Identify intermediates and catalysts in mechanisms
Track species through elementary steps to distinguish intermediates from catalysts.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 26. Rate laws are empirical unless an elementary step is explicitly stated. Written practice is self-checked
Before you begin
- Use AS collision theory, concentration, graph gradients, and balanced reaction equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What identifies an intermediate?
What you will learn
- Identify an intermediate as formed then consumed
- Identify a catalyst as used then regenerated
- Check both roles against the net equation
Track appearance across the full sequence
An intermediate is formed in one step and consumed in a later step. A catalyst is consumed or changed in one step and regenerated later, so it is not used up overall.
When mechanism steps are added, intermediates and catalysts cancel. Reactants and products that remain in the net equation are not intermediates.
Use coefficients and charges when cancelling
Add steps with their stoichiometric coefficients, then cancel the same species on opposite sides. A species that remains after cancellation belongs to the overall reaction.
Do not label a substance a catalyst simply because it appears in a mechanism. Regeneration is essential; a species that is produced and then consumed is an intermediate.
Pause and explain
What identifies a catalyst in a mechanism?
Worked example
Step 1: X + C → I. Step 2: I + Y → Z + C. Identify the intermediate and catalyst.
Show the worked solution
- Find the species formed then consumed.
- Find the species consumed then regenerated.
- Add the steps to check cancellation.
Answer I is the intermediate because it forms then reacts. C is the catalyst because it is consumed then regenerated. The net equation is X + Y → Z.
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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.
Step 1: A + C → I. Step 2: I + B → P + C. Identify I and C.
Give me a hint
Check which species cancels and how it is produced or restored.
Compare my reasoning
- I is made then used.
- C is used then regenerated.
- Add steps to obtain A+B→P.
I is an intermediate; C is a catalyst. The net equation is A + B → P.
Look for these in your work
- Intermediate identified
- Catalyst identified
- Net equation checked
A mechanism produces Q in step 1 and consumes Q in step 2. Is Q an intermediate or catalyst based on that information?
Give me a hint
Ask whether Q itself is regenerated or only formed then used.
Compare my reasoning
- Q is produced then consumed.
- No evidence says Q catalyses a step and is regenerated.
- Classify Q as an intermediate.
Q is an intermediate. To identify a catalyst, a species must be used in one step and regenerated in a later step.
Look for these in your work
- Appearance sequence tracked
- Regeneration criterion applied
- Classification justified
When mechanism steps are summed, species C appears on both sides and cancels; I also cancels. Explain why cancellation alone does not distinguish catalyst from intermediate.
Give me a hint
Use the direction in which each species appears in individual steps.
Compare my reasoning
- A catalyst is consumed then regenerated.
- An intermediate is formed then consumed.
- Their step-by-step roles differ even though both cancel overall.
Both cancel from the net equation, but C is a catalyst if it is used then regenerated, whereas I is an intermediate if it is formed then consumed.
Look for these in your work
- Cancellation recognized
- Role sequences differentiated
- Terminology precise
Common mistakes
- Calling a regenerated catalyst an intermediate
- Calling every short-lived species a catalyst
- Ignoring cancellation when steps are added
- Failing to balance the summed mechanism
What can you explain now?
What identifies an intermediate?
Compare with the explanation
It is formed in one step and consumed in a later step
It cancels when the mechanism steps are summed.
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Make it stick
How do step-by-step formation and regeneration distinguish an intermediate from a catalyst?
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.