Basics of Bond Enthalpies

Bond enthalpy (or bond energy) is defined as the energy required to break one mole of a bond in a gaseous molecule under standard conditions.
It is always expressed in kJ/mol, kilojoules per mole.
Bond breaking requires energy (is endothermic) while bond forming releases energy (is exothermic).
The bond enthalpy for a specific bond varies slightly depending on the molecule, hence we often use an average bond enthalpy.

Determining Bond Enthalpies

Calculating the enthalpy change for a reaction involves considering the energy needed to break the reactant bonds and the energy released when product bonds are formed.
The equation used is ∆H = Σ (Bond enthalpies of bonds broken) - Σ (Bond enthalpies of bonds formed).
If the overall change is positive, the reaction is endothermic. If negative, it is exothermic.
These calculations are just approximations because they rely on the average bond enthalpies which do not always accurately represent the actual bonds in the reaction.

The type of atoms bonded together has a significant impact on bond enthalpy. For example, triple bonds (e.g., N≡N) are stronger and have higher bond enthalpies than double bonds (O=O) which are in turn stronger than single bonds (H–H).
The distance between the nuclei of the bonded atoms also affects bond enthalpy. Shorter bonds are stronger and have higher bond enthalpies.
The environment of the molecule also influences bond enthalpy. For example, water molecules have higher bond enthalpies than gaseous hydrogen and oxygen due to their polar nature and hydrogen bonding.

Understanding bond enthalpies is crucial for determining the feasibility of a reaction.
If the enthalpy change is highly positive, the reaction is less likely to occur spontaneously because it requires a significant amount of energy.
If the enthalpy change is negative, the reaction is more likely to be spontaneous as it releases energy.
Bond enthalpies also allow us to predict the kinetics of a reaction – reactions with high bond enthalpies in the reactants often proceed more slowly because they require more energy to initiate.