SEC G3 Combined Science Chemistry component K326/K328
C10: Rate of Reactions
Explain concentration, pressure, particle-size, and temperature effects through collisions and rate data.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Reaction rate changes when the frequency of successful collisions changes, and rate data show how quickly reactants disappear or products form.
Concentration and gas pressure
Higher solution concentration places more reacting particles in a given volume. Higher gas pressure brings gas particles closer. Both usually increase collision frequency and therefore reaction rate.
State that other variables are held constant when attributing a rate change to one factor.
Particle size and temperature
Smaller solid particles provide a larger total exposed surface area, allowing more collisions at once. Higher temperature makes particles move faster, increasing collision frequency and the proportion of collisions that can react.
Do not say that temperature increases particle size or concentration.
Interpreting rate data
A steeper product-time graph has a greater rate. A plateau can mean a limiting reactant has been used up, so the final amount and initial gradient answer different questions.
Compare rates over equivalent intervals or use a tangent for an instantaneous rate when the data and graph permit it.
Formulae and relationships
| Relationship | Use |
|---|---|
| Calculate an average reaction rate. |
Worked examples
Example 1: Experiment A produces gas in ; B produces in . Compare average rates.
- A: .
- B: .
- Compare the calculated rates rather than final volumes.
Answer: B is faster over the stated intervals.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Concentration and gas pressure with the named terms, evidence, and causal links kept distinct.
- Use Particle size and temperature to interpret the evidence given and justify each conclusion.
- Apply Interpreting rate data to a new example, then check the conclusion against the information given.
Official outcome coverage
K326 C10: 2 mapped outcomes, references C10(a), C10(b). Check the official K326 syllabus.
K328 C10: 2 mapped outcomes, references C10(a), C10(b). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Measure reaction rate from gas volume, mass loss, temperature, colour or another stated quantity. Change one of concentration, gas pressure, particle size or temperature, control the rest, repeat trials, and compare gradients or equal-interval changes consistently.
Exam traps and retrieval check
Avoid these traps
- Using final product amount as the rate.
- Saying smaller particles collide harder.
- Changing two variables but attributing the result to one.
Check from memory
Why can higher concentration increase rate?
It increases collision frequency per volume.
Why does powder react faster than equal-mass lumps?
It has greater exposed surface area.
What graph feature indicates a greater rate?
A steeper gradient.
Official Combined Science scope
This shared Combined Chemistry owner serves both K326 and K328. Concentration, gas pressure, particle size, temperature, collision reasoning, and interpretation of rate data.