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

Heats of reactions are additive in exactly the same way that the reactions they belong to are additive. From the information in the heat of combustion table, we can calculate how much heat would be given off by the combustion of a mole of water in fluorine gas, even though this reaction is not found in the table:

This calculation tells us that 60 calories more heat are obtained by burning hydrogen in F2 (Reaction 1) than in O2 (reverse of Reaction 2). Even the "garbage" from O2 combustion (H2O) still is a fuel for F2 combustion (Reaction 3), because F is more electronegative than O.

This additivity of heats of reaction is a tremendous labour-saving device. One might think that, to have available complete heat of reaction data for all possible chemical reactions, it would be necessary to measure and tabulate all of these heats. This is not the case. It is necessary to tabulate heats only for the minimum number of reactions from which all other reactions can be obtained by suitable combinations.

   Page 20 of 39 HomeGlossary