AP Chemistry Unit 5 Study Notes

AP Chemistry 5.6: Reaction Energy Profiles

Interpret transition states, activation barriers, intermediates, and reaction energy changes.

Aligned to Kinetics from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

Study these notes

Start with each main idea, then follow the indented explanations and worked examples. Try the next calculation before reading its answer.

Organized from the provided Unit 5 study document. Further study: Khan Academy.

Energy Profiles and Mechanisms

  • Each elementary step has its own activation-energy barrier.

    • Therefore a two-step reaction typically has an energy diagram with:
    • two peaks

  • The valleys between peaks represent intermediates.

  • The peaks correspond to transition states.

  • The step with the largest relevant activation-energy barrier is generally the slowest and can act as the rate-determining step.

Transition State

  • The transition state, sometimes represented by a double dagger (‡), is the high-energy arrangement of atoms at the top of an activation-energy barrier.

    • It is extremely unstable and short-lived.
    • On a reaction-energy diagram:
    • reactants → rise to transition state → fall to products

  • The vertical energy difference between reactants and the transition state is the forward activation energy.

Reaction Energy Diagrams

  • A reaction-energy diagram usually plots:

    • Potential Energy
    • on the y-axis
    • and:
    • Reaction Progress / Reaction Coordinate
    • on the x-axis.

  • For a one-step reaction, the graph has one major peak.

  • The activation energy of the forward reaction is:

    • Eₐ(forward) = energy of transition state − energy of reactants

  • For the reverse reaction:

    • Eₐ(reverse) = energy of transition state − energy of products
Exothermic Reactions

  • For an exothermic reaction, products are lower in energy than reactants.

    • Therefore:
    • ΔE < 0
    • or depending on context:
    • ΔH < 0
    • The reaction releases energy overall.
    • However, it still requires activation energy to get started.
    • In an exothermic energy profile:
    • products are lower than reactants.
Endothermic Reactions

  • For an endothermic reaction, products are higher in energy than reactants.

    • Therefore:
    • ΔE > 0
    • or:
    • ΔH > 0
    • The reaction absorbs energy overall.
Forward and Reverse Activation Energy

  • Suppose a forward reaction has:

    • Eₐ(forward) = 132 kJ/mol
    • and:
    • ΔE = −226 kJ/mol
    • Products are 226 kJ/mol below reactants.
    • To go backward, particles must climb from the lower-energy products to the same transition state.
    • Therefore:
    • Eₐ(reverse) = 132 + 226
    • Eₐ(reverse) = 358 kJ/mol
    • This relationship can also be understood from the energy diagram instead of memorizing a formula.
Multistep Energy Profiles

  • A multistep reaction has multiple peaks and valleys.

    • For example, a three-step reaction generally has:
    • three transition-state peaks
    • and intermediate valleys between them.

  • Each step has its own activation energy.

  • The step with the largest relevant activation-energy barrier is generally the slowest step and can be the rate-determining step.

  • The overall energy change still depends only on the energy difference between the initial reactants and final products.

Intermediates on an Energy Diagram

  • Intermediates correspond to local minima between transition states.

    • Suppose the graph looks like:
    • Reactants → Peak 1 → Valley → Peak 2 → Products
    • The valley between Peak 1 and Peak 2 represents an intermediate.
    • Peak 1 and Peak 2 represent transition states.

  • The intermediate has a real, though often short, lifetime. A transition state is the extremely unstable configuration at the top of an energy barrier.