1. What rate of reaction means
The rate of reaction is the change in concentration of a reactant or product per unit time.
In the lab we often follow something easier to measure, such as gas volume (cm3 s−1) or mass lost (g s−1). These are useful for comparing rates in the same set-up, but they are different quantities from a rate in mol dm−3 s−1. Don't swap the units between them.
2. Collision theory and activation energy
Reactions happen when particles collide. A collision only leads to reaction (a successful collision) if the particles:
- collide with at least the activation energy, and
- collide with a suitable orientation, so the right bonds can break.
The activation energy, Ea, is the minimum energy that colliding particles need for a reaction to happen. Many collisions do not have enough energy, so they are not successful.
Diagram placeholder
Reaction profile (exothermic) showing Ea and ΔH
Labels to include:
- Vertical axis: Enthalpy (or energy)
- Horizontal axis: Progress of reaction
- Reactants level (left) and products level (right, lower)
- Single peak between them
- Ea: vertical arrow from the reactants level up to the peak
- ΔH: vertical arrow from the reactants level down to the products level
The curve rises from the reactants to a peak and falls to the products. Ea is measured from the reactants to the peak. The same diagram, with an endothermic version, is explained in Energetics Part 1.
3. Concentration
Increasing the concentration of a reactant in solution means there are more reactant particles per unit volume. Particles collide more often, so there are more successful collisions per unit time and the rate increases.
At the same temperature, the fraction of collisions with energy ≥ Ea is unchanged. Only the number of collisions per second goes up.
4. Pressure of gases
Raising the pressure of a gaseous reaction by compressing it into a smaller volume at constant temperature has the same effect as raising concentration: more gas particles per unit volume, so more frequent collisions and more successful collisions per unit time.
Be careful about what “higher pressure” means. Adding an unreactive gas to a fixed volume raises the total pressure, but the number of reactant particles per unit volume does not change, so this does not by itself increase the rate. Having more particles in total does not guarantee a faster reaction; what matters is reactant particles per unit volume.
5. Surface area
For a solid reacting with a liquid or gas, only the particles on the surface can be hit. A powder exposes far more reactant particles than the same mass in lumps. So there are more collisions per unit time at the surface and more successful collisions per unit time.
Grinding a solid does not lower Ea and does not give the particles more kinetic energy.
6. Temperature
At a higher temperature, particles have a higher average kinetic energy. The main reason the rate increases is that a much larger fraction of particles have energy ≥ Ea, so a larger proportion of collisions are successful. Particles also collide slightly more often, but this is a much smaller effect.
So “particles move faster” is not a complete answer: link it to more particles having at least the activation energy, giving more successful collisions per unit time. Part 2 shows this with the Maxwell–Boltzmann distribution.
7. Catalysts
A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. It is regenerated at the end, so it is not used up overall. A catalyst may take part in intermediate steps; it is not true that it “never reacts”.
A catalyst does not change ΔH or the position of equilibrium. For the same reaction under the same conditions, it does not change the amount of product at equilibrium or at completion; it only gets there faster. The yield obtained in a fixed, limited time can increase, because more reaction happens in that time.
8. Summary of cause chains
| Change | What changes | What stays the same | Result |
|---|---|---|---|
| Higher concentration | Particles per unit volume → collision frequency | Fraction with E ≥ Ea; Ea | More successful collisions per unit time |
| Higher pressure (compression) | Gas particles per unit volume → collision frequency | Fraction with E ≥ Ea; Ea | More successful collisions per unit time |
| Larger surface area | Exposed particles → collisions at surface | Ea; particle energies | More successful collisions per unit time |
| Higher temperature | Average KE; fraction with E ≥ Ea (main effect) | Ea | Many more successful collisions per unit time |
| Catalyst | Ea is lower (new pathway) → larger fraction can react | Particle energies; ΔH; equilibrium/completion yield | More successful collisions per unit time |
Quick checks
These are Finesse practice questions. The step-by-step answers are indicative worked solutions, not official AQA mark allocations.
Q1. The concentration of a product rises from 0.56 to 0.80 mol dm−3 between 40 s and 100 s. Calculate the average rate.Show answer
Step 1: Δ[product] = 0.80 − 0.56 = 0.24 mol dm−3; Δt = 60 s.
Step 2: rate = 0.24 ÷ 60 = 4.0 × 10−3 mol dm−3 s−1.
Q2. Explain why 2.0 g of powdered zinc reacts faster with excess dilute acid than 2.0 g of zinc granules.Show answer
The powder has a larger surface area, so more zinc particles are exposed to the acid.
There are more collisions per unit time between acid particles and zinc, so more successful collisions per unit time.
Ea and the particles' energies are unchanged.
Q3. Explain why raising the temperature increases the rate of a reaction.Show answer
Particles have a higher average kinetic energy.
A much larger fraction of particles have energy greater than or equal to Ea.
So there are more successful collisions per unit time. (Collision frequency also rises slightly, but that is the smaller effect.)
Q4. Argon is added at constant temperature to a sealed, fixed-volume flask in which two gases are reacting. The total pressure rises. A student says the rate must increase. Evaluate this.Show answer
The volume and the amount of each reactant are unchanged, so the number of reactant particles per unit volume is unchanged.
The frequency of collisions between reactant particles does not increase for this reason, so the rate is not expected to increase just because the total pressure rose. The student is wrong.
Q5. State three things a catalyst does not change, and explain how it increases rate.Show answer
Does not change: ΔH, the position of equilibrium, and (same reaction and conditions) the amount of product at equilibrium or completion, though yield in a fixed time can rise. It is also not used up overall.
It provides an alternative pathway with a lower Ea, so a larger fraction of particles have enough energy to react, giving more successful collisions per unit time.
Sources
Sources and examiner guidance (reviewed 1 October 2026)
- Chemrevise — AQA 1.5 Reaction kinetics revision guide (N. Goalby) — Primary checklist for collision theory and rate factors (Feb 2023 edition).
- AQA 7405 specification — 3.1.5 Kinetics (incl. Required Practical 3) — 3.1.5.1 Collision theory, 3.1.5.3 Effect of temperature, 3.1.5.4 Concentration and pressure, 3.1.5.5 Catalysts.
- AQA 7404/2 mark scheme, June 2022 — Q01.6: many more particles with enough energy and more successful collisions per unit time (used in Part 2).
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.