Exothermic and Endothermic Reactions (C5.1)
Energy transfers in reactions
All chemical reactions involve breaking bonds (requires energy — endothermic step) and forming bonds (releases energy — exothermic step). The overall energy change determines whether the reaction is exothermic or endothermic.
- Exothermic: energy released to surroundings > energy absorbed. Temperature of surroundings RISES. ΔH is negative.
- Endothermic: energy absorbed from surroundings > energy released. Temperature of surroundings FALLS. ΔH is positive.
Examples:
- Exothermic: combustion, respiration, neutralisation, oxidation (rusting), hand warmers.
- Endothermic: photosynthesis, thermal decomposition, sports cold packs, dissolving ammonium nitrate.
Reaction profiles (energy diagrams)
A reaction profile shows the energy changes along the reaction pathway.
Exothermic profile:
- Reactants at higher energy than products.
- The difference = energy released (ΔH negative).
- The peak = transition state; height above reactants = activation energy (Eₐ).
Endothermic profile:
- Products at higher energy than reactants.
- ΔH positive.
- Eₐ = energy needed to start the reaction.
Activation energy (Eₐ): the minimum energy reactant particles must have to react.
A catalyst lowers Eₐ — provides an alternative reaction pathway.
Bond energies
Breaking bonds requires energy (endothermic process, positive value).
Forming bonds releases energy (exothermic process, negative value).
Overall energy change:
ΔH = Σ(bond energies broken) − Σ(bond energies formed)
If ΔH negative → exothermic. If ΔH positive → endothermic.
Worked example: H₂ + Cl₂ → 2HCl
Bond energies (kJ/mol): H–H = 436, Cl–Cl = 242, H–Cl = 431
- Energy to break: 436 + 242 = 678 kJ
- Energy released forming: 2 × 431 = 862 kJ
- ΔH = 678 − 862 = −184 kJ/mol (exothermic) ✓
Common exam errors
- Saying "breaking bonds is exothermic" — it is endothermic. Only forming bonds releases energy.
- Mixing up ΔH sign: exothermic = negative ΔH; endothermic = positive ΔH.
- Forgetting to multiply bond energies by the number of bonds broken/formed.
- Drawing an endothermic profile with products at lower energy than reactants — wrong.
AI-generated · claude-opus-4-7 · v3-deep-combined-science