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Two-step reaction energy profile with an intermediate

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Reaction energy profile of a two-step mechanism: two transition states with an intermediate in the dip between them, each step’s Ea marked, and ΔH
The energy profile of a reaction that happens in two steps. Each step has its own transition state, a peak, at 120 and 100 kJ/mol, with an intermediate in the dip between them at 70 kJ/mol. Each step’s activation energy runs up from the level before it, so the first step, with the larger activation energy, is the rate-determining step. ΔH runs from the reactants to the products.
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Answer key

  • ΔH = −30 kJ/mol: exothermic
  • Step 1: Eₐ = 80 kJ/mol (reverse 50 kJ/mol)
  • Intermediate at 70 kJ/mol
  • Step 2: Eₐ = 30 kJ/mol (reverse 90 kJ/mol)
  • Rate-determining step: step 1, with the highest activation energy

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