Edexcel IAL Chemistry · Unit 4
Kinetics
The big idea: a reaction's rate depends on the concentration of certain reactants — and the way it depends on them (the "order") reveals exactly which molecules are colliding in the slow, bottleneck step of the mechanism.
Summary — What This Chapter Covers
- Rate of reaction = how fast reactants disappear or products appear, measured in mol dm⁻³ s⁻¹.
- Rate equations & orders — rate = k[A]ᵐ[B]ⁿ, and orders can only be found by experiment, never from the balanced equation.
- Graphs — concentration-time and rate-concentration graphs have distinct shapes for zero, first, and second order.
- Obtaining rate data — titration, colorimetry, mass loss, gas volume: matching the method to the reaction.
- Initial rate method & clock reactions — measuring the very start of a reaction, using tangents or 1/t.
- Continuous monitoring — tracking concentration throughout the whole reaction (e.g. iodination of propanone by colorimetry).
- Rate-determining step — the slowest step controls the rate; only species in this step appear in the rate equation.
- Reaction mechanisms — deducing plausible step-by-step mechanisms, including SN1 and SN2 nucleophilic substitution.
- Activation energy & the Arrhenius equation — linking rate constant, temperature, and activation energy; using ln k vs 1/T plots.
1. Kinetic Rates — The Basics
The rate of reaction is simply how quickly a reactant is used up, or a product is formed, per unit time. Its units are always mol dm⁻³ s⁻¹ — concentration divided by time.
Think of it like: filling (or draining) a bathtub. The "rate" is how many litres change per second. It doesn't matter whether you're watching the tap (product forming) or the plug (reactant disappearing) — both tell you the same story about how fast the process is happening.
You can measure rate by watching almost any property that changes during a reaction:
- Mass lost over time (e.g. gas escaping from an open flask)
- Volume of gas produced over time
- Colour changes (using a colorimeter)
- pH changes over time
- Changes in electrical conductivity
Here's the key insight this chapter builds on: if you plot rate against concentration of a reactant and get a straight line through the origin, that tells you rate is directly proportional to concentration — i.e. Rate ∝ [reactant], or Rate = k[reactant]. Doubling the concentration doubles the rate; halving it halves the rate. This one relationship is the seed of everything else in the chapter.
Practice Question
A reaction has the rate expression Rate = k[X]. If [X] is tripled, what happens to the rate? What if [X] is reduced to a quarter of its original value?
2. Rate Equations & Orders
For a general reaction A(aq) + B(aq) → C(aq) + D(g), the rate equation is written:
⚠ Critical Rule
only
never
What "Order" Actually Means
The order with respect to a reactant is the power that reactant's concentration is raised to in the rate equation. It tells you how sensitive the rate is to changes in that reactant's concentration.
| Order | What happens to rate | In rate equation? |
|---|
N
N