BDE in Methane

From WikidChem

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Bond Dissociation Energy in Methane

The heat of atomization for Methane can be calculated through its heats of combustion and formation to be 397.5 kcal/mol.

The average bond energy here is simply that heat of atomization divided by four, for the four bonds; this yields 99.4 kcal/mol as the average bond energy.

These measurements provide a basis for comparison with the bond dissociation energies, which are experimentally available from spectroscopy as extremely precise (but as noted earlier, sometimes uncertain) figures. The bond dissociation energy for each hydrogen is different, since the hybridization and a number of other chemical properties change as each hydrogen is dissociated, but an examination of the figures introduces an intriguing result:

Bond Bond Dissociation Energy

CH3-H ~104.99 kcal/mol CH2-H ~ 110.4 kcal/mol CH-H ~101.3 kcal/mol C-H ~80.9 kcal/mol

We note that each bond differs from the average bond energy; none of these dissociation energies can be considered an "average" C-H bond.

However, what is perhaps most striking here is that the sum in Bond Dissociation Energies equals the heat of atomization, 397.5 kcal/mol, even though they are measured in completely independent experiments. One implication is that the spectroscopy and the other experiments used to calculate Bond Dissociation Energy are quite precise. Another, which is discussed on the following slide, is that the sum of the average bond energies may accurately predict the heats of atomization of a variety of other molecules (even though here the agreement was a matter of definition).