# Bond enthalpy

Bond enthalpy is the energy needed to break one mole of a particular bond in the gas phase. Breaking bonds costs energy and forming bonds releases it. ΔH ≈ Σ(bonds broken) − Σ(bonds formed). It gives only an estimate.

Chemistry (university) · Part 05: Thermochemistry and thermodynamics · Card 6 of 9 · Last updated 2026-10-19

## Analogy

Pulling apart LEGO bricks takes effort, and snapping new ones together gives it back.

## Example

**H₂ + Cl₂ → 2HCl**

Broken: H–H 436 + Cl–Cl 243 = 679 kJ
Formed: 2 × H–Cl 431 = 862 kJ
ΔH = 679 − 862 = −183 kJ

## In practice

A fuel is a good one when the new bonds it forms (like in CO₂ and H₂O) are much stronger than the bonds it breaks.

## Practice

### Flashcards

- **Does bond breaking absorb or release energy?** It absorbs energy. Bond forming releases it.

## Questions people ask

### Why are bond enthalpies only approximate?

A bond's strength changes slightly depending on the rest of the molecule. The tables give an average over many compounds. The O–H bond in water is not exactly the same as in ethanol. For accurate answers, use enthalpies of formation instead.

### Does breaking a bond release or absorb energy?

Breaking a bond always absorbs energy, because the atoms are held together by attraction and you must pull them apart. Forming a bond always releases energy. A reaction is exothermic when the energy released forming bonds is greater than the energy absorbed breaking bonds.

## Related concepts

- [Hess's law](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-5/chemistry-university-part-5-concept-4)
- [Standard enthalpy of formation](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-5/chemistry-university-part-5-concept-5)

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