Chemistry · Common misconception
Breaking bonds needs energy—so why do fuels release it?
Compare energy taken in to break bonds with energy released when new bonds form.
The short answer
Breaking chemical bonds requires energy. Forming chemical bonds releases energy. An exothermic reaction transfers energy to the surroundings because forming the product bonds releases more energy than breaking the reactant bonds requires.
Conceptual illustration. Colours distinguish parts of the model.
Where the confusion starts
Saying “energy is released when bonds break” reverses the bond-energy model. A reaction can release energy overall while still requiring an initial energy input to get started. Activation energy and overall energy change describe different parts of the process.
WORK IT THROUGH
Suppose breaking reactant bonds requires 600 kJ/mol and forming product bonds releases 850 kJ/mol.
- Energy taken in: +600 kJ/mol.
- Energy released: −850 kJ/mol.
- Estimated overall change = bonds broken − bonds formed = 600 − 850 = −250 kJ/mol. The negative sign indicates an exothermic change.
CHECK YOUR UNDERSTANDING
Breaking bonds requires 900 kJ/mol and bond formation releases 700 kJ/mol. Is the reaction exothermic?
Make a prediction or write an answer before opening the explanation.
Reveal the explanation
No. The estimated energy change is +200 kJ/mol, so it is endothermic. More energy is taken in than released. Mean bond-energy calculations give estimates.
SEE IT. CHANGE IT. EXPLAIN IT.
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