Breaking needs energy. Forming releases it.
Breaking bonds is endothermic: energy must be supplied to separate bonded atoms. Forming bonds is exothermic: energy is released as new bonds form. Never say that breaking a bond releases energy.
Lesson 5.6 · AQA Combined Science
Breaking and forming bonds
Higher Tier only. This is an optional extension after the core lessons. Go to exam practice.
Compare the two totals
In an exothermic reaction, forming product bonds releases more energy than breaking reactant bonds needs. In an endothermic reaction, breaking reactant bonds needs more energy than forming product bonds releases. Both types involve energy in and energy out; the difference decides the overall change.
Keep track of the atoms and bonds
In H₂ + Cl₂ → 2HCl, break one H–H bond and one Cl–Cl bond. Form two H–Cl bonds. There are two H atoms and two Cl atoms before and after. The large 2 in front of HCl means two molecules, each with one bond.
Remember
Breaking = energy in. Forming = energy out. Compare both totals to decide the overall change.
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