Page 140 - Chemical equilibria Volume 4

116 Chemical Equilibria
and their standard enthalpies. We shall see later on that the reactions and
total heats of combustion are very practical for that use – particularly in
organic chemistry, where syntheses from simple substances are rarely
possible.
– The third method is to apply relation [4.20] to a reaction involving the
compound under study, and for which we know the standard enthalpy of
reaction and the enthalpies of formation of the other components involved in
the reaction.

NOTE 4.2.– By combining relations [4.19] and [4.20], we can see that, by
definition, the standard enthalpies of formation of simple substances such as
C, O 2, H 2, Cl 2, Fe, Na, etc., are null at all temperatures.

4.2.5.4. Enthalpies of formation and temperature

The fact that the enthalpies of formation of the simple substances are
chosen as equal to zero at all temperatures means that the enthalpy of
formation will obey relation [4.7] with temperature. This is consistent with
the fact that the enthalpies of formation represent a set of values of the
enthalpies of the pure substances.

4.2.6. Enthalpies of combustion

As we noted earlier, it is not always possible to synthesize a pure substance
directly, with no secondary reactions, from its elementary substances, and it is
then not possible to determine an enthalpy of formation by the direct method.
This obstacle is often circumvented by considering the enthalpies of combustion
of the species in the presence of excess oxygen.

4.2.6.1. Definitions
The enthalpy of combustion of a pure substance is the enthalpy associated
with the reaction of complete combustion of a mole of that pure substance in the
presence of excess oxygen.

The standard enthalpy of combustion at a temperature T is the enthalpy of
combustion at the standard pressure of 1 bar and temperature T.