How Concentration, Pressure and Temperature Affect Reaction Rate Revision Notes | GCSE AQA Higher Chemistry

1. Why Some Reactions Are Lightning Fast and Others Take Forever

Picture a firework exploding in milliseconds versus rust forming over months. Both are chemical reactions, but their rates are wildly different. The secret lies in how often particles smash into each other — and how hard they hit.

Three factors control this molecular demolition derby: concentration (how crowded the particles are), pressure (how squashed together they get), and temperature (how fast they're moving). Master these, and you can predict whether a reaction will crawl or sprint.

2. The Collision Theory Playbook

Think of particles as bumper cars at a fairground. Not every crash results in a reaction — just like not every bump breaks the cars. For a successful reaction, two things must happen: particles must collide with enough energy (activation energy) and hit each other at the right angle.

Most collisions are gentle taps that bounce particles apart unchanged. Only the high-energy head-on crashes break bonds and form new products. This is why reaction rates depend on collision frequency AND collision energy.

3. Concentration - The Crowding Effect

Imagine trying to cross a busy shopping centre versus an empty car park. In the crowded centre, you bump into people constantly. In the empty car park, you might not meet anyone. Particle concentration works exactly the same way.

Higher concentration means more particles crammed into the same space. More particles equals more collisions per second. More collisions means more chances for successful reactions. It's a numbers game — double the concentration, roughly double the collision frequency.

Worked example: Calculating the Effect of Concentration on Rate

A reaction has a rate of 0.02 mol/dm³/s when the concentration is 0.5 mol/dm³. Predict the rate when concentration increases to 1.0 mol/dm³.

1. Rate ∝ concentration (for this reaction) — Doubling concentration should double the rate
2. New concentration ÷ Old concentration = 1.0 ÷ 0.5 = 2 — Concentration has doubled
3. New rate = 0.02 × 2 = 0.04 mol/dm³/s — Rate doubles when concentration doubles

Answer: 0.04 mol/dm³/s

4. Pressure - Squashing Gas Particles Together

Pressure only affects reactions involving gases. Picture a syringe full of gas particles — push the plunger in, and you force the same number of particles into a smaller space. The result? They collide more often because they're squashed closer together.

Increasing pressure has the same effect as increasing concentration for gases. Double the pressure, and you halve the volume, which doubles the particle density. More particles per unit volume means more collisions per second.

5. Temperature - The Energy Booster

Temperature is the game-changer. Concentration and pressure increase collision frequency only, not collision energy. Temperature increases both collision frequency and the energy of the collisions.: it makes particles move faster (more collisions) AND hit harder (more energetic collisions).

Here's the crucial insight: at higher temperatures, many more particles have energy above the activation energy threshold. A small temperature rise can dramatically increase the fraction of successful collisions. This is why a 10°C increase often doubles or triples reaction rate.

Worked example: Temperature Effect on Collision Energy

Explain why increasing temperature from 20°C to 30°C roughly doubles the rate of many reactions.

1. 20°C = 293K, 30°C = 303K — Always use absolute temperature for energy calculations
2. 303K ÷ 293K = 1.034 — Only 3.4% increase in absolute temperature
3. Small temperature rise → many more particles exceed activation energy — The energy distribution curve shifts, dramatically increasing successful collisions

Answer: A small absolute temperature increase causes an exponential increase in particles with sufficient energy to react.

6. Exam Mastery - Getting Full Marks on Rate Questions

Examiners love testing collision theory because it separates students who memorise from those who understand. The key is linking every answer back to particle collisions and energy.

Common exam trap: students say temperature 'speeds up particles' but forget to explain why this increases reaction rate. Always complete the chain: faster particles → more frequent collisions → more energetic collisions → more successful reactions.

Exam precision note: In How Concentration, Pressure and Temperature Affect Reaction Rate - Collision theory and activation energy - Rate of reaction, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Surface area matters when one of the reactants is a solid. Breaking a solid into smaller pieces or using a powder exposes more particles at the surface, so collisions happen more often each second. The particles themselves are not moving faster, but more of them are available to collide. This is why powdered calcium carbonate reacts faster with acid than marble chips of the same mass. Students should describe surface area as increasing the number of exposed particles and therefore increasing collision frequency.