Bond energy and bond length

Chemists measure a bond's strength by what it costs to break it. Break a mole of H–H bonds and you have to put in 436 kJ:

$\text{H}_2\text{(g)} \rightarrow 2\text{H(g)} \qquad \Delta H = +436\ \text{kJ mol}^{-1}$

Breaking bonds always takes in energy, so bond energies are always positive.

In a liquid or solid you'd also have to pull the molecules apart from each other. That extra energy isn't part of the bond.

In the gas, molecules are already separate, so the energy measured is just the bond.

• For a diatomic molecule like $\text{H}_2$ or $\text{HCl}$, the bond energy is exact: there's only one bond to break.
• A C–H bond in methane is slightly different from one in ethane. The Data Booklet value (C–H, 410 kJ mol⁻¹) is an average over many compounds.