Bond Energies and Calculating Energy Changes — AQA GCSE Chemistry (Higher Tier)
The overall energy change of a reaction can be calculated from the energy needed to break and make bonds.
Breaking and making bonds
- Breaking bonds is endothermic — energy must be supplied (taken in).
- Making bonds is exothermic — energy is released.
Calculating the energy change
$$\Delta E = (\text{energy to break bonds}) - (\text{energy released making bonds})$$
- If more energy is released making bonds than is used breaking them → the reaction is exothermic (ΔE is negative).
- If more energy is needed to break bonds than is released → the reaction is endothermic (ΔE is positive).
Worked method
- Add up the bond energies of all the bonds broken in the reactants.
- Add up the bond energies of all the bonds made in the products.
- Subtract: energy in (broken) − energy out (made).
Example outline (H₂ + Cl₂ → 2HCl)
- Bonds broken: 1 H–H + 1 Cl–Cl.
- Bonds made: 2 H–Cl.
- A negative answer means exothermic.
Exam tips
- Breaking = endothermic (take in); making = exothermic (give out) — "break = take, make = give".
- ΔE = energy to break − energy released making.
- A negative ΔE means exothermic; a positive ΔE means endothermic.
- Count every bond carefully, including all bonds in larger molecules.