Order vs Molecularity: Stop Confusing Them
Order of Reaction
Experimentally determined. Sum of powers of concentration terms in the rate law. Can be 0, 1, 2, or even fractional. Changes with conditions. e.g., Rate = k[A]²[B]¹ → Order = 3.
Molecularity
Theoretical. Number of molecules colliding in a single elementary step. Always a positive integer (1, 2, or 3). Never fractional. Applies only to elementary reactions.
Half-Life Formulas by Order
| Order | Rate Law | t½ Formula |
|---|---|---|
| Zero | Rate = k | [A]₀ / 2k |
| First | Rate = k[A] | 0.693 / k (independent of [A]₀) |
| Second | Rate = k[A]² | 1 / (k[A]₀) |
The Arrhenius Equation
k = A·e^(−Ea/RT)
ln(k₂/k₁) = (Ea/R) × (1/T₁ − 1/T₂). If temperature increases by 10°C, rate roughly doubles (Rule of Thumb). Catalyst reduces Ea — it does NOT change ΔH or ΔG.
ChemistryKineticsNEETJEERate LawHalf Life
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