Difficulty: Intermediate | Prerequisites: Mole concept, basic algebra, logarithms.
Chemical kinetics is about how fast reactions happen and what controls that speed. It sits alongside thermodynamics (which tells you whether a reaction can happen) and equilibrium (which tells you where it settles). Kinetics tells you the route and the clock. CBSE tests this chapter heavily: expect MCQs on molecularity, assertion-reason on half-life, and numerical problems on zero- and first-order rate laws in Sections B and C.
TL;DR
The rate law links reaction rate to reactant concentration raised to a power (the order). For first-order reactions the half-life is constant regardless of concentration. Molecularity counts how many molecules collide in one elementary step and can never be zero or fractional.
Rate law (rate equation)
An expression that relates the rate of a reaction to the concentration of reactants, each raised to a power: Rate = k[A]^x[B]^y. The powers x and y are determined experimentally.
Order of reaction
The sum of the powers to which reactant concentrations are raised in the rate law (x + y). It can be zero, fractional, or integer, and is determined experimentally, not from the balanced equation.
Molecularity
The number of reacting species (atoms, ions, molecules) that collide simultaneously in an elementary reaction step. Always a positive integer (1, 2, or 3). It cannot be zero, fractional, or greater than 3. In simple terms, molecularity asks: how many particles crash together in one step?
Half-life (t1/2)
The time required for the concentration of a reactant to fall to half its initial value.
Zero-order reaction
A reaction whose rate is independent of reactant concentration: Rate = k. Concentration decreases linearly with time.
First-order reaction
A reaction whose rate is directly proportional to the concentration of one reactant: Rate = k[A]. Its half-life (t1/2 = 0.693/k) is constant, independent of initial concentration.
Second-order reaction
A reaction whose rate depends on the square of one reactant's concentration (Rate = k[A]^2) or on the product of two concentrations. Doubling the concentration quadruples the rate.
Rate constant (k)
The proportionality constant in the rate law. Its units depend on the overall order of the reaction.
Integrated rate law
The rearranged form of the rate law that relates concentration to time directly, used for calculating how long a reaction takes or what concentration remains at a given time.
Activation energy (Ea)
The minimum energy that colliding molecules must possess for a reaction to occur. A higher Ea means a slower reaction at a given temperature.
Molecularity applies only to elementary (single-step) reactions. Order applies to overall reactions and is found experimentally.
Molecularity is always a positive integer: unimolecular (1), bimolecular (2), trimolecular (3). It cannot be zero.
Order can be zero, fractional (e.g. 1.5), or negative. It is the sum of the exponents in the experimentally determined rate law.
The statement "molecularity of a reaction may be zero" is false, and this is a standard CBSE MCQ distractor.
Zero-order: Rate = k. Changing concentration has no effect on rate.
First-order: Rate = k[A]. Doubling [A] doubles the rate.
Second-order: Rate = k[A]^2. Doubling [A] quadruples the rate (2^2 = 4). Halving [A] reduces rate to one-quarter (1/2)^2 = 1/4.
Zero-order: t1/2 = [A]0 / 2k. Half-life depends on initial concentration and is directly proportional to it.
First-order: t1/2 = 0.693 / k. Half-life is constant, independent of [A]0. This is a defining feature tested in assertion-reason questions.
The assertion "if concentration is doubled, half-life is also doubled" is false for first-order reactions. CBSE tests this directly.
For a zero-order reaction A --> P, the integrated rate law is:
[A] = [A]0 - kt
Worked problem from the 2025 paper: k = 0.0030 mol/L/s, [A]0 = 0.10 M, [A] = 0.075 M. Find t.
Substitute: 0.075 = 0.10 - (0.0030)(t)
Rearrange: 0.0030t = 0.10 - 0.075 = 0.025
Solve: t = 0.025 / 0.0030 = 8.33 seconds
The concentration drops linearly, so this is a straightforward subtraction problem.
For the decomposition of NH3 on a platinum surface (zero-order), the rate of product formation equals the rate constant k. If 2NH3 --> N2 + 3H2 and k = 2.5 x 10^-4 mol/L/s, the rates of production of N2 and H2 are both equal to k (since the rate of reaction = k for zero order, and stoichiometric ratios apply to express individual rates).
Quantity | Zero-Order | First-Order | Second-Order |
|---|---|---|---|
Rate law | Rate = k | Rate = k[A] | Rate = k[A]^2 |
Integrated rate law | [A] = [A]0 - kt | ln[A] = ln[A]0 - kt | 1/[A] = 1/[A]0 + kt |
Half-life | [A]0 / 2k | 0.693 / k | 1 / k[A]0 |
Units of k | mol L^-1 s^-1 | s^-1 | L mol^-1 s^-1 |
Half-life depends on [A]0? | Yes (proportional) | No (constant) | Yes (inversely proportional) |
Students often confuse molecularity with order. Molecularity is a theoretical count for a single elementary step; order is experimentally determined for the overall reaction.
Students assume that doubling concentration always doubles the rate. It does not: the effect depends on the order. For second-order, it quadruples; for zero-order, there is no change.
The statement "half-life of a zero-order reaction is inversely proportional to the rate constant" is true (t1/2 = [A]0 / 2k). Students sometimes mark this as false, confusing it with first-order.
⚠️ Assertion-reason on first-order half-life: "If concentration doubles, half-life doubles" (false) paired with "Half-life does not depend on concentration" (true). Know the correct combination: A is false, R is true.
⚠️ "Molecularity may be zero" is the standard false statement in "which is false" MCQs.
⚠️ Numerical on zero-order integrated rate law: plug into [A] = [A]0 - kt. Keep units consistent.
⚠️ Second-order rate effects: doubling concentration gives 4x rate, halving gives 1/4 rate. This is a standard 2-mark calculation.
True or false: For a first-order reaction, doubling the initial concentration doubles the half-life. ______
The molecularity of a reaction can never be ______ or ______.
For a second-order reaction, if [A] is halved, the rate becomes ______ of the original.
The units of the rate constant for a zero-order reaction are ______.
True or false: The rate of a zero-order reaction depends on the concentration of the reactant. ______
Q: A reaction is second order with respect to a reactant. How is the rate affected if the concentration is (i) doubled, (ii) reduced to half?
A: Rate = k[A]^2. (i) New rate = k(2[A])^2 = 4k[A]^2, so the rate quadruples. (ii) New rate = k([A]/2)^2 = k[A]^2/4, so the rate becomes one-quarter.
Q: Which among the following is a false statement? (1) Rate of zero-order reaction is independent of concentration. (2) Half-life of zero-order reaction is inversely proportional to k. (3) Molecularity may be zero. (4) For first-order, t1/2 = 0.693/k.
A: (3). Molecularity is the number of reacting species in an elementary step and must be a positive integer. It cannot be zero.
Q: The rate constant for a zero-order reaction A --> P is 0.0030 mol/L/s. How long for [A] to fall from 0.10 M to 0.075 M?
A: Using [A] = [A]0 - kt: t = (0.10 - 0.075) / 0.0030 = 0.025 / 0.0030 = 8.33 seconds.
Chemical kinetics connects to Electrochemistry (Ch. 4) through the temperature dependence of reaction rates and the Arrhenius equation. The concept of activation energy reappears when discussing overpotential in electrolytic cells. It also connects to Surface Chemistry (Ch. 5): catalysts work by providing an alternative pathway with lower activation energy, which is a kinetics concept.
Tags: chemical kinetics, rate law, order of reaction, molecularity, half-life, zero order, first order, second order, rate constant, integrated rate law, activation energy, Arrhenius equation, CBSE Class 12 Chemistry, Chapter 3