Bond Enthalpy gives the strength of a chemical bond in a gaseous molecule.
Some bonds can have different strengths in different environments, so Data Books will list the average or mean bond enthalpies.
Determining Enthalpy Changes using bond enthalpy
Chemical reactions are bond breaking and then bond making. Bond breaking requires energy and is an Endothermic process. Bond making releases energy and is an exothermic process.
The enthalpy change can be determined by using the average bond enthalpies in an energy cycle.
Enthalpy required to break bonds = ∑(bond enthalpies in reactants)
Enthalpy required to make bonds = ∑(bond enthalpies in products)
ΔH = ∑(bond enthalpies in reactants) - ∑(bond enthalpies in products)
ΔH is the Change in Enthalpy.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
4(C-H) + 2(O=O) → 2(C=O) + 4(O-H)
(4 x 413) + (2 x 497) (2 x 805) + (4 x 463)
∴ ΔH = [(4 x 413) + (2 x 497)] - [(2 x 805) + (4 x 463)]
= -816 kj mol-1
These values are always negative.