Identify a rate-determining step
Use a measured rate law and a proposed mechanism to identify which step controls the reaction rate.
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 evidence helps identify the rate-determining step?
What you will learn
- Compare rate-law orders with species in mechanism steps
- Recognise a slow step that predicts the observed rate dependence
- State when available data are insufficient to identify a unique step
The rate-determining step must explain the measured law
For a mechanism with an elementary slow step, the concentrations of its reactants determine the immediate rate form. If it contains an intermediate, use a preceding fast equilibrium to express that intermediate in measurable reactants.
Compare the resulting orders with the experimental rate equation. A step that predicts the measured law can be identified as rate-determining within the proposed mechanism.
Evidence may not identify one unique mechanism
Several mechanisms can sometimes fit the same rate equation. If the data do not distinguish them, state that the evidence supports a rate dependence but does not prove one unique sequence.
A catalyst mechanism or balanced overall equation alone does not identify the rate-determining step; kinetic evidence is needed.
Pause and explain
If a proposed slow step predicts a different order from experiment, what follows?
Worked example
A mechanism has fast A + B ⇌ I, then slow I + B → P. Experiments show rate = k[A][B]². Which step is consistent as rate-determining?
Show the worked solution
- Use the slow step to write rate∝[I][B].
- Use the fast equilibrium [I]∝[A][B].
- Compare the derived expression with experiment.
Answer The slow I+B step predicts rate∝[A][B]² after substituting the fast equilibrium, so it is consistent with the rate-determining step for the measured law.
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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 slow elementary step is A + 2B → products. What rate form does this step predict, and what caveat applies?
Give me a hint
Use reactant coefficients only for this stated elementary step.
Compare my reasoning
- Write rate∝[A][B]².
- Compare it with measured orders.
- Remember the expression is justified by the elementary-step assumption.
The step predicts rate∝[A][B]². It is a valid step-level prediction because the step is specified as elementary; it must still agree with experiment.
Look for these in your work
- Molecularity interpreted
- Orders stated
- Assumption qualified
A measured rate law is first order in A. A proposed mechanism's slow step, after equilibrium substitution, predicts second order in A. Can that mechanism explain the data?
Give me a hint
Compare the two A exponents.
Compare my reasoning
- Measured order is 1.
- Predicted order is 2.
- State whether the proposed step fits.
No. The proposed mechanism predicts second order in A, which conflicts with the measured first-order dependence.
Look for these in your work
- Data compared
- Mismatch identified
- Conclusion stated
Two different proposed mechanisms both reproduce the same measured rate law. What can be concluded about the rate-determining step?
Give me a hint
Rate-law consistency is necessary evidence but may not be unique.
Compare my reasoning
- Both proposals are consistent with this kinetic evidence.
- The rate law alone does not select one uniquely.
- Additional evidence is needed to distinguish them.
The measured law does not distinguish the two mechanisms by itself. Additional experimental or mechanistic evidence is needed before claiming a unique rate-determining sequence.
Look for these in your work
- Evidence scope understood
- Uniqueness not overclaimed
- Need for discriminating data stated
Common mistakes
- Selecting the slowest-looking step without comparing orders
- Leaving intermediate concentrations in the predicted law
- Claiming a unique mechanism from insufficient evidence
- Using an overall reaction coefficient as an order
What can you explain now?
What evidence helps identify the rate-determining step?
Compare with the explanation
Agreement between its predicted and measured rate law
The rate dependence tests the proposed slow step.
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Make it stick
How do predicted and measured rate laws test a proposed rate-determining step, and what can the data not prove?
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.