Reaction Energy Profile
Read forward and reverse barriers separately from endpoint energy change; the coordinate is not time.
AP Chemistry Topic 5.6 · CED printed p. 100 / PDF p. 107. 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
An elementary chemical step rearranges bonds. Breaking interactions costs energy and forming interactions releases it; these contributions overlap along an actual pathway rather than occurring as two isolated laboratory stages. The energy profile summarizes the energetic route to a rearranged product configuration.
The horizontal reaction coordinate represents progress through the set of particle motions involved in rearrangement. It is not elapsed time and its spacing does not tell you how long a configuration persists. The vertical axis is an energy measure; labeled numerical profiles should include energy units. Reactants and products are endpoint configurations, and a maximum marks the transition-state configuration.
The forward activation energy is Etransition −Ereactants. The reverse barrier is Etransition −Eproducts. The overall energy change is Eproducts −Ereactants. An exothermic step can still have a large forward barrier and therefore proceed slowly. Draw barrier arrows from the correct starting level, not from a convenient zero on the page.
For a fixed elementary pathway, increasing temperature increases the fraction of collisions that can reach the transition-state region, so k increases. The Arrhenius relation expresses this dependence qualitatively: higher T and lower activation barrier favor larger k under the model’s assumptions. AP assessment here does not require Arrhenius arithmetic. A catalyst can change the route and barrier while preserving the energies of the same initial and final species. The transition state is a high-energy configuration along a step, not a stable intermediate that accumulates in a valley.
Let's work through it
An elementary-step energy profile places reactants at 25 kJ mol−1, the transition-state peak at 95 kJ mol−1, and products at 5 kJ mol−1, relative to the same reference.
- Forward Ea =95 −25 =70 kJ mol−1.
- Reverse Ea =95 −5 =90 kJ mol−1; ΔE =5 −25 =−20 kJ mol−1.
- The profile is exothermic, but product formation still requires crossing the 70 kJ mol−1 forward barrier. Heating changes the energetic collision fraction; it does not change ΔE or the drawn barrier for this same idealized route.
For a fixed elementary pathway, the reactant and peak energies stay the same in the qualitative model while temperature rises.
- The activation barrier remains peak minus reactant energy.
- The energy distribution shifts so a larger fraction of collisions can reach the transition state.
- Thus k and rate generally rise at fixed reactant concentrations; no Arrhenius numerical calculation is needed.
In the schematic elementary event A–B +C–D →A–C +B–D, compare the starting bonded pairs with the final pairs.
- The old A–B and C–D interactions must be reorganized; new A–C and B–D bonds form. The elemental/chemical-unit inventory stays fixed.
- Breaking the old attractions requires input. Formation of the new attractions releases energy. These changes can occur together along a transition-state pathway, not necessarily as separated stages.
- An endothermic endpoint difference means the net input is larger than the formation release under the modeled conditions. The activation barrier measures the additional rise from reactants to the transition-state configuration.
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
Why can an exothermic reaction be slow?
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
Why can an exothermic reaction be slow?
Worked answer and reason
Its products can be lower in energy while the route still has a high activation barrier.
Overall energy change and pathway barrier answer different questions.
Retrieval 2
Is reaction coordinate a clock?
Worked answer and reason
No. It labels progression through molecular rearrangement configurations; horizontal spacing is not elapsed time.
A pathway diagram does not encode time directly.
Catch the tempting mistake
- Ea is peak minus zero.
- Subtract the energy of the reactant configuration for that direction.
- A peak is an intermediate.
- A peak is a transition state; an intermediate is associated with a local minimum between steps.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 13 · ap application · multiple
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For the teacher
- Have students annotate forward and reverse barriers separately.
- Keep Arrhenius numerical fitting out of assessed practice in this AP edition.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.6.A
- Represent the activation energy and overall energy change in an elementary reaction using a reaction energy profile.
- 5.6.A.1
- Elementary reactions typically involve the breaking of some bonds and the forming of new ones.
- 5.6.A.2
- The reaction coordinate is the axis along which the complex set of motions involved in rearranging reactants to form products can be plotted.
- 5.6.A.3
- The energy profile gives the energy along the reaction coordinate, which typically proceeds from reactants, through a transition state, to products. The energy difference between the reactants and the transition state is the activation energy for the forward reaction.
- 5.6.A.4
- The rate of an elementary reaction is temperature dependent because the proportion of particle collisions that are energetic enough to reach the transition state varies with temperature. The Arrhenius equation relates the temperature dependence of the rate of an elementary reaction to the activation energy needed by molecular collisions to reach the transition state.
- Calculations involving the Arrhenius equation will not be assessed on the AP Exam.
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 · 6 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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Activation energy
Activation energy is the energy barrier associated with a reaction pathway in an introductory energy-profile model. It concerns the pathway and is distinct from the reactant-to-product energy change. For example, a thermodynamically favored reaction can remain slow when its activation barrier is large.
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Exothermic process
An exothermic process transfers heat from the chosen system to its surroundings. Under constant-pressure conditions with only pressure–volume work, its enthalpy change is negative. For example, freezing water releases heat to the surroundings.
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Catalyst
A catalyst participates in a reaction pathway and is regenerated overall. It can increase the reaction rate by providing a different pathway without changing the reaction's equilibrium constant at fixed temperature. For example, a catalyst can help a mixture reach equilibrium sooner without changing the final equilibrium composition.
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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.