12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Heats of Reactions from Bond Energies
How good are these approximate bond-energy values? How close can we come to reproducing measurable heats of reaction? As an example, let us calculate the heat of formation of ethanol vapor:
2C(graphite) + 3H2(g) + ŻO2(g) « CH3CH2OH(g)

This is the heat of the hypothetical reaction

CH3CH2OH(g) « 2C(g) + 6H(g) + O(g)  DH = +771.7 kcal mole-1 ethanol

We must add to this the heat involved in atomizing solid graphite, and H2 and O2 gases:
This sum is the heat of the reaction
2C(s) + 3H2(g) + ŻO2(g) « 2C(g) + 6H(g) + O(g) DH = +715.1 kcal

Ethanol molecule (C2H5OH)

   Page 30 of 39 HomeGlossary