Class 12 Chemistry Chapter 3 Chemical Kinetics – Revision Notes

Quick revision notes for Class 12 Chemistry Chapter 3 – Chemical Kinetics, covering rate of reaction, rate laws and order, integrated rate equations, half-life, and the Arrhenius equation. Ideal for last-minute board exam revision.

Rate of Reaction and Rate Law

The rate of a reaction is the change in concentration of a reactant or product per unit time. The rate law expresses rate as a function of reactant concentrations raised to experimentally determined powers, e.g. Rate=k[A]^x[B]^y, where k is the rate constant. The order of reaction is the sum of these powers (x+y) — it can be zero, fractional or a whole number, and must be found experimentally, unlike molecularity (the number of species colliding in an elementary step), which is always a whole number and applies only to a single mechanistic step.

Zero-Order and First-Order Reactions

For a zero-order reaction, rate=k (independent of concentration); integrating gives [A]_t=[A]₀−kt, so concentration falls linearly with time, and k has the same units as rate (e.g. mol L⁻¹s⁻¹). For a first-order reaction, rate=k[A]; integrating gives k=(1/t)ln([A]₀/[A]_t), and k has units of (time)⁻¹. Radioactive decay and many pseudo-first-order reactions (like ester hydrolysis in excess water) follow first-order kinetics.

Half-Life

The half-life (t_1/2) is the time taken for the concentration of a reactant to fall to half its initial value. For a zero-order reaction, t_1/2=[A]₀/2k (depends on initial concentration). For a first-order reaction, t_1/2=0.693/k — notably independent of initial concentration, which is why radioactive decay (always first order) has a fixed, well-defined half-life regardless of sample size.

Pseudo First-Order Reactions

A reaction that is intrinsically of higher order can behave as pseudo first order when one reactant is present in large excess, so its concentration stays effectively constant during the reaction. A classic example is the acid hydrolysis of an ester (CH₃COOC₂H₅+H₂O→CH₃COOH+C₂H₅OH), which is genuinely second order but, since water is the solvent and present in vast excess, follows first-order kinetics with respect to the ester alone.

Temperature Dependence: The Arrhenius Equation

The rate constant of most reactions increases with temperature, quantified by the Arrhenius equation: k=Ae^−E_a/RT, where A is the pre-exponential (frequency) factor, E_a is the activation energy (the minimum extra energy reactant molecules need to react successfully), R is the gas constant, and T is the absolute temperature. Taking logarithms gives the linear form ln k=ln A−E_a/RT, so plotting ln k against 1/T gives a straight line of slope −E_a/R, letting E_a be found graphically from rate-constant data at different temperatures.

One-Line Summary

Chapter 3 explains how reaction rate is captured by an experimentally determined rate law and order, how zero- and first-order reactions integrate to give concentration-time relationships and a characteristic half-life, and how the Arrhenius equation quantifies why raising the temperature speeds up a reaction by increasing the fraction of molecules with enough energy to cross the activation-energy barrier.