4.1.7Unit 4. Rates, Equilibria and Further Organic Chemistry
Rate-Determining Steps from Equations — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 16 of 56 Unit quiz
Section 1 of 6The Rate-Determining Step
Many reactions happen in several steps, but only one step sets the pace.
Definition
- The rate-determining step is the slowest step in a multi-step mechanism and it controls the overall rate.
Analogy
- Water leaving a funnel cannot flow faster than its narrowest part, just as the whole reaction cannot go faster than its slowest step.
Fast steps
- Steps after the slow step do not affect the rate and their species do not appear in the rate equation.
Molecularity
- The number of particles colliding in one step is usually one or two, rarely three.
Section 2 of 6Reading the Rate Equation
Reading the Rate Equation
Appears in equation
- A species in the rate equation is present in the rate-determining step or in an earlier fast step.
Order links to number
- If the order is two in a species, two particles of it are involved up to and including the slow step.
Missing species
- A reactant absent from the rate equation is zero order and takes part only after the slow step.
Catalysts
- A catalyst that appears in the rate equation is involved in the slow step even though it is not in the overall equation.
| Rate equation | Species in rate-determining step |
|---|---|
| rate = k[A] | One A |
| rate = k[A][B] | One A and one B |
| rate = k[A]² | Two A |
| rate = k[A][B]² | One A and two B |
Section 3 of 6Testing a Proposed Mechanism
1
Write down the steps of the proposed mechanism and mark which one is slow.
2
Add all the steps together and cancel any species that appears on both sides.
3
Check that the sum is the same as the overall balanced equation.
4
Check that the species in the slow step match the species and orders in the experimental rate equation.
5
If both conditions are met the mechanism is consistent with the evidence, although it is not proved.
Section 4 of 6Intermediates
Definition
- An intermediate is formed in one step and used up in a later step, so it does not appear in the overall equation.
Not in rate equation
- Intermediates are normally eliminated from the final rate equation by relating them to reactants.
Contrast with catalyst
- A catalyst is used and then regenerated, but it is present at the start of the reaction as well.
Evidence
- Intermediates are rarely isolated, but their detection by spectroscopy supports a mechanism.
Section 5 of 6Worked Example
Question
- For NO₂(g) + CO(g) → NO(g) + CO₂(g) the experimental rate equation is rate = k[NO₂]².
- A mechanism is proposed: step 1 (slow) 2NO₂ → NO + NO₃;
- step 2 (fast) NO₃ + CO → NO₂ + CO₂.
- Is it consistent?
- Add the steps
- 2NO₂ + NO₃ + CO → NO + NO₃ + NO₂ + CO₂;
- cancel NO₃ and one NO₂ to give NO₂ + CO → NO + CO₂, which matches the overall equation.
- 2NO₂ + NO₃ + CO → NO + NO₃ + NO₂ + CO₂;
- The slow step contains two NO₂ and no CO, so the predicted rate equation is rate = k[NO₂]².
- This matches experiment, and NO₃ is an intermediate.
Answer The mechanism is consistent with the rate equation and the overall equation
Section 6 of 6Exam Checklist & Tips
4.1.7 – Key Points for the Exam
- Rate-determining step The slowest step in a reaction mechanism, which controls the overall rate.
- Intermediate A species formed in one step and consumed in a later one.
- Mechanism The sequence of steps by which reactants are converted into products.
- Molecularity The number of particles reacting together in a single step.
- Overall equation The balanced equation obtained by adding all the steps.
Common Mistakes
- Writing the rate equation from the coefficients of the overall equation rather than from experiment.
- Including an intermediate in the final overall equation.
- Saying a mechanism is proved because it matches one rate equation.
- Putting a species in the rate-determining step when it is zero order.
Last updated:
\n