Rate Law and Order of Reaction for JEE Main 2026

Chemical kinetics describes the rate at which reactions proceed and the factors controlling that rate. JEE Main draws three to four questions from this chapter — integrated rate laws, half-life calculations, temperature dependence (Arrhenius equation), and the experimental determination of order. The formulas are few but their applications require genuine understanding of what each quantity means.

Rate Law and Order

The rate law Rate = k[A]^m[B]^n is determined experimentally, not from the balanced equation. The order with respect to A is m, with respect to B is n, and the overall order is m + n. Units of k depend on overall order: for first order, k has units s⁻¹; for second order, L/(mol·s); for zero order, mol/(L·s). Determining order from data: in the initial rate method, doubling [A] and observing the rate change tells you m directly. If rate doubles when [A] doubles, m = 1; if rate quadruples, m = 2. For the broader kinetics treatment see our chemical kinetics guide.

Integrated Rate Laws and Half-Lives

Zero-order: [A] = [A]₀ − kt; t½ = [A]₀/(2k) (half-life decreases as concentration decreases). First-order: [A] = [A]₀ e^(−kt); t½ = ln2/k = 0.693/k (half-life constant, independent of concentration). Second-order: 1/[A] = 1/[A]₀ + kt; t½ = 1/(k[A]₀) (half-life increases as concentration decreases). A first-order reaction whose half-life is 10 minutes will show 87.5% conversion in 30 minutes (three half-lives: (½)³ = 1/8 remaining, so 7/8 converted). This power-of-half shortcut works for any first-order problem that gives time as a multiple of the half-life.

Pseudo-First-Order Reactions

When one reactant is in large excess, its concentration is essentially constant, reducing an overall second-order reaction to an apparent first-order reaction. Classic example: hydrolysis of ethyl acetate in excess water. Rate = k[CH₃COOC₂H₅][H₂O] simplifies to Rate = k'[CH₃COOC₂H₅] where k' = k[H₂O] is the pseudo-first-order rate constant. JEE tests this concept by asking why certain reactions follow first-order kinetics even though the stoichiometry suggests a higher order.

Arrhenius Equation and Activation Energy

The Arrhenius equation: k = Ae^(−Ea/RT), where A is the pre-exponential factor (frequency factor), Ea is the activation energy, R = 8.314 J/(mol·K), T is temperature in Kelvin. Taking logarithm: ln k = ln A − Ea/(RT). Plotting ln k vs 1/T gives a straight line with slope −Ea/R. If the rate constant at two temperatures T₁ and T₂ is given: ln(k₂/k₁) = (Ea/R)(1/T₁ − 1/T₂). A common numerical: find Ea given that the rate doubles when temperature rises from 300 K to 310 K. Substitute k₂/k₁ = 2 and solve for Ea.

Collision Theory and Transition State

Collision theory: molecules must collide with sufficient energy (≥ Ea) and correct orientation. The fraction of molecules with energy ≥ Ea is e^(−Ea/RT) — this appears directly in the Arrhenius equation. Transition state theory: the reactants pass through a high-energy transition state (activated complex) at the top of the energy barrier. The energy difference between reactants and the transition state is Ea. The enthalpy of reaction ΔH = Ea(forward) − Ea(reverse) — this Hess-like connection often appears in JEE diagrams. After mastering kinetics, take a free mock test on chemical kinetics and equilibrium.