Predict concentration and pressure effects on rate
Use particles per unit volume and effective-collision frequency to explain rate changes.
Section 8 · Reaction kinetics
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Rate data and diagrams are illustrative unless a question supplies measurements. A diagram is qualitative unless labelled values are provided. Written practice is self-checked
Before you begin
- Rearrange simple ratios and use consistent units for time, volume, mass and concentration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to particles per unit volume when a gas is compressed at constant temperature?
What you will learn
- Explain the concentration effect on rate
- Explain the pressure effect for gaseous reactants
- Use effective-collision frequency in each explanation
More concentrated solutions increase collision opportunities
At a higher reactant concentration, more reactant particles occupy the same solution volume. Particles therefore meet more often, so the frequency of effective collisions per second generally increases and the reaction rate rises.
The explanation is about collision frequency in the reacting mixture. It does not mean each individual collision has more energy or that the activation energy has changed.
Higher gas pressure brings particles closer
For gaseous reactants, increasing pressure at constant temperature compresses the gas into a smaller volume. More particles occupy each unit volume, so collisions occur more frequently and effective collisions generally occur more often per second.
Pressure is used here for gases. Do not apply the gas-pressure explanation to a solid reactant or treat pressure as changing the activation energy. A rate change does not by itself tell you the final equilibrium yield.
Pause and explain
Why does increased reactant concentration usually increase rate?
Worked example
Explain why increasing the pressure of gaseous reactants at constant temperature can increase the reaction rate.
Show the worked solution
- The gas is compressed, so the particles are closer together.
- More particles occupy each unit volume and collide more frequently.
- The frequency of effective collisions per second increases.
Answer Higher pressure increases the frequency of effective collisions between gas particles, so the reaction rate increases.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A solution reaction is repeated using a more concentrated solution at the same temperature. Explain the expected rate change.
Give me a hint
Focus on the number of reactant particles in the same volume.
Compare my reasoning
- Higher concentration means more reactant particles per unit volume.
- Collision frequency increases.
- More effective collisions occur each second, so rate rises.
The more concentrated solution usually reacts faster because more frequent collisions produce more effective collisions per second.
Look for these in your work
- Particles per volume stated
- Collision frequency linked
- Rate prediction justified
A gaseous reaction is run at 100 kPa and then at 200 kPa, with temperature unchanged. Explain the collision-theory prediction.
Give me a hint
State what the pressure change does to particle spacing and collision frequency.
Compare my reasoning
- The gas occupies less volume at higher pressure.
- There are more particles in each unit volume.
- Collisions and effective collisions become more frequent, increasing rate.
At 200 kPa, gas particles are closer together, so effective collisions occur more frequently and the reaction is faster.
Look for these in your work
- Gas compression described
- Per-volume particle count compared
- Effective collisions linked to rate
A student claims that doubling pressure doubles the reaction rate for every gas reaction. Evaluate this claim using only the qualitative collision model.
Give me a hint
The syllabus asks for a qualitative effect, not a universal numerical rate law.
Compare my reasoning
- Higher pressure generally increases collision frequency for gaseous reactants.
- The fraction of collisions that are effective and the reaction's kinetic law are not supplied.
- So a qualitative rate increase is justified, but an exact doubling is not.
The qualitative model predicts a higher rate because effective collisions become more frequent. It does not justify an exact doubling for every reaction.
Look for these in your work
- State qualitative trend
- Avoid unsupported quantitative law
- Identify what else would be needed
Common mistakes
- Saying concentration gives particles more kinetic energy
- Applying gas pressure to solids
- Claiming every extra collision is effective
- Confusing rate with equilibrium position
What can you explain now?
What happens to particles per unit volume when a gas is compressed at constant temperature?
Compare with the explanation
The number per unit volume increases
The same gas occupies less volume.
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Make it stick
How do concentration and gas pressure alter the frequency of effective collisions?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.