O-Level and SEC G3 Chemistry K324
C10: Rate of Reactions
Link collision reasoning, variables, graphs, experimental design, and catalyst effects to measured rate.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Rate of Reactions connects a measured change per unit time to successful particle collisions. Explain each factor through collision frequency or collision energy, distinguish rate from final amount, and design investigations that change one variable while measuring a suitable dependent quantity.
Collision explanation of rate factors
Higher concentration places more reacting particles in a given volume, while higher gas pressure packs particles more closely. Both increase collision frequency. Smaller solid particles provide greater total surface area for contact. Higher temperature increases particle speed, collision frequency, and the fraction of collisions with enough energy to react.
A faster reaction reaches completion sooner but does not necessarily make more product when reactant amounts are unchanged. Explain the factor named in the question and keep other variables controlled. Do not say particles collide harder without relating the change to successful collisions.
Catalysts and activation energy
A catalyst increases reaction rate and is chemically unchanged overall. It provides an alternative pathway with lower activation energy, so a greater fraction of collisions can lead to reaction at the same temperature. It does not increase particle energy or change the enthalpy difference between reactants and products.
Transition-element compounds can catalyse industrial reactions, and enzymes are biological catalysts. Enzyme activity depends on conditions, but do not add biological detail beyond the data and required K324 link. Name the catalyst only when it is given or part of the required example.
Investigating and interpreting rate
Measure gas volume, mass loss, precipitate formation, colour change, or concentration at regular times, depending on the reaction. State the independent variable, dependent measurement, controlled variables, apparatus, repeat strategy, and safety. A gas syringe often gives more quantitative data than counting bubbles.
On a product-time graph, gradient gives rate and the plateau gives final product amount. A steeper initial gradient means faster initial rate. Compare equal time intervals or draw tangents when required. An unchanged plateau with a steeper curve indicates the same final amount reached sooner.
Formulae and relationships
| Relationship | Use |
|---|---|
| Calculate an average reaction rate. | |
| Estimate rate at one time from a graph. |
Worked examples
Example 1: A reaction produces of gas in the first . Find the average rate and state why this is not necessarily the rate at .
- Average rate .
- Calculate .
- The reaction rate may change during the interval; the rate at needs a tangent gradient.
Answer: Average rate .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Define rate from change in a measurable quantity per unit time.
- Use collision frequency and successful-collision energy to explain concentration, pressure, surface area, temperature, and catalyst effects.
- Interpret gradients and plateaus without assuming every graph segment has a constant rate.
Official outcome coverage
K324 C10: 6 mapped outcomes, references C10(a), C10(b), C10(c), C10(d), C10(e), C10(f). Check the official K324 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Choose gas volume, mass loss, colour, or another observable response, then control quantities and compare initial or average rates consistently.
Exam traps and retrieval check
Avoid these traps
- Using final product amount as a measure of initial rate.
- Saying a catalyst raises temperature or gives particles more energy.
- Changing particle size and mass of solid at the same time in a fair test.
Check from memory
Why does powder react faster than large lumps?
It has greater total surface area, increasing collision frequency.
What does a catalyst lower?
Activation energy for an alternative pathway.
What does a graph plateau show?
The final amount of measured product or reactant change.
Pure versus Combined scope
Combined Chemistry uses the same top-level topic label but a narrower outcome set. Use the Combined component checklist before removing or adding detail.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K324. Open the related IP explanation.