Bond energies · Year 13
In a reaction, bonds in the reactants break and new bonds form in the products. Breaking bonds takes in energy (endothermic) and making bonds gives out energy (exothermic). The bond energy is the energy needed to break one mole of a bond. Adding up the bonds broken and the bonds made gives the overall energy change of the reaction.
Key facts
- Breaking bonds is endothermic: energy is taken in. Making bonds is exothermic: energy is given out.
- Bond energy: the energy needed to break one mole of a particular bond, in kJ/mol.
- Energy change = energy needed to break bonds − energy released making bonds.
- If more energy is released making bonds than is needed to break them, the reaction is exothermic and the energy change is negative.
- Count every bond: draw the displayed formulae, and multiply by the numbers in front of each formula.
- Bond energies are mean values, so the answer is an estimate.