Multistep Reaction Energy Profile
Measure each local multistep barrier from its starting valley rather than an arbitrary zero.
AP Chemistry Topic 5.10 · CED printed p. 104 / PDF p. 111. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Balanced equations and stoichiometric ratios
- Molarity and concentration versus amount
- Bond rearrangement and energy conservation
- Slopes, logarithms and reciprocal graphs
A multistep profile has one peak for each elementary transition state and valleys for intermediate configurations. Build it from the energy levels of reactants, each intermediate and products. For each forward step, measure the barrier from that step’s starting valley to its next peak. The tallest peak measured from an arbitrary zero is not necessarily the largest local barrier. The overall energy change still compares final products to initial reactants; intermediate peaks do not enter that subtraction. A large local barrier often suggests a slow step under comparable prefactors and conditions, but actual rate control also depends on concentrations and reversibility. Do not read elapsed time from horizontal widths. A catalyst can create a different number of steps and intermediate valleys, so its route need not be the same curve with every peak simply pushed downward.
Let's work through it
A two-step profile has energies: reactants 0, TS1 45, intermediate −20, TS2 35, products −30 kJ mol−1.
- Step 1 forward barrier is 45 −0 =45 kJ mol−1. Step 2 forward barrier is 35 −(−20) =55 kJ mol−1.
- TS2 is lower in absolute height, but step 2 has the larger local forward barrier. Under comparable kinetic conditions it can be slower.
- Overall ΔE =−30 −0 =−30 kJ mol−1. The intermediate at −20 is a valley, not a transition state.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Can the highest plotted peak alone identify the slowest step?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Can the highest plotted peak alone identify the slowest step?
Worked answer and reason
No. Compare each peak to its preceding local minimum, then consider kinetic conditions.
A local activation barrier uses its own starting configuration.
Retrieval 2
What two levels determine the overall energy change?
Worked answer and reason
Initial reactants and final products, regardless of the number or height of intermediate peaks.
Intermediate energetic steps sum to the endpoint difference.
Catch the tempting mistake
- Add peak heights to find overall ΔE.
- Subtract endpoint energies; peaks describe barriers rather than net energy change.
- Every valley is a product of the whole reaction.
- A valley can be an intermediate consumed by a later step.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 18 · ap synthesis · multiple
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For the teacher
- Use a lower absolute second peak with a larger local barrier to expose the common error.
- Have students label intermediate versus transition states on the same diagram.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.10.A
- Represent the activation energy and overall energy change in a multistep reaction with a reaction energy profile.
- 5.10.A.1
- Knowledge of the energetics of each elementary reaction in a mechanism allows for the construction of an energy profile for a multistep reaction.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Collision Theory — Scientific check: collision orientation, energetic fraction and elementary activation barriers. Arrhenius arithmetic is excluded from AP assessment.
- OpenStax Chemistry 2e: Reaction Mechanisms — Scientific check: intermediates, elementary molecularity, slow steps and rapid-equilibrium substitution.
Glossary and deeper reading · 4 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Stoichiometry
Stoichiometry uses a balanced chemical equation to relate amounts of reactants and products. Equation coefficients provide mole ratios rather than direct mass ratios. For example, forming water from hydrogen and oxygen consumes two moles of hydrogen molecules per mole of oxygen molecules.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Transition state
A transition state is the high-energy configuration at a barrier along a reaction pathway. It is not a stable intermediate that accumulates as a separately observable species in the basic model. For example, the top of an elementary step's energy-profile peak represents its transition state.
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Reaction intermediate
A reaction intermediate is produced in one mechanism step and consumed in another. It cancels from the overall equation and differs from a catalyst, which is present before it is consumed and then regenerated. For example, a short-lived species formed in step one and used in step two can be an intermediate.