Theory
Bond Enthalpies:
•The bond enthalpy of a specific bond is the enthalpy change needed to break that covalent bond into gaseous atoms. All substances being gases.
•So for HCl gas the bond enthalpy for the H-Cl refers to this change
H-Cl (g) → H(g) + Cl (g)
•Bond enthalpies are always positive because it will always require energy to overcome the attractive forces in the bond.
•Mean bond enthalpy is the enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules.
In CH4 there are 4 C-H bonds. Breaking each one will require a different amount of energy. However, we use an average value for the C-H bond for all hydrocarbons.
Applying Hess’s law to bond enthalpies:
