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5.2.11Reactivity 3 — What are the mechanisms of chemical change?

Reaction Mechanisms (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 13 of 13 Unit quiz

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 4Mechanism Vocabulary

5.2.11 Reaction Mechanisms (HL)

A mechanism is a proposal consistent with the rate equation, not a proven fact.

Elementary step
  • A single molecular event in a mechanism, with a rate equation that follows directly from its reactants.
Intermediate
  • A species formed in one step and used up in a later step, so it does not appear in the overall equation.
Rate-determining step
  • The slowest step in the mechanism, which controls how fast the overall reaction can occur.
Overall equation
  • The sum of all elementary steps after cancelling intermediates.

Part 2 of 4From Mechanism to Rate Equation

1
Write the steps of the proposed mechanism and mark which one is slow.
2
The rate equation includes the reactants in the slow step, each raised to its coefficient in that step.
3
If an intermediate is in the slow step, express it through earlier fast steps in terms of reactants.
4
Add the steps and cancel intermediates to check that the sum matches the overall equation.
5
Compare the derived rate equation with the experimental one to judge whether the mechanism is plausible.

Part 3 of 4Example: NO₂ and CO

Example: NO₂ and CO
Overall
  • NO₂(g) + CO(g) → NO(g) + CO₂(g), but the experimental rate equation is rate = k[NO₂]².
Step 1 (slow)
  • NO₂ + NO₂ → NO₃ + NO, so only NO₂ appears in the rate equation, squared.
Step 2 (fast)
  • NO₃ + CO → NO₂ + CO₂, which does not affect the overall rate.
Intermediate
  • NO₃ is formed in step 1 and consumed in step 2, so it cancels from the overall equation.

Part 4 of 4Exam Checklist & Tips

5.2.11 – Key Points for the Exam

  • Mechanism The sequence of elementary steps by which reactants become products.
  • Intermediate A species formed and consumed during the mechanism, not present in the overall equation.
  • Rate-determining step The slowest elementary step in a mechanism.
  • Elementary step One simple step in which bonds break or form.

Common Mistakes

  • Including a species that appears only in a fast step in the rate equation.
  • Calling an intermediate a catalyst; a catalyst is regenerated and appears in the starting species.
  • Writing the overall equation without cancelling intermediates.
  • Believing that the balanced equation tells us the mechanism.

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