Bond-Breaking & Bond-Forming
Bond breaking = endothermic (energy absorbed to overcome electrostatic attraction)
Bond forming = exothermic (energy released as new attractions form)
The net ΔH = energy to break reactant bonds + energy released forming product bonds.
Exothermic vs Endothermic
| Exothermic | Endothermic | |
|---|---|---|
| Energy flow | System → Surroundings | Surroundings → System |
| ΔH sign | Negative (ΔH < 0) | Positive (ΔH > 0) |
| Temperature | Rises | Falls |
| Product stability | Products more stable | Reactants more stable |
| Examples | Combustion, neutralisation | Thermal decomposition, photosynthesis |
Energy Profile Diagrams
Energy Profile Diagrams
📐 Reading the Diagram
- ΔH = net vertical difference between reactants and products
- Ea = vertical distance from reactants to the transition state (peak)
- Exothermic: products sit lower than reactants
- Endothermic: products sit higher than reactants
⚠️ Examiner Trap
Do not confuse ΔH with Ea on the diagram. ΔH is the difference between start and end (reactants → products). Ea is the height of the activation barrier above the reactants.
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