Page 139 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 115
A particular class of enthalpies of formation is constituted by standard
0
enthalpies of formation at a temperature T (written as H ), which are those
T
obtained when the compound and its elements are all in their standard state
at that temperature. Thus, we have the relation between standard enthalpy of
formation and enthalpy associated with the reaction of formation f:
0
H = Δ h T 0 [4.19]
T
f
We shall see later on that the tables of thermodynamic data give all the
values of the standard enthalpies of formation at the temperature of 25°C or
298.15 K: H 0 298 .

4.2.5.2. Use of the enthalpies of formation to calculate the standard
enthalpy of a reaction
If we know the enthalpies of formation of all the components of a
reaction, it is easy, by application of Hess’s law (see section 4.2.4.3) to
calculate the standard enthalpy of the reaction at the same temperature.
Application of relation [4.19] gives us:

T ∑
Δ h = ν k H T 0 () k [4.20]
0
r
k
NOTE 4.1.– standard enthalpies of formation constitute a set of values of the
enthalpies of substances; it is the set of enthalpies at standard pressure for
which the origin zero is posited for the enthalpy of simple substances.

4.2.5.3. Determination of the enthalpies of formation
We can use three methods to determine the standard enthalpy of
formation of a compound at a given temperature T:
– The first method is to use calorimetry to experimentally measure the
direct synthesis reaction and then to use relation [4.19]. This method, which
is apparently simple, is in fact often difficult or even impossible to apply.
For example, in the case of methane synthesis, reaction [4R.1] is impossible
to perform directly.
– The second family of methods is the use of Hess’s theorem, as we
saw in section 4.2.4.3. In order to do this, we need to have reactions
which will give us the formation reaction when combined linearly,