12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Benzene and Resonance

The energy needed to make the same number of isolated atoms from graphite and H2 gas is


The difference between these two numbers should be the heat of formation, as shown below:

        DH0 = 1343 - 1283 = +60 kcal per mole of benzene

By this calculation, heat is absorbed, not emitted, when benzene is formed from its elements. We encounter trouble as soon as we try to compare this calculated value of +60 kcal mole-1 with reality. It is true that heat is absorbed when benzene is made from carbon and hydrogen, but only 20 kcal mole-1, not 60.

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