Chemical Kinetics Class 12 Notes PDF | Rate Laws, Graphs & Numericals Guide

Mastering reaction speeds and mechanisms is fundamental to physical chemistry. This chemical kinetics class 12 notes pdf guide provides a deep dive into how chemical reactions occur and the factors that control their speed. For NEET aspirants, this chapter is high-yield, offering scoring opportunities through both conceptual questions on order and molecularity and numerical problems on half-life and the Arrhenius equation.

01
Introduction to Chemical Kinetics

Chemical kinetics is the branch of chemistry that deals with the study of reaction rates and the mechanisms by which they occur. While thermodynamics tells us if a reaction is feasible (ΔG < 0), kinetics tells us how fast that reaction will actually proceed. Understanding this distinction is vital for any comprehensive chemical kinetics class 12 notes pdf study resource.

SCOPE Measurement of reaction rates under varying conditions such as concentration, temperature, and pressure.

MECHANISM The stepwise path (elementary steps) that leads from reactants to the final products.

02
Rate of Reaction

The rate of a chemical reaction is defined as the change in the concentration of any one of the reactants or products per unit time. It can be expressed as the rate of disappearance of reactants or the rate of appearance of products.

AVERAGE RATE FORMULA

Rate = −Δ[R] / Δt = +Δ[P] / Δt

Type of Rate	Definition	Expression
Average Rate	Rate over a measurable time interval.	Δ[C] / Δt
Instantaneous Rate	Rate at a specific instant of time.	d[C] / dt
Unit	Standard SI unit of reaction rate.	mol L⁻¹ s⁻¹

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03
Factors Affecting Rate of Reaction

Several variables influence the speed of a reaction. In your chemical kinetics class 12 notes pdf, make sure to categorize these as intensive and extensive factors.

CONCENTRATION Rate generally increases with concentration as the number of effective collisions per unit volume increases.

TEMPERATURE For most reactions, a 10°C rise in temperature nearly doubles the rate constant.

CATALYST Provides an alternative pathway with lower activation energy (E_a), thus increasing the rate.

SURFACE AREA In heterogeneous reactions, increasing surface area (by grinding solids) increases the rate.

04
Rate Law and Order of Reaction

The rate law is the mathematical expression that relates the rate of reaction to the molar concentration of reactants. Unlike stoichiometric coefficients, the exponents in a rate law must be determined experimentally.

Rate = k [A]^m [B]ⁿ | Overall Order = m + n

WARN

Order can be zero, fractional, or even negative. Molecularity, however, is always a whole number and can never be zero or fractional.

Order vs Molecularity

Property	Order of Reaction	Molecularity
Basis	Experimental value.	Theoretical value.
Applicability	Elementary and complex reactions.	Only elementary reactions.
Values	Can be zero or fractional.	Whole numbers only (1, 2, 3).

05
Integrated Rate Equations

Integrating the differential rate laws allows us to calculate the concentration of reactants at any time. This is a core focus of the chemical kinetics class 12 notes pdf guide.

1. Zero Order Reaction

ZERO ORDER FORMULA

[A] = [A]₀ − kt

2. First Order Reaction

k = (2.303 / t) log([A]₀ / [A])

TIP

The plot of log[A] vs time for a first-order reaction is a straight line with a slope equal to −k / 2.303.

06
Half-Life of Reactions (t_1/2)

Half-life is the time required for the concentration of a reactant to decrease to half of its initial value. This is one of the most frequently tested numerical topics in the chemical kinetics class 12 notes pdf.

Zero Order: t_1/2 = [A]₀ / 2k (Depends on initial concentration).
First Order: t_1/2 = 0.693 / k (Independent of initial concentration).

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07
Temperature Dependence: Arrhenius Equation

The rate constant (k) is highly sensitive to temperature changes. This relationship is quantified by the Arrhenius Equation. Mastery of this formula is essential for solving advanced chemical kinetics class 12 notes pdf problems.

ARRHENIUS FORMULA

k = A e^−E_a/RT

Logarithmic form for calculations at two different temperatures:

log(k₂ / k₁) = (E_a / 2.303R) [(T₂ − T₁) / (T₁T₂)]

08
Collision Theory and Catalysis

According to Collision Theory, a chemical reaction occurs only when reactant molecules undergo “effective collisions.” This requires two conditions: Activation Energy Threshold and Proper Orientation.

HOMOGENEOUS CATALYSIS Reactants and catalyst are in the same phase (e.g., Lead chamber process).

HETEROGENEOUS CATALYSIS Reactants and catalyst are in different phases (e.g., Haber’s process).

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Quick Revision Summary

Reaction Rate = ± (1/n) d[C]/dt. Unit: mol L⁻¹ s⁻¹.
Rate Law is determined experimentally, not stoichiometrically.
Order can be zero, fraction, or whole number.
First order t_1/2 = 0.693/k. Note it is concentration-independent.
Arrhenius Equation: k = A exp(−E_a/RT).
Catalysts lower E_a without affecting ΔG or equilibrium constant.
Threshold Energy = Activation Energy + Average Kinetic Energy.
Radioactive decay follows First Order kinetics.
Integrated Rate Equation for nth order (n≠1): k = [1/t(n−1)] [1/Aⁿ⁻¹ − 1/A₀ⁿ⁻¹].
Collision frequency (Z) increases with temperature.

Download Chemical Kinetics Formula Sheet

09
Frequently Asked Questions

What is the physical significance of Activation Energy?

Activation energy (E_a) is the minimum extra energy that reactant molecules must absorb to reach the “activated complex” state and form products. A higher E_a means a slower reaction at a given temperature.

Why are third-order reactions rare?

For a reaction to occur, multiple molecules must collide simultaneously with sufficient energy and proper orientation. The probability of three molecules colliding at the exact same point and time is extremely low, making high-order reactions very rare.

Does temperature affect activation energy?

Generally, no. E_a is a characteristic of the reaction pathway. Temperature increases the kinetic energy of molecules, meaning more molecules possess energy greater than E_a, but it doesn’t change the E_a value itself unless a catalyst is used.

What is a pseudo-first-order reaction?

It is a reaction that is actually higher-order (e.g., second-order) but behaves like a first-order reaction because one of the reactants is present in large excess, making its concentration virtually constant. Example: Hydrolysis of ester.

How does a catalyst increase reaction rate?

A catalyst provides an alternative reaction mechanism or “shortcut” with a lower activation energy barrier. This allows a much larger fraction of molecules to have enough energy to cross the barrier and form products.

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Table of Contents — Chemistry Class 12

Chemistry — Class 12

01	Solutions	Go to page
02	Electrochemistry	Go to page
03	Chemical Kinetics	Go to page
04	d- and f-Block Elements	Go to page
05	Coordination Compounds	Go to page
06	Haloalkanes and Haloarenes	Go to page
07	Alcohols, Phenols and Ethers	Go to page
08	Aldehydes, Ketones and Carboxylic Acids	Go to page
09	Amines	Go to page
10	Biomolecules	Go to page

Chemical Kinetics Class 12 Notes PDF: Complete NEET Revision & Formula Sheet

01
Introduction to Chemical Kinetics

02
Rate of Reaction

03
Factors Affecting Rate of Reaction