Identify a catalyst from reaction-rate evidence
Explain faster product formation without treating the catalyst as a consumed reactant.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall protein building blocks and the distinction between a molecule and a living cell.
- Use the supplied observations, quantities and units rather than assuming an optimum for every enzyme.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which quantity describes product-formation rate?
What you will learn
- Explain faster product formation without treating the catalyst as a consumed reactant.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Compare reactions fairly
A catalyst increases the rate of a chemical reaction and is not changed by that reaction. Compare catalysed and uncatalysed reactions under matched conditions: the same starting reactants, quantities and temperature. A faster reaction makes a specified product amount in less time, or more product in the same initial interval.
A rate comparison needs a defined measurement. Average product-formation rate is product amount divided by elapsed time, with units such as mg/s. Comparing completion times alone is useful only when the same amount and endpoint are being compared.
Separate catalyst from reactant
The catalyst participates in the process but is recovered unchanged overall; reactants are converted into products. An unchanged recovered substance is not automatically a catalyst: there must also be evidence that it speeds the reaction.
Catalysts do not create matter or energy. Faster production does not mean the final product amount must be larger when identical reactant amounts are eventually used up. The supplied comparison concerns reaction rate, not an unlimited product supply.
Sources for this lesson
Pause and explain
Is unchanged recovery alone enough to identify a catalyst?
Worked example
Matched reactions make 20 mg product in 40 s without X and 10 s with X. X is recovered unchanged. What supports calling X a catalyst?
Show the worked solution
- Calculate 20 ÷ 40 = 0.5 mg/s.
- Calculate 20 ÷ 10 = 2.0 mg/s: four times the average rate.
- Combine faster formation with unchanged recovery.
Answer X increases the average rate from 0.5 to 2.0 mg/s and is unchanged overall.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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 makes 12 mg product in 6 s. Calculate its average product-formation rate.
Give me a hint
Divide amount by time.
Compare my reasoning
- 12 ÷ 6 = 2.
- Keep the units mg/s.
2 mg/s.
Look for these in your work
- Divide in the correct direction.
- Include mg/s.
Matched reactions each make 30 mg product, taking 60 s without Y and 15 s with Y. Calculate both average rates and the rate multiplier.
Give me a hint
Use the same product amount for both.
Compare my reasoning
- 30 ÷ 60 = 0.5 mg/s.
- 30 ÷ 15 = 2 mg/s.
- 2 ÷ 0.5 = 4.
0.5 mg/s and 2 mg/s; the average rate is four times greater with Y.
Look for these in your work
- Check both rates.
- Compare rates rather than final amounts.
A solid is recovered unchanged, but matched reactions take identical times to the same endpoint with and without it. Does this evidence demonstrate catalysis?
Give me a hint
Look for the missing rate effect.
Compare my reasoning
- Recovery supports no overall change.
- The comparison shows no rate increase.
- The two conditions together do not establish the catalyst definition.
No; unchanged recovery without increased rate does not demonstrate catalysis.
Look for these in your work
- Use both observations.
- Avoid assuming every unchanged solid is a catalyst.
Common mistakes
- Do not call a consumed reactant the catalyst.
- The same final amount can be produced at different rates.
What can you explain now?
Is unchanged recovery alone enough to identify a catalyst?
Compare with the explanation
No; increased reaction rate must also be shown.
Both defining features are needed.
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Make it stick
Retrieve both defining features of a catalyst and one fair rate comparison.
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.