Explain reaction rates and effective collisions
Define reaction rate and distinguish collisions that lead to reaction from collisions that do not.
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 does reaction rate describe?
What you will learn
- Explain rate of reaction
- Describe collision frequency
- Distinguish effective from non-effective collisions
Rate describes change per unit time
The rate of a reaction describes how quickly reactants are used or products are formed. It can be measured by tracking a suitable quantity such as concentration, gas volume or mass as time passes.
Collision frequency is the number of particle collisions in a stated time. A high collision frequency alone does not guarantee a high reaction rate: collisions must also be effective.
Only some collisions are effective
An effective collision is one that results in reaction. A non-effective collision does not produce the reaction. In collision theory, particles need sufficient energy to overcome the activation-energy barrier; the reacting particles must also meet in a way that allows the required bonds to change.
The rate is linked to the frequency of effective collisions. If more effective collisions occur each second, reactant particles are converted to products more quickly. Do not call every collision an effective collision.
Pause and explain
Which collisions count as effective?
Worked example
A mixture has many collisions each second, but only a small fraction produce products. Explain what controls the reaction rate.
Show the worked solution
- The total number of collisions per second is the collision frequency.
- Only collisions that result in reaction are effective.
- The reaction rate depends on how frequently effective collisions occur.
Answer The reaction rate depends on the frequency of effective collisions, not simply on the total collision frequency.
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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 reaction's rate increases while the total number of collisions per second stays the same. What must have changed?
Give me a hint
Compare all collisions with the subset that produce reaction.
Compare my reasoning
- Total collision frequency is unchanged.
- The number of collisions that result in reaction must have risen.
- Thus the frequency of effective collisions increased.
The frequency of effective collisions increased, even though the total collision frequency stayed the same.
Look for these in your work
- Distinguish total from effective collisions
- Relate effective collisions to rate
In one second, 240 collisions occur and 18 lead to reaction. State the total collision frequency and effective-collision frequency.
Give me a hint
Count each set per the same one-second interval.
Compare my reasoning
- Total collision frequency is 240 s⁻¹.
- Effective collisions are the 18 that produce reaction.
- Their frequency is 18 s⁻¹.
Total collision frequency = 240 s⁻¹; effective-collision frequency = 18 s⁻¹.
Look for these in your work
- Total counted
- Successful subset identified
- Both rates use s⁻¹
A student says, ‘The fastest reaction is always the one with the most total collisions.’ Explain the missing condition.
Give me a hint
A high collision count matters only if enough collisions produce chemical change.
Compare my reasoning
- Some collisions are non-effective.
- Rate depends on effective collisions per unit time.
- Compare the effective-collision frequency, not just all collisions.
Total collisions alone do not determine rate; the fastest reaction has the greater frequency of effective collisions.
Look for these in your work
- Reject total-collision shortcut
- Name effective-collision frequency
- Connect it to rate
Common mistakes
- Treating collision frequency and reaction rate as identical
- Calling a collision effective when it produces no reaction
- Assuming every collision has enough energy to react
- Describing rate only as the time a reaction takes
What can you explain now?
What does reaction rate describe?
Compare with the explanation
Change in a measurable amount per unit time
A rate compares how much changes with the time taken.
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Make it stick
How are collision frequency, effective collisions and reaction rate related?
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