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5.10Kinetics

Multistep Reaction Energy Profile — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 14 1 quiz

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Lesson summary

5.10 Multistep Reaction Energy Profile — AP CHEMISTRY notes: Read a multistep energy profile to find steps, intermediates, activation energies, the rate-limiting step and overall ΔH.Peaks — each peak is a transition state, so the number of peaks equals the number of elementary st…

  • Three peaks and two troughs
  • Products above reactants
  • Largest rise from trough to peak
  • Multistep profile
  • Enthalpy change (ΔH)
  • Peaks — each peak is a transition state, so the number of peaks equals the number of elementary steps

Section 1 of 8Features of a multistep profile

1Peaks
  • Peaks each peak is a transition state, so the number of peaks equals the number of elementary steps
2Troughs
  • Troughs each trough between peaks is an intermediate
3Overall ΔH
  • Overall ΔH the energy of the products minus the energy of the reactants

Section 2 of 8Activation energy of each step

1Step Ea
  • Step Ea measured from the energy just before the step up to its peak
    • (reactant or intermediate)
2Rate-limiting step
  • Rate-limiting step the step with the largest activation energy
  • It is not necessarily the step with the highest peak overall.

Section 3 of 8Exothermic or endothermic overall

ObservationConclusion
Products lower than reactantsExothermic, ΔH negative
Products higher than reactantsEndothermic, ΔH positive

Section 4 of 8Reading an example

Three peaks and two troughs
  • Three peaks and two troughs a three-step mechanism with two intermediates
Products above reactants
  • Products above reactants overall endothermic
Largest rise from trough to peak
  • Largest rise from trough to peak that step is rate-limiting

Section 5 of 8Links to rate laws

1Fast step
  • Fast step low barrier
2Slow step
  • Slow step high barrier, and it sets the rate law of the overall reaction

Section 6 of 8More from the specification

More from the specification
Pre-equilibrium approximation
  • Assumption that a fast reversible step is at equilibrium.
Chemical equation
  • A statement using formulas to show reactants and products.
Approximation
  • A value close to but not exactly the true value.

Section 7 of 8Key terms

TermMeaning
Multistep profileEnergy profile of a mechanism with several elementary steps.
PeakTransition state of an elementary step.
TroughIntermediate between two steps.
Rate-limiting stepStep with the largest activation energy.
Enthalpy change (ΔH)Product energy minus reactant energy.

Section 8 of 8Exam technique

Avoid these mistakes
Choosing the highest peak as the slow step — compare the energy rise of each step from its starting level.
Counting peaks as intermediates — peaks are transition states and troughs are intermediates.
Reading ΔH from a peak — ΔH is the product level minus the reactant level.

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