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.
System vs Surroundings
In chemistry, the system is the chemical reaction itself (the reactants and products). The surroundings are everything else, including the solvent, container, and air. Energy is transferred between the system and the surroundings, but the total energy is always conserved.
📘 Heat vs Temperature
Temperature is a measure of the average kinetic energy of particles. It tells you how hot something is.
Heat is the transfer of energy between objects at different temperatures. It is measured in joules (J) or kilojoules (kJ).
A large body of water and a small cup of water at the same temperature have the same average kinetic energy, but the large body contains more heat energy overall.
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