Rates & Mechanisms

AQA Specification

Rate Equation Builder

The rate equation can only be determined experimentally — you cannot deduce it from the balanced equation. It takes the form: rate = k[A]ᵐ[B]ⁿ

Build your rate equation

Identifying Reaction Orders

Use concentration-time and rate-concentration graphs to determine the order with respect to each reactant.

Concentration vs Time graphs

[ ]time
Zero order
Linear decrease. Constant rate — doesn't depend on concentration.
[ ]time
First order
Exponential decay. Constant half-life.
[ ]time
Second order
Steeper initial drop. Half-life increases over time.

Rate vs Concentration graphs

rate[ ]
Zero order
Horizontal line. Rate is constant regardless of concentration.
rate[ ]
First order
Straight line through origin. Rate ∝ [A].
rate[ ]
Second order
Curve through origin. Rate ∝ [A]².
AQA exam tip: To find order from data: if doubling [A] has no effect on rate → zero order. If doubling [A] doubles rate → first order. If doubling [A] quadruples rate → second order. Show your working.

Reaction Mechanisms

Many reactions occur in multiple steps. The rate equation tells us about the rate-determining step (RDS) — the slowest step that controls the overall rate.

1
Rate-determining step (RDS) is the slowest step. Only species that appear in the RDS appear in the rate equation.
2
Overall order = sum of individual orders. If rate = k[A][B], overall order = 2. The RDS involves one molecule of A and one of B.
3
Intermediates are species formed in one step and consumed in another — they don't appear in the overall equation but may appear in the mechanism.
4
Molecularity of the RDS must match the rate equation. If rate = k[A]², the RDS is bimolecular involving two molecules of A.

Example: SN1 vs SN2

SN1 (two-step)

Step 1 (slow): R-X → R⁺ + X⁻
Step 2 (fast): R⁺ + :Nu⁻ → R-Nu

Rate = k[RX]
First order — only the halogenoalkane is in the RDS.

SN2 (one-step)

Step 1: Nu⁻ + R-X → Nu-R + X⁻

Rate = k[RX][Nu⁻]
Second order — both reactants are in the single (rate-determining) step.

AQA exam tip: When proposing a mechanism from a rate equation: (1) identify the species and their orders in the rate equation, (2) these must appear in the RDS, (3) ensure all steps add up to the overall balanced equation, (4) label slow and fast steps.