Class 12 Chemistry Chapter 4 Chemical Kinetics Notes

Chemical Kinetics is a crucial chapter in Class 12 Chemistry syllabus that deals with the rate of chemical reactions, the factors affecting them, and the mechanisms by which reactions occur. This chapter is numerical-heavy and conceptual, forming the backbone of both board exam questions and competitive exams like JEE and NEET.

These notes are curated to simplify key concepts like rate laws, order of reaction, activation energy, Arrhenius equation, and integrated rate equations for different orders. Download and revise these notes to master the core formulas, derivations, and tricks.

Chemical Kinetics Class 12 Notes Material PDF Download

This study material for Class 12 explains the basics of Chemical Kinetics in an easy-to-understand way. Download the PDF to learn key concepts and prepare well for your exams.

Below we have provided the links to downloadable PDFs of class 12 chemistry Ch 4 notes and get an in-depth explanation and understanding of the chapter.

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1. Rate of Reaction

The change in concentration of reactants or products per unit time.

• Rate = Δ [Concentration] / ΔTime
• Average Rate: Over a time interval.
• Instantaneous Rate: At a particular moment.
• Units of rate: mol L⁻¹ s⁻¹

2. Factors Affecting Reaction Rate

• Concentration: Higher concentration → faster rate.
• Temperature: Increases kinetic energy, leading to faster reactions.
• Presence of Catalyst: Lowers activation energy.
• Surface Area: Larger area (in solids) increases reaction rate.
• Nature of Reactants: Ionic reactions are faster than covalent ones.

3. Rate Law and Order of Reaction

Rate Law:
Rate = k[A]^m[B]^n
Where:

• k = rate constant
• m, n = order with respect to A and B

Order of Reaction: Sum of powers of concentration terms in rate law (m+n).

• Determined experimentally, not from balanced chemical equation.
• Can be 0, 1, 2, or even fractional.

4. Molecularity of Reaction

• The number of molecules or species that collide simultaneously in an elementary step.
• Always a whole number.
• Molecularity ≠ Order (except in elementary reactions).

5. Integrated Rate Law

a) For Zero Order Reactions:

Rate = k
[A] = [A]₀ – kt
Graph: [A] vs t is a straight line.
Unit of k: mol L⁻¹ s⁻¹

b) For First Order Reactions:

Rate = k [A]
ln[A] = ln [A]₀ – kt
Or
k = (2.303 / t) × log ( [A]₀ / [A] )
Half-life (t½) = 0.693/k
Unit of k: s⁻¹

c) For Second Order Reactions:

Rate = k[A]²1 / [A] = 1 / [A]₀ + ktHalf-life (t½) = 1 / ( k [A]₀ )Unit of k: L mol⁻¹ s⁻¹

6. Half-Life Period (t½)

• Time required for half of the reactant to be consumed.
• For first-order: Independent of initial concentration.
• For other orders: Depends on [A]₀.

7. Arrhenius Equation

Shows the effect of temperature on rate constant: k = Ae^(-Ea/RT)

Where:

• k = rate constant
• A = frequency factor
• Ea = activation energy (J/mol or kJ/mol)
• R = universal gas constant
• T = temperature in Kelvin

Taking logarithm:
log k = log A – (Ea / 2.303RT)

Arrhenius plot: log k vs 1/T gives a straight line with slope = –Ea / 2.303R

8. Collision Theory

• Reactant molecules must collide with proper orientation and enough energy.
• Not all collisions are successful → only effective collisions lead to product formation.
• Catalyst increases frequency of effective collisions.

Important Questions from Chemical Kinetics Class 12

1. Very Short Answer Questions (1 Mark Each)

Q: Define rate of a reaction.
Ans: Rate of a reaction is the change in concentration of a reactant or product per unit time.

Q: Write the unit of rate constant for a first-order reaction.
Ans: The unit of rate constant for a first-order reaction is s⁻¹.

Q: What is the half-life formula for a zero-order reaction?
Ans: Half-life for a zero-order reaction: t ½ = [ R₀ ] / 2k

2. Short Answer Questions (2–3 Marks Each)

Q: Differentiate between order and molecularity.
Ans: Order is the sum of powers of concentration terms in the rate law; molecularity is the number of reacting species involved in an elementary step.

Q: Derive the expression for rate constant of a first-order reaction.
Ans: For A → Product

• d[A] / dt = -k [A]
• Integrating:
• In [A] = [A]₀ - kt or K = 1/t ln ( [A₀] / [A] )

Q: Define activation energy and its significance.
Ans: Activation Energy (Ea) is the minimum energy required for reactants to convert into products. It determines the reaction speed—higher Ea means slower reaction.

3. Long Answer Questions (4–5 Marks Each)

Q: Explain the derivation of integrated rate law for a zero-order and first-order reaction.
Ans: 

Q: What is Arrhenius equation? Show how Ea can be calculated from a plot.
Ans: The Arrhenius equation connects the rate constant of a chemical reaction to temperature and activation energy. It is a mathematical relationship that describes how the rate constant (k) varies with temperature and activation energy (Ea). 

The Arrhenius equation is: k = A * e^(-Ea/RT)

• k = rate constant
• A = pre-exponential factor or frequency factor
• Ea = activation energy
• R = universal gas constant (8.314 J/mol·K)
• T = absolute temperature (Kelvin) 

Activation energy (Ea) can be determined from an Arrhenius plot by calculating the slope of the line when plotting ln(k) against 1/T.

• Taking log
• In k = A - Ea/Rt
• Plotting in k vs 1/t, slope = -Ea/R
• Thus, Ea= - slope * R

Q: Describe the factors influencing rate of chemical reactions with examples.

Ans: Factors Influencing Rate of Reaction:

• Concentration: Higher concentration increases rate (e.g., acid + metal).
• Temperature: Higher temperature increases kinetic energy (e.g., cooking faster at high temp).
• Catalyst: Lowers activation energy (e.g., MnO₂ in H₂O₂ decomposition).
• Surface Area: More area increases rate (e.g., powdered zinc reacts faster than a lump).
• Nature of Reactants: Ionic reactions are faster than covalent ones.

Common Mistakes to Avoid

🚫 Confusing order of reaction with molecularity.

🚫 Using wrong units for rate constant (k) for different orders.

🚫 Ignoring log vs ln in integrated rate laws.

🚫 Forgetting to convert Celsius to Kelvin in Arrhenius calculations.

🚫 Mixing up rate law with stoichiometric coefficients in balanced reactions.

Creative Ways to Make Notes for Chemical Kinetics Chapter

1. Graph Summaries: Include sketches of [A] vs t, ln[A] vs t for different orders.
2. Order-Specific Formula Charts: One table with formulas, units of k, t½ expressions for 0, 1st, 2nd order reactions.
3. Concept-Flow Diagrams: Visual links between rate, rate constant, and reaction mechanisms.
4. Derivation Boxes: Side notes with step-by-step derivations for rate laws.
5. Activation Energy Shortcut Table: Highlight Ea calculation tricks and log forms.

How Can Notes Help?

• Simplify complex derivations and graphs into quick reference pages.
• Make numerical practice easier with direct formula application.
• Provide clarity between conceptually confusing terms like order and molecularity.
• Aid in visual learning through reaction plots and charts.
• Perfect for last-minute revision before board or JEE/NEET Chemistry exams.

Chemical Kinetics is all about understanding the "how fast" and "why fast" of chemical reactions. With these well-structured Class 12 notes, you’ll grasp not only the formulas but also their real-world applications. Whether you're a visual learner or a number cruncher, having organized notes helps consolidate your learning effectively and confidently tackle both board and entrance exam questions.

Frequently Asked Questions

1. What is Chemical Kinetics?

Answer: Chemical kinetics is the branch of chemistry that deals with the study of the speed or rate of chemical reactions and the factors that influence it. It involves understanding how different conditions like temperature, concentration, and catalysts affect reaction rates.

2. What is the Rate of a Chemical Reaction?

Answer: The rate of a chemical reaction refers to the change in concentration of a reactant or product per unit time. In simple terms, it measures how fast or slow a reaction occurs.

3. What Factors Affect the Rate of a Chemical Reaction?

Answer: Several factors can influence the rate of a reaction, including:

• Concentration of reactants: Higher concentration typically increases the rate.
• Temperature: Higher temperatures generally increase the reaction rate.
• Presence of a catalyst: Catalysts speed up reactions without being consumed.
• Surface area of reactants: More surface area leads to faster reactions.

4. What is Activation Energy?

Answer: Activation energy is the minimum energy required to start a chemical reaction. It is the energy needed to break bonds in the reactants so that new bonds can form in the products.

5. What is a Catalyst?

Answer: A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. It works by lowering the activation energy, making the reaction proceed faster, but it is not used up in the reaction.

6. What is the Difference Between First-Order and Second-Order Reactions?

Answer: The order of a reaction refers to the relationship between the concentration of reactants and the rate of the reaction:

• First-order reaction: The rate depends on the concentration of one reactant raised to the first power.
• Second-order reaction: The rate depends on the concentration of one reactant raised to the second power, or the product of concentrations of two reactants.

7. What is Half-Life in Chemical Kinetics?

Answer: Half-life is the time required for half of the reactant in a reaction to be consumed or to decrease to half of its initial concentration. It is an important concept in reactions that follow first-order kinetics.

8. What is a Rate-Determining Step?

Answer: The rate-determining step is the slowest step in a multi-step chemical reaction. This step controls the overall rate of the reaction because it takes the longest time.

9. What is the Arrhenius Equation?

Answer: The Arrhenius equation is a formula used to express the dependence of reaction rate on temperature. While it involves some specific terms, the basic idea is that the reaction rate increases with temperature due to higher energy, making the reactants more likely to overcome the activation energy.

10. What is the Collision Theory?

Answer: The collision theory suggests that for a reaction to occur, the reactant molecules must collide with each other with sufficient energy and proper orientation. More frequent and energetic collisions increase the chances of a successful reaction.