12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Heats of Formation

To summarize what has been said so far about heats of reaction, heats are always quoted per mole of an individual reactant or product. For the ethanol combustion in the table on the previous page,

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)

the enthalpy change, or heat of reaction, can be quoted in three equivalent ways:

DH=-327 kcal per mole of ethanol consumed,
DH=-164 kcal per mole of CO2 produced, and
DH=-109 kcal per mole of O2 used, or liquid water produced, since the numbers of moles are the same.

Moreover, each of the above heats would be different if any one of the reactants or products was in a different physical state: ethanol vapor, water vapor, ice, solid CO2, and so on. The reverse reaction, synthesis of ethanol from carbon dioxide and water, would have the same heat but with opposite sign: +327 kcal per mole of ethanol produced.

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