Rate Law Formula
Express the speed of a chemical reaction based on reactant concentrations.
Fundamental to chemical kinetics.
The Formula
Rate = k × [A]ᵐ × [B]ⁿ
The rate law expresses how fast a reaction proceeds based on reactant concentrations. The exponents m and n are determined experimentally — they are not necessarily the stoichiometric coefficients.
Variables
| Symbol | Meaning |
|---|---|
| Rate | Speed of the reaction (mol/L/s) |
| k | Rate constant (units vary with reaction order) |
| [A], [B] | Concentrations of reactants (mol/L) |
| m, n | Reaction orders with respect to each reactant |
Example 1
Rate = 0.02 × [A]² × [B]. Find the rate when [A] = 0.5 M and [B] = 0.3 M.
Rate = 0.02 × (0.5)² × (0.3)
Rate = 0.02 × 0.25 × 0.3
Rate = 0.0015 mol/L/s
Example 2
If [A] is doubled while [B] stays the same (second order in A):
New rate = 0.02 × (1.0)² × (0.3) = 0.02 × 1.0 × 0.3
New rate = 0.006 mol/L/s
The rate quadrupled (2² = 4 times faster)
When to Use It
Use the rate law when:
- Predicting how changes in concentration affect reaction speed
- Determining the order of a reaction from experimental data
- Calculating the rate constant from measured rates
- Designing industrial chemical processes for optimal speed