Knowledge Organiser: Bond Energies
Key Terms
- Bond energy — energy to break 1 mol of a bond (kJ/mol)
- ΔH — energy in (breaking) minus energy out (making)
- Breaking bonds — endothermic (requires energy)
- Making bonds — exothermic (releases energy)
Must-Know Facts
- Breaking bonds ALWAYS requires energy (endothermic)
- Making bonds ALWAYS releases energy (exothermic)
- ΔH = Σ(breaking) − Σ(making)
- Negative ΔH → exothermic; Positive ΔH → endothermic
- Double/triple bonds have higher bond energies
- Count ALL bonds × stoichiometric coefficients
Key Equations
- ΔH = Σ(bond energies broken) − Σ(bond energies formed)
- Negative result → exothermic; Positive result → endothermic
- Bond energy units: kJ/mol (energy to break 1 mole of that bond)
Common Mistakes
- Doing the subtraction backwards: ΔH = bonds BROKEN minus bonds FORMED — not the other way round; getting this backwards flips the sign and the answer
- Forgetting to multiply bond energies by coefficients: In CH₄ + 2O₂ → CO₂ + 2H₂O, there are 4 C-H bonds and 2 O=O bonds to break — always count every bond in every molecule
- Saying bond making requires energy: Bond BREAKING requires energy; bond MAKING releases energy — the direction always trips students up
- Using bond energies for ionic compounds: Bond energy calculations only apply to covalent bonds — lattice energy applies to ionic compounds (A-level only)
Memory Aid
- Breaking = Bad (needs energy), Making = Magic (releases energy)
- This is the most important thing to remember for bond energy questions. Break = ENDO (energy in). Make = EXO (energy out).
- The formula: "IN minus OUT" = ΔH
- ΔH = energy IN (breaking) − energy OUT (making)
- If ΔH is negative → more OUT than IN → exothermic (more energy released than absorbed)
- If ΔH is positive → more IN than OUT → endothermic (more energy absorbed than released)
- Double check rule: "Negative ΔH = nice and warm (exothermic). Positive ΔH = pulls energy from surroundings (endothermic)."