Breaking and making bonds (Higher)
- During a chemical reaction, energy must be supplied to break bonds in the reactants. Bond breaking is endothermic.
- Energy is released when bonds form in the products. Bond making is exothermic.
- If more energy is released making new bonds than is needed to break the old ones, the reaction is exothermic. If less is released, it is endothermic.
Calculating the energy change (Higher)
- Energy change = energy needed to break bonds − energy released making bonds.
- A negative answer means the reaction is exothermic; a positive answer means it is endothermic.
- Bond energies are measured in kJ/mol.
Worked example (Higher)
- H₂ + Cl₂ → 2HCl. Bond energies: H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol.
- Bonds broken: 436 + 242 = 678 kJ/mol.
- Bonds made: 2 × 431 = 862 kJ/mol.
- Energy change = 678 − 862 = −184 kJ/mol, so the reaction is exothermic.
Tips (Higher)
- Draw the displayed formula of each molecule to count every bond, including double bonds such as O=O and C=O.
- Multiply by the numbers in the balanced equation: 2H₂O contains four O–H bonds.
Key terms
- Bond energy
- The energy needed to break one mole of a particular bond, in kJ/mol.
- Bond breaking
- An endothermic process that needs energy.
- Bond making
- An exothermic process that releases energy.
- Overall energy change
- Energy to break bonds minus energy released making bonds.
- Displayed formula
- A formula that shows every atom and every bond in a molecule.
- kJ/mol
- Kilojoules per mole, the unit of bond energy.