5.7 Mechanisms

Reaction Mechanisms

Step through 4 multi-step mechanisms with species pills and an energy profile. Intermediates and catalysts are identified, and the rate law is derived from the slow step.

Energy ProfileRate-Determining StepIntermediatesRate Law Derivation
Topic 5.7

Introduction to Reaction Mechanisms

Identify the components of a reaction mechanism.

A reaction mechanism is a series of elementary reactions, or steps, that occur in sequence. Its components may include reactants, intermediates, products, and catalysts.

Two validity requirements:

  1. The elementary steps, when combined, must align with the overall balanced equation.
  2. The mechanism must predict the experimentally observed rate law (see 5.8).

A mechanism can never be proven — only supported, or ruled out.

Intermediate: a species produced by some elementary steps and consumed by others, so it is present only while the reaction is occurring. It appears as a product first, then a reactant, and cancels out when the steps are summed. It does not appear in the overall equation.

Catalyst: the mirror image. It appears as a reactant first, then a product. It also cancels from the overall equation, but its net concentration is unchanged (see 5.11).

Telling them apart is a matter of which comes first:

  • Consumed then regenerated → catalyst
  • Produced then consumed → intermediate

EK 5.7.A.4 notes that experimental detection of a reaction intermediate is a common way to build evidence supporting one mechanism over an alternative — though the CED excludes assessing the data-collection techniques themselves.

Key points

  • Steps must sum to the overall equation and reproduce the observed rate law.
  • Intermediate: produced first, consumed later. Catalyst: consumed first, regenerated later.
  • Neither appears in the overall balanced equation.
  • An intermediate is a valley on the energy profile; a transition state is a peak.

Common mistakes

  • Confusing intermediate with transition state. An intermediate is a real molecule with a lifetime; a transition state is not.
  • Including an intermediate in the overall equation. It cancels.
  • Assuming a mechanism is proven because it sums correctly. It must also match the rate law.
  • Missing a catalyst because it cancels — check whether any species appears as a reactant in an early step and a product later.

Worked example

For the mechanism below, determine the overall equation and identify any intermediates and catalysts. Step 1: NO₂ + NO₂ → NO₃ + NO Step 2: NO₃ + CO → NO₂ + CO₂

Sum the steps:
NO₂ + NO₂ + NO₃ + CO → NO₃ + NO + NO₂ + CO₂

Cancel species on both sides: NO₃ appears on both sides and cancels completely. One NO₂ appears on both sides and cancels, leaving one NO₂ on the left.

Overall: NO₂ + CO → NO + CO₂

Intermediate: NO₃. It is produced in step 1 and consumed in step 2 — produced first, so it is an intermediate.

Catalyst: none. Although NO₂ cancels once, it is a genuine reactant in the overall equation (one NO₂ is consumed net), so it is not regenerated in the catalytic sense.

Full notes for topic 5.7 →