Topic 4 of 4
Breaking bonds costs energy; making bonds releases it
The net change depends on both parts of the rearrangement.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Pure onlyBreaking bonds costs energy; making bonds releases it
The net change depends on both parts of the rearrangement.
Breaking a covalent bond separates atoms that attract one another, so energy must be supplied: it is endothermic. Forming a bond releases energy as atoms become more strongly held together: it is exothermic. A chemical reaction usually does both; never explain combustion by saying that breaking fuel bonds releases energy.
- Break reactant bonds
Energy is absorbed.
- Form product bonds
Energy is released.
- Compare the amounts
More released than absorbed gives an exothermic reaction; more absorbed than released gives an endothermic reaction.
Worked example
Explain hydrogen combustion qualitatively
Why can hydrogen reacting with oxygen release energy even though bonds must first break?
- H-H and O=O bonds must be broken, requiring energy.
- O-H bonds form in water, releasing energy.
- The energy released in making product bonds exceeds that needed to break reactant bonds.
The net change is exothermic. This explanation compares totals, not just whether a bond is strong.