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

The individual formation reactions for the substances appearing in the combustion process and their molar heats of formation, DHo298 are:

1. 2C(s) + 3H2(g) + ŻO2(g) « C2H6OH(l) -66.4 kcal
2. O2(g) « O2 (g) (already in standard state) 0
3. C(s) + O2(g) « CO2(g) -94.1 kcal
4. H2(g) + ŻO2(g) « H2O(l) -68.3 kcal

The standard state for carbon is solid graphite, and that for all other elements on the left side of the equations is the gas at one atmosphere.

The superscript zero on DH indicates standard states for reactants and products, and the subscript 298 (often omitted and assumed) indicates that the heats quoted are for reactions begun at 298K, with the products brought back to the starting temperature at the conclusion of the reaction.

The heats are expressed, as they usually are, as kilocalories per mole of compound formed. Values for some representative substances are shown in this table.

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