[Benzoic Acid]

I have a question in relation to the difference between ‘bond energy’ and ‘heats of combustion’. I was reviewing my organic and inorganic chemistry and my inorganic books states – “Bond energy is a measure of strength of a bond: the larger the bond energy the stronger the chemical bond.”

In my organic chemistry book I was reviewing monosubstitued, disubstitued, etc. bonds and it shows a ‘heats of combustion’ chart and the most unstable compound has the highest enthalpy change.

So I’m confused, if ‘bond energy’ is represented by enthalpy change why then in this ‘heats of combustion’ chart does the highest enthalpy change represent the most unstable compound?

Thanks!

[Yggdrasil]

In terms of energetics of a reaction, you can think of two steps.  The first step is breaking bonds which requires energy.  The next step is forming bonds, which releases energy.  Therefore, you can make a reaction highly exothermic by two means.  The first means is starting with reactants that have very weak (i.e. unstable) bonds.  Since breaking these bonds requires little energy, more energy will be released by the reaction overall.  Alternatively, you could form products with very stable bonds, which release a lot of energy when formed.

Since combustion refers to the reaction:

X + O₂ --> CO₂ + H₂O

and the bonds in CO₂ and H₂O are fairly stable bonds, the more unstable the bonds in X are, the more exothermic your combustion will be.