Reaction rate factors and catalysts
Predict the usual rate changes while keeping amount of product separate from how quickly it forms.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Collision-theory explanations and method evaluation are taught in separate Extended lessons. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Interpret concentration as amount per solution volume.
- Distinguish a gas's pressure and a solid's exposed surface area.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For equal masses of the same solid, which usually has greater exposed surface area?
What you will learn
- Describe rate effects of concentration, gas pressure, solid surface area, temperature and catalysts.
- State what a catalyst does and distinguish rate from final product amount.
State the factor and the direction of the effect
For the suitable reactions studied, increasing a solution reactant's concentration usually increases rate; increasing pressure for reacting gases usually increases rate; breaking a reacting solid into smaller pieces increases exposed surface area and usually increases rate; and increasing temperature usually increases rate over the stated working range. Reverse these changes to describe the corresponding slower cases, while keeping the other relevant conditions controlled.
A catalyst increases the rate of a reaction and is unchanged at the end of it. It can take part in intermediate steps but is regenerated overall. Enzymes are biological catalysts. Removing a catalyst slows the catalysed route; it does not necessarily stop every possible reaction. Detailed collision and activation-barrier explanations belong to the Extended lesson that follows.
A faster reaction need not make more final product
Reaction rate describes how quickly reactants are used or products form. Final product amount also depends on reactant amounts and the balanced reaction. Equal masses of the same limiting solid can give equal final gas volumes but reach that total at different times when surface area changes, assuming the other reactant remains in excess and the reaction completes.
Changing concentration does not always preserve reactant amount: the same volume of a more concentrated solution contains more solute. State which quantity is controlled before concluding that final yield remains equal. Enzyme activity also has a working temperature range; excessive heating can damage its structure, so do not extrapolate a simple temperature-rate trend without considering the stated catalyst and conditions.
Pause and explain
A catalyst makes a reaction faster. What can be concluded about final product amount?
Worked example
Equal masses of the same carbonate react fully with excess acid, one as large chips and one as small chips. Compare initial rate and final gas volume, with other conditions controlled.
Show the worked solution
- Small chips have more exposed surface area for the same solid mass.
- They usually produce gas faster initially than the large chips.
- Equal amounts of the limiting carbonate form equal final gas amounts if both reactions complete with acid still in excess.
Answer Small chips: faster initial rate; equal final gas volume under the stated conditions.
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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.
Predict the usual rate changes when a solution reactant is more concentrated and when reacting gases are compressed at the same temperature. Name one condition to control in each comparison.
Give me a hint
Treat solution concentration and gas pressure as separate experiments.
Compare my reasoning
- Increasing reactant concentration usually increases the rate; temperature should be controlled.
- Increasing reacting-gas pressure usually increases the rate at the same temperature; compare the same reaction and catalyst conditions.
- Both predictions concern speed; extra information is needed to compare total product amounts.
Usually faster for greater concentration or reacting-gas pressure, with other relevant conditions controlled.
Look for these in your work
- Both stated factors have a clear rate direction.
- Controlled conditions and final-amount limits are included.
Compare the usual effects of higher temperature, smaller solid pieces and adding a suitable catalyst on rate. State the catalyst's condition at the end.
Give me a hint
Give one rate direction for each change rather than one vague claim about energy.
Compare my reasoning
- Higher temperature usually speeds the reaction over the stated range.
- Smaller pieces increase exposed surface area and usually speed reaction; a suitable catalyst also increases rate.
- The catalyst is unchanged overall at the end of the reaction.
All three usually increase rate in the stated suitable conditions; the catalyst remains unchanged overall.
Look for these in your work
- Temperature, surface area and catalyst effects are distinguished.
- Catalyst regeneration is stated.
Two equal volumes of acid have different concentrations and react with the same carbonate amount. Explain why you cannot assume the same final gas volume unless you know which reactant limits.
Give me a hint
Equal volume with different concentration gives different acid amounts.
Compare my reasoning
- The more concentrated equal-volume acid contains more acid solute.
- One solution might contain insufficient acid while the other is in excess.
- Final gas amounts depend on the limiting reactant; rate and final amount therefore need separate evidence.
Different acid amounts can change the limiting reactant, so equal final gas volume is not guaranteed.
Look for these in your work
- The concentration-to-amount connection is explained.
- The limiting-reactant condition is checked.
Common mistakes
- Confusing faster production with more final product.
- Calling a catalyst unchanged at every intermediate step instead of unchanged overall.
- Applying a gas-pressure trend to a solid's amount without a gas reactant.
- Assuming equal solution volumes contain equal amounts when concentrations differ.
What can you explain now?
What happens to a catalyst overall at the end of a reaction it speeds up?
Compare with the explanation
It is unchanged overall.
It increases rate and is regenerated; participation in intermediate steps does not mean it is consumed overall.
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Make it stick
List the five syllabus rate factors and their usual effects. Add one example where the rate changes but final amount does not, with the necessary conditions stated.
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.