JEE Main Chemistry Chemical Kinetics 2027: Rate Laws, Half-Life & Arrhenius Equation
Rate law, order versus molecularity, first-order kinetics, half-life and the Arrhenius equation, taught with worked examples and clear formulas.
Edurack
September 28, 2026
Thermodynamics tells you whether a reaction can happen. Kinetics tells you how fast it will. This chapter is one of the most formula-friendly in Physical Chemistry, and first-order kinetics alone can win you a question or two in almost every paper.
A catalyst lowers the activation energy. It does not change ΔH or the equilibrium constant.
Chapter at a Glance
| Snapshot | Detail |
|---|---|
| NTA unit | Unit 8 of 20: Chemical Kinetics |
| Priority (trend-based) | Moderate |
| Typical question style | First-order numericals, order from data and Arrhenius calculations |
| Best first step | Learn first-order equations and half-lives, then Arrhenius |
Priority reflects past-paper trends, not an official NTA weightage.
What the NTA Syllabus Covers
- Rate of reaction and factors affecting it: concentration, temperature, pressure and catalyst
- Elementary and complex reactions, order and molecularity, rate law, rate constant and its units
- Differential and integrated rate equations for zero and first order reactions, half-lives
- Effect of temperature, Arrhenius theory, activation energy, collision theory of bimolecular gaseous reactions (no derivation)
Master These Topics
1. Rate Law, Order and Molecularity
For aA + bB → products, the rate law is rate = k [A]^x [B]^y, where x and y are found by experiment. Order is x + y, and it can be zero, fractional or negative. Molecularity is the number of molecules colliding in an elementary step, so it is always a positive whole number (1, 2, occasionally 3).
Units of k for an nth-order reaction: (mol/L)^(1−n) s⁻¹. First order gives s⁻¹, second order gives L mol⁻¹ s⁻¹.
Finding order from data: if doubling [A] doubles the rate, the order in A is 1. If it quadruples the rate, the order is 2. If the rate is unchanged, the order is 0.
Trap: Order comes from experiment and need not match the stoichiometric coefficients. Molecularity applies only to elementary steps and never to a complex reaction as a whole.
2. First-Order and Zero-Order Kinetics
| Zero order | First order | |
|---|---|---|
| Integrated law | [A] = [A]₀ − kt | k = (2.303/t) log([A]₀/[A]) |
| Half-life | [A]₀ / 2k | 0.693 / k |
| Half-life depends on initial concentration? | Yes | No |
Worked example: For a first-order reaction with k = 0.0693 min⁻¹, t½ = 0.693 / 0.0693 = 10 min. After 30 min (three half-lives), the fraction remaining is (½)³ = 1/8, or 12.5%.
3. The Arrhenius Equation
k = A e^(−Ea/RT), or in logarithmic form log(k₂/k₁) = (Ea / 2.303R)(1/T₁ − 1/T₂).
Worked example: If the rate doubles between 300 K and 310 K, then Ea = 2.303 × 8.314 × log 2 × (300 × 310 / 10) ≈ 53.6 kJ/mol. This matches the everyday rule that many reactions double in rate per 10 K rise.
A catalyst provides an alternative pathway with lower activation energy. It speeds up both directions equally and does not change ΔH or K.
4. Collision Theory
Only collisions with sufficient energy (at least Ea) and proper orientation lead to reaction. The rate is proportional to the collision frequency Z times a steric factor times e^(−Ea/RT). Raising temperature increases the fraction of molecules with enough energy, which is why rate rises steeply.
Common Traps to Avoid
- Writing the rate law directly from the balanced equation for a complex reaction.
- Assuming half-life is independent of concentration for all orders. That holds only for first order.
- Treating molecularity as fractional or zero.
- Thinking a catalyst changes the equilibrium constant or reaction enthalpy.
60-Second Revision Sheet
rate = k [A]^x [B]^y; units of k(mol/L)^(1−n) s⁻¹- First order
k = (2.303/t) log([A]₀/[A]),t½ = 0.693/k - Zero order
t½ = [A]₀/2k - Arrhenius
k = A e^(−Ea/RT)
Your Study Plan
- Day 1: rate law, order, molecularity and units of k.
- Day 2: order from experimental data tables.
- Day 3: first-order and zero-order integrated equations and half-lives.
- Day 4: Arrhenius equation, collision theory and a timed set.
Practice Chemical Kinetics Questions Free → (opens in a new tab)
Continue Your Chemistry Journey
- Previous chapter: Redox Reactions & Electrochemistry
- Next chapter: Classification of Elements & Periodicity
- All JEE Main Chemistry chapters
- Complete JEE Main Syllabus 2027 guide
Frequently Asked Questions
What is the difference between order and molecularity?
Order is an experimental quantity from the rate law and can be zero or fractional. Molecularity is theoretical, applies to elementary steps and is always a whole number.
Which reaction order has a constant half-life?
First order, because t½ = 0.693/k does not depend on the initial concentration.