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

For a general reaction of the type A + 4B 3C + 2D, in which A, B, C, and D are chemical substances, the heat of reaction can be calculated from DH0 = 3DH0C + 2DH0D - DH0A - 4DH0B, in which DH0C is the heat of formation of Compound C from its elements in their standard states.

For example, if the reaction is the oxidation of glucose by nitrate, as is carried out by some nitrate-respiring bacteria

5C6H12O6+24KNO3 6CO2+24KHCO3+12N2+18H2O

then the heat of reaction is obtained from the following combination of heats of formation of products and reactants:

Tables of standard heats of formation can be found in the CRC Handbook of Chemistry and Physics, in Lange's Handbook of Chemistry. With such tables, the heats of all reactions involving the compounds can be calculated, including reactions that never have been carried out, or that for various reasons we cannot carry out easily in the laboratory.

Nitrogen fixing bacteria
Nitrogen fixing nodules on a bean root
formed by the Rhizobium etli bacterium

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