Last Updated: 11 May 2026
JEE Main 2027 dedicates 3-4 questions to Chemical Kinetics from Physical Chemistry — high-yield, formula-heavy and almost entirely numerical.
Rate of Reaction — Basic Definitions
- Rate = -d[R]/dt = +d[P]/dt.
- For aA + bB → cC + dD: rate = -1/a · d[A]/dt = +1/c · d[C]/dt.
- Average rate vs Instantaneous rate.
- Units: mol L⁻¹ s⁻¹.
Order vs Molecularity
| Parameter | Order | Molecularity |
|---|---|---|
| Definition | Sum of powers in rate law | Number of species in elementary step |
| Determined by | Experiment | Stoichiometry of elementary step |
| Value | Can be fractional, zero, negative | Whole number (1, 2, 3) |
| Example | 2H₂ + 2NO → N₂ + 2H₂O, order = 3 | Same step, molecularity = 4 |
Integrated Rate Laws
Zero Order: R → P
- [R] = [R]₀ – kt
- t½ = [R]₀ / 2k
- Units of k: mol L⁻¹ s⁻¹
First Order: R → P
- [R] = [R]₀ e^(-kt)
- ln([R]₀/[R]) = kt; k = (2.303/t) log([R]₀/[R])
- t½ = 0.693/k (independent of [R]₀)
- Units of k: s⁻¹
Second Order
- 1/[R] – 1/[R]₀ = kt
- t½ = 1/(k[R]₀)
Pseudo First-Order Reactions
When one reactant is in excess (e.g., hydrolysis of ester in water), apparent order = 1.
- Ester hydrolysis: CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH
- Inversion of cane sugar: C₁₂H₂₂O₁₁ + H₂O → glucose + fructose
Arrhenius Equation
k = A · e^(-Ea/RT)
- Logarithmic form: ln k = ln A – Ea/RT.
- Two-temperature form: log(k₂/k₁) = (Ea/2.303R) · (T₂-T₁)/(T₁T₂).
- A = Pre-exponential / frequency factor; Ea = Activation energy.
- Rate doubles when T rises by 10 K (for Ea ≈ 50 kJ/mol).
Collision Theory (Bimolecular Gas-Phase)
k = P · Z · e^(-Ea/RT) where P = steric factor, Z = collision frequency.
Catalysis
- Lowers Ea, does not change ΔH.
- Homogeneous: same phase as reactants.
- Heterogeneous: different phase (e.g., Pt for hydrogenation).
- Enzymes: biological catalysts with high specificity.
JEE Main 2026 Snapshot (3 Qs)
- Q1: Arrhenius — find Ea from k at two temperatures
- Q2: t½ calculation for 1st order radioactive decay
- Q3: Half-life ratio for zero vs first order
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FAQ
Average questions in JEE Main?
3-4 questions; almost all numerical.
Most-asked formula?
Arrhenius two-temperature form — appears every alternate year.
Is unit-conversion important?
Critical — JEE often gives Ea in kJ/mol and expects R = 8.314 J/K/mol.
Practice Quiz
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