Rate Law
Reaction rate as a function of reactant concentrations.
Formula
Variables
Example
If a reaction is first order in A, doubling [A] doubles the reaction rate.
How it works
This rate law says that the speed of a reaction depends on the concentrations of the reactants raised to powers called orders, with the proportionality set by the rate constant k. The exponents m and n are not usually taken from the balanced equation; instead, they come from experiment and reflect how strongly each reactant affects the rate through the reaction mechanism. As concentrations change, the rate changes in a predictable way: if a reactant appears with a higher order, small concentration changes have a larger effect on the rate.
When to use it
Use this when analyzing experimental kinetics data, predicting how a reaction rate changes if reactant concentrations are altered, or comparing different proposed mechanisms. It is especially useful in chemistry problems involving initial-rate data and reaction-order determination.
Worked example, step by step
Suppose a reaction follows r = k[A]^2[B]. Given: k = 0.50 M^-2 s^-1, [A] = 0.20 M, [B] = 0.30 M Step 1: Substitute into the rate law r = 0.50(0.20)^2(0.30) Step 2: Square [A] (0.20)^2 = 0.040 Step 3: Multiply by [B] 0.040 × 0.30 = 0.012 Step 4: Multiply by k r = 0.50 × 0.012 = 0.0060 M s^-1 Final answer: r = 6.0 × 10^-3 M s^-1
Common mistakes to avoid
- Assuming the exponents must match the balanced chemical equation
- Forgetting that the units of k change with the overall reaction order
- Treating concentration changes as additive instead of multiplying by powers
Did You Know?
A reaction's rate law can only be found by experiment, never guessed from the balanced equation.
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