12. Heat, Energy, and Chemical Bonds
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) + 3H
2
(g) + �O
2
(g)
�
CH
3
CH
2
OH(g)
This is the heat of the hypothetical reaction
CH
3
CH
2
OH(g)
�
2C(g) + 6H(g) + O(g)
D
H = +771.7 kcal mole
-1
ethanol
We must add to this the heat involved in atomizing solid graphite, and H
2
and O
2
gases:
This sum is the heat of the reaction
2C(s) + 3H
2
(g) + �O
2
(g)
�
2C(g) + 6H(g) + O(g)
D
H = +715.1 kcal
Ethanol molecule (C
2
H
5
OH)
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