Reaction profiles
A reaction profile is a graph showing the relative energies of reactants, transition state and products through the course of a reaction. It tells you at a glance:
- Whether the reaction is exo- or endothermic.
- The activation energy Ea — the energy "hill" reactants must climb.
- The overall energy change ΔE.
Exothermic profile
- Reactants on the left, higher energy.
- Products on the right, lower energy.
- A "hump" between them = transition state.
- Activation energy Ea = height of hump above reactants.
- Overall energy change = energy(products) − energy(reactants), which is negative for exothermic.
Endothermic profile
- Reactants on the left, lower energy.
- Products on the right, higher energy.
- Hump in between (transition state).
- Ea = height of hump above reactants.
- Overall energy change = positive for endothermic.
Activation energy
The minimum energy colliding particles must have to react. Particles below Ea bounce off harmlessly. Increasing temperature gives more particles enough energy to react (C6.2).
Drawing a profile
- y-axis: energy. x-axis: progress of reaction.
- Mark reactants and products at appropriate heights.
- Draw a curve up to the transition state, then down to products.
- Label Ea (reactants → top of hump).
- Label overall energy change (reactants → products, with sign).
✦Worked example
For 2H₂O₂ → 2H₂O + O₂ (exothermic), draw the profile:
- Reactants H₂O₂ at high energy.
- Curve rises to the transition state (Ea = +75 kJ/mol roughly).
- Curve falls to products at much lower energy.
- ΔE = −196 kJ/mol (negative).
Catalysts on profiles
A catalyst lowers Ea by providing an alternative pathway. The product and reactant energies are unchanged — only the hump is lower.
A second curve drawn lower than the first shows the catalysed pathway.
⚠Common mistakes
- Drawing reactants and products at the same height for exothermic — they aren't.
- Saying Ea is between products and transition state — Ea is from reactants to the transition state.
- Forgetting Ea applies in both directions — for an exo reaction, the reverse Ea is bigger.
- Drawing a catalyst as moving the products — only the activation barrier shifts.
Links
Builds on C5.1. Sets up C5.3 (bond energies HT) and C6.2 (effect of temperature on rate).
AI-generated · claude-opus-4-7 · v3-deep-chemistry