Bond dissociation enthalpy (D) is the energy required to break a particular bond between two atoms in the gas phase. Because breaking a bond requires energy and forming a bond releases it, an estimate of a reaction's enthalpy can be made from the bonds broken and formed. It is always a positive quantity, since bond breaking is endothermic.

键解离焓(D)是在气相中断裂两原子间某一特定化学键所需的能量。由于断键需要能量而成键释放能量,可由断裂和形成的键来估算反应的焓变。它总是正值,因为断键是吸热的。