5.6Kinetics
Reaction Energy Profile — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 14 1 quiz
Lesson summary
5.6 Reaction Energy Profile — AP CHEMISTRY notes: Interpret an energy profile: activation energy, enthalpy change, transition state and the effect of Ea on rate.Activated complex (transition state) — an unstable high-energy arrangement formed at the top of the energy barrierOld b…
- Why Ea is small
- Activation energy Ea
- Enthalpy change ΔH
- Reverse Ea
- Rate and ΔH
- Enthalpy change (ΔH)
Section 1 of 8Transition state theory
1Activated complex (transition state)
- Activated complex (transition state) an unstable high-energy arrangement formed at the top of the energy barrier
- Old bonds are partly broken and new bonds are partly formed.
- It cannot be isolated.
2Why Ea is small
- Why Ea is small only enough energy to weaken bonds is needed, not enough to break them completely
Section 2 of 8Parts of an energy profile
Parts of an energy profile
Axes
- Axes potential energy on the y-axis and progress of the reaction on the x-axis
- (reaction coordinate)
Activation energy Ea
- Activation energy Ea the energy from the reactants to the top of the peak
Enthalpy change ΔH
- Enthalpy change ΔH the energy difference between products and reactants
Peak
- Peak represents the transition state or activated complex
Section 3 of 8Exothermic and endothermic profiles
| Type | Products relative to reactants | Sign of ΔH |
|---|---|---|
| Exothermic | Lower in energy | Negative |
| Endothermic | Higher in energy | Positive |
Section 4 of 8Forward and reverse activation energies
Reverse Ea
- Reverse Ea the energy from the products to the top of the peak
Relation
- Ea(reverse) = Ea(forward) − ΔH
- For CO + NO2 the forward Ea is 134 kJ and ΔH = −226 kJ, so the reverse Ea is 360 kJ.
Section 5 of 8Activation energy and rate
High Ea
- High Ea few particles have enough energy, so the reaction is slow
Low Ea
- Low Ea many particles have enough energy, so the reaction is fast
Rate and ΔH
- Rate and ΔH rate depends on Ea, not on ΔH
- A very exothermic reaction can be slow and an endothermic reaction can be fast.
Section 6 of 8More from the specification
More from the specification
Frequency
- Number of wave cycles per second.
Elementary reaction
- A single molecular event; one step of a mechanism.
Collision model
- Reactions occur when particles collide with enough energy and correct orientation.
Arrhenius equation
- K = Ae^−Ea/RT, relating the rate constant to temperature and activation energy.
Maxwell-Boltzmann distribution
- A graph showing how particle energies are spread at a given temperature.
Carbon monoxide
- A toxic gas that binds to haemoglobin.
Steric factor
- Fraction of collisions with the correct orientation.
Molecule
- Two or more atoms held together by covalent bonds.
Temperature
- A measure of the average kinetic energy of the particles.
Fraction
- A group of hydrocarbons with similar chain length and boiling points.
Theory
- A tested hypothesis that is widely accepted.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Energy profile | Graph of potential energy against reaction progress. |
| Activation energy | Energy barrier between reactants and the transition state. |
| Transition state | Highest-energy arrangement on the pathway; the activated complex. |
| Enthalpy change (ΔH) | Energy difference between products and reactants. |
| Reaction coordinate | The progress of the reaction from reactants to products. |
Section 8 of 8Exam technique
Avoid these mistakes
Saying a more exothermic reaction is faster — rate depends on Ea, not ΔH.
Measuring Ea from the reactants to the products — it is measured from the reactants to the peak.
Treating the transition state as an intermediate — the transition state is a peak and cannot be isolated.
Marked practiceQuizzes for this lesson
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