Bond Breaking – IGCSE Chemistry Definition

Bond breaking is the process of separating two atoms that are held together by a chemical bond. It is always an endothermic process, meaning it requires an input of energy. The energy needed to break a specific bond is called the bond energy (or bond dissociation energy) and is measured in kJ/mol.

Why bond breaking requires energy

A chemical bond represents a region of lower energy where atoms are more stable together than apart. To pull the atoms apart, energy must be supplied to overcome the attractive forces. The stronger the bond, the more energy is required. For example, breaking the triple bond in nitrogen (N≡N, 944 kJ/mol) requires much more energy than breaking the single bond in chlorine (Cl-Cl, 242 kJ/mol).

Role in chemical reactions

Every chemical reaction involves two stages: breaking bonds in the reactants and making new bonds in the products. Bond breaking is the first stage and always absorbs energy. If the energy absorbed by breaking bonds is greater than the energy released by making bonds, the reaction is endothermic overall. If the energy released by making bonds is greater, the reaction is exothermic overall.

Bond energy calculations

In a bond energy calculation, candidates list all the bonds broken in the reactants and add up their bond energies. This gives the total energy input. For example, in H₂ + Cl₂ → 2HCl, the bonds broken are one H-H bond (436 kJ/mol) and one Cl-Cl bond (242 kJ/mol), giving a total of 678 kJ/mol.

Exam context

Paper 4 questions on bond energies require candidates to distinguish between the energy needed to break bonds and the energy released when bonds form. A common error is applying the wrong sign. Remember: breaking is always positive (energy in), making is always negative (energy out).

Worked exam question

Explain, in terms of bond breaking and bond making, why the reaction between hydrogen and fluorine is exothermic. (H-H = 436 kJ/mol, F-F = 158 kJ/mol, H-F = 568 kJ/mol) (3 marks)

Bonds broken: H-H + F-F = 436 + 158 = 594 kJ/mol [1]. Bonds made: 2 x H-F = 2 x 568 = 1136 kJ/mol [1]. More energy is released making bonds than is needed to break bonds, so the reaction is exothermic [1].

Common mistakes

Candidates sometimes say “bonds are broken, releasing energy.” This is incorrect. Breaking bonds always absorbs energy. Only making bonds releases energy. Mixing these up inverts the logic and leads to wrong conclusions about whether a reaction is exothermic or endothermic.