CBSE Class 12 Chemistry: Chemical Kinetics — Rate Laws Notes 2026

Rate of Reaction

• Rate = −(1/stoich) × Δ[reactant]/Δt = (1/stoich) × Δ[product]/Δt
• Average rate over time interval; instantaneous rate = slope of concentration-time graph
• Factors: concentration, temperature, catalyst, surface area, pressure (gases)

Rate Law and Order of Reaction

• Rate = k[A]ᵐ[B]ⁿ; m and n are orders (experimentally determined, not from equation coefficients)
• Overall order = m + n; units of k depend on order
• Zero order: rate = k (independent of concentration); first order: rate = k[A]

Integrated Rate Laws

• Zero order: [A] = [A]₀ − kt; t½ = [A]₀/2k
• First order: ln[A] = ln[A]₀ − kt; t½ = 0.693/k (independent of initial concentration)
• Plot of ln[A] vs t: straight line for first order; verify order by linearity

Temperature Dependence — Arrhenius Equation

• k = Ae^(−Ea/RT); A = pre-exponential factor (frequency of collisions with proper orientation)
• ln k = ln A − Ea/RT; plot of ln k vs 1/T gives slope = −Ea/R
• Rule of thumb: rate doubles for every 10°C rise (Q₁₀ rule)

Collision Theory and Transition State Theory

• Collision theory: reaction rate = collision frequency × fraction with Ea × steric factor
• Transition state: activated complex at top of energy barrier; intermediate between reactants and products
• Catalysts lower Ea; do not change ΔH of reaction; increase rate in both directions equally

Catalysis

• Homogeneous catalysis: catalyst and reactants in same phase; e.g., NO in oxidation of SO₂
• Heterogeneous catalysis: different phases; Haber process (Fe catalyst), catalytic converter (Pt/Pd)
• Enzyme catalysis: lock-and-key model; highly specific; optimum pH and temperature

CBSE Exam — Kinetics

• For first order reaction: show that t½ is independent of initial concentration
• Rate doubles when temperature raised from 290 K to 300 K; find Ea
• Define: order, molecularity, pseudo-first order reaction; give example of each