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Bond energy is a measure of the stability of a molecule. It is the enthalpy change required to break a particular bond in 1 mole of gaseous molecules.
The experimentally determined bond energy of the diatomic molecules are directly measurable quantities. For example, the bond energy for H-Cl is 431.9 kJ/mol and for Cl-Cl is 242.7 kJ/mol. H-Cl bond energy is higher due to hydrogen bonding.
However when we speak of the bond energy for O-H and C-H bond, we are actually measuring the strength of covalent bonds in polyatomic molecules since there are alot of compounds that contain O-H bond and C-H bond.
Example CH4 (methane), C6H6 (benzene), CH3OH (methanol)
The environment of each C-H bond per molecule is not the same. Thus for polyatomic molecule we speak of the average bond energy of a particular bond.
For example, we can measure the energy of the C-H bond in 10 different polyatomic molecules and obtain the average C-H bond energy which is 414 kJ/mol. This value is large if we only consider the C-H bond per se. But since the value is averaged over many compounds, we expect a value that is not accurate.
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