9701 · 26.1
Simple rate equations, orders of reaction and rate constants
A rate equation is like a recipe that tells you how changing the amount of each reactant (concentration) affects the speed of the reaction. The orders of reaction are exponents in this recipe, which we can only find through experiments.
Need to know
What you need to know
- **[A]** and **[B]** represent the concentrations of reactants A and B in mol dm⁻³.
- **k** is the rate constant, a value specific to a reaction at a particular temperature.
- **m** is the order of reaction with respect to reactant A.
- **n** is the order of reaction with respect to reactant B.
- The **overall order** of the reaction is the sum of the individual orders, m + n.
- Orders of reaction (m and n) are usually integers (0, 1, or 2) but can be fractions. They must be determined from experimental data.
Explanation
The Recipe for Reaction Speed
- Rate = k[A]^m[B]^n — orders m, n from experiment only. | Sim hint: Initial rates method or half-life method.
- Overall order = m + n; units of k depend on order. | Sim hint: First order: k in s⁻¹; second order: dm³ mol⁻¹ s⁻¹.
- Half-life constant for first order: t_{½} = ln 2 / k. | Sim hint: Independent of initial concentration.
- Arrhenius: k = Ae^{−Ea/RT} — temperature effect on rate. | Sim hint: Plot ln k vs 1/T for Ea.