Rate Expression & Method of Initial Rates
- Rate expression: Rate = k[A]m[B]n where m, n are the
orders with respect to each reactant
- Overall order = m + n (sum of individual orders)
- Orders are determined experimentally — they cannot be deduced from the balanced
equation
- Rate constant k: temperature-dependent; units depend on overall order
- The method of initial rates compares experiments where only one concentration
changes at a time
IB Exam Strategies
- Finding order: Compare two experiments where only [X] changes. If [X]×2 and
rate×1 → order 0. If rate×2 → order 1. If rate×4 → order 2
- Units of k: For overall order n, units = mol(1-n)
dm3(n-1) s-1
- Zero order: Rate is independent of that reactant's concentration
- Common mistake: Assuming the coefficient in the equation equals the order
- Half-life: Only constant for first-order reactions
Step-by-Step Method
- Step 1: Find two experiments where only ONE reactant's concentration changes
- Step 2: Calculate the ratio of concentrations: [X]₂/[X]₁
- Step 3: Calculate the ratio of rates: Rate₂/Rate₁
- Step 4: Solve (concentration ratio)order = rate ratio
- Step 5: Repeat for each reactant
- Step 6: Substitute back into rate = k[A]m[B]n using any
experiment's data to find k