Bond energies

During a reaction, bonds in the reactants are broken and new bonds in the products are made.

• Breaking bonds takes in energy. It is always endothermic.
• Making bonds releases energy. It is always exothermic.

This is true in every reaction. Whether a whole reaction is exothermic or endothermic depends on which is bigger:

• If more energy is released making bonds than is taken in breaking them, the reaction is exothermic and the energy change is negative.
• If more energy is taken in breaking bonds than is released making them, the reaction is endothermic and the energy change is positive.

energy change = energy taken in breaking bonds − energy released making bonds

Example: H₂ + Cl₂ → 2HCl

Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol

1. Bonds broken: one H–H and one Cl–Cl = 436 + 242 = 678 kJ/mol
2. Bonds made: two H–Cl = 2 × 431 = 862 kJ/mol
3. Energy change = 678 − 862 = −184 kJ/mol

The answer is negative, so the reaction is exothermic.

Tip: draw out the displayed formula of each molecule so you count every bond, and remember to multiply by the balancing numbers.

The answer to a bond energy calculation is the overall energy change shown on the reaction profile:

• The negative energy change of an exothermic reaction appears as products below the reactants.
• The positive energy change of an endothermic reaction appears as products above the reactants.