
   Rate approximately doubles for every 10°C rise

3. Catalyst
   Increases rate without being consumed

4. Nature of reactants
   Ionic reactions are faster than covalent

5. Surface area
   Powdered solids react faster than lumps

6. Radiation (photochemical reactions)
   Light provides activation energy

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4. Rate Law & Order of Reaction

Rate Law

$$\text{Rate}=k[A{]}^m[B{]}^n$$

• k = rate constant

• m + n = order of reaction

Order of Reaction

• Can be zero, fractional, or whole number

• Determined experimentally

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5. Integrated Rate Equations

Zero Order Reaction

$$[A]=[A{]}_0-kt$$

• Rate independent of concentration

• Straight line graph of [A] vs t

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First Order Reaction

$$\ln \frac{[A{]}_0}{[A]}=kt$$

Half-life

$$t_{1\mathrm{/}2}=\frac{0.693}{k}$$

• Independent of initial concentration

• Most radioactive reactions follow first order

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Second Order Reaction

$$\frac{1}{[A]}=kt+\frac{1}{[A{]}_0}$$

• Rate ∝ square of concentration

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6. Pseudo First Order Reaction

• Reaction appears first order because one reactant is in large excess

• Example: Acidic hydrolysis of ester in water

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7. Temperature Dependence of Rate

Arrhenius Equation

$$k=A{e}^{-E_a\mathrm{/}RT}$$

Where:

• A = frequency factor

• Eₐ = activation energy

• R = gas constant

• T = temperature (K)

Key Points

• Higher Eₐ → slower reaction

• Plot of ln k vs 1/T is a straight line

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8. Activation Energy (Eₐ)

• Minimum energy required for effective collisions

• Catalyst lowers Eₐ but does not change ΔH

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9. Collision Theory

For a reaction to occur:

1. Molecules must collide

2. Collision must have proper orientation

3. Energy ≥ activation energy

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10. Half-life (t₁/₂)

Order	Half-life
Zero order	Depends on initial concentration
First order	Independent of concentration
Second order	Depends on concentration

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11. Important Exam Formulas

• Rate = −Δ[A]/Δt

• Rate law: r = k[A]^n

• First order: $t_{1\mathrm{/}2}=0.693\mathrm{/}k$

• Arrhenius: $k=A{e}^{-E_a\mathrm{/}RT}$

• ln k vs 1/T → straight line

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12. Typical Exam Conclusions

✔ If concentration doubled in 2nd order, rate becomes 4 times
✔ First order → most common in nature
✔ Temperature affects k, not order
✔ Catalyst lowers activation energy

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13. Where This Chapter Is Asked

• 📌 Board numericals (very high weightage)

• 📌 Assertion–Reason

• 📌 Graph-based questions

• 📌 Competitive exam basics

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What is chemical kinetics?
   It is the branch of chemistry that deals with rate of reactions and factors affecting it.

2. Define rate of reaction.
   Rate of reaction is the change in concentration per unit time.

3. Write the unit of rate of reaction.
   mol L⁻¹ s⁻¹

4. Why does rate of reaction decrease with time?
   Because concentration of reactants decreases as reaction proceeds.

5. What is rate law?
   It is the mathematical expression relating rate with concentration of reactants.

6. What is order of reaction?
   Sum of powers of concentration terms in the rate law.

7. What is half-life of a reaction?
   Time required for concentration of reactant to become half of its initial value.

8. Which reactions follow first order kinetics?
   Most radioactive reactions.

9. What is activation energy?
   Minimum energy required for effective collision between reactant molecules.

10. What is the effect of catalyst on reaction rate?
    It increases rate by lowering activation energy.

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📝 Long Answer Questions

1. Explain factors affecting the rate of a chemical reaction.

The rate of a chemical reaction depends on the following factors:

• Concentration: Higher concentration increases rate of reaction

• Temperature: Rate roughly doubles for every 10°C rise

• Catalyst: Increases rate without being consumed

• Nature of reactants: Ionic reactions are faster than covalent

• Surface area: Powdered solids react faster

• Radiation: Light supplies activation energy in photochemical reactions

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2. Explain zero order and first order reactions with equations.

Zero Order Reaction

• Rate is independent of concentration

• Integrated rate law:
  $[A]=[A{]}_0-kt$

• Graph of [A] vs t is a straight line

• Half-life depends on initial concentration

First Order Reaction

• Rate depends on concentration

• Integrated rate law:
  $\ln \frac{[A{]}_0}{[A]}=kt$

• Half-life:
  $t_{1\mathrm{/}2}=\frac{0.693}{k}$

• Independent of initial concentration

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3. Explain Arrhenius equation and activation energy.

Arrhenius equation:

$$k=A{e}^{-E_a\mathrm{/}RT}$$

Where:

• k = rate constant

• A = frequency factor

• Eₐ = activation energy

• R = gas constant

• T = temperature

Activation energy:

• Minimum energy required for reaction to occur

• Higher Eₐ → slower reaction

• Catalyst lowers Eₐ but does not change ΔH

• Plot of ln k vs 1/T is a straight line

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❓ FAQs (Concept Clarity)

1. Why is order of reaction determined experimentally?
   Because it cannot be predicted from stoichiometry.

2. Why does temperature affect rate constant but not order?
   Because temperature changes kinetic energy, not reaction mechanism.

3. Why is half-life constant for first order reactions?
   Because it is independent of initial concentration.

4. Why are powdered solids more reactive?
   Due to larger surface area.

5. Why catalyst does not change equilibrium position?
   Because it speeds up both forward and reverse reactions equally.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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