Page 138 - Chemical equilibria Volume 4

114 Chemical Equilibria
4.2.4.3. Calculation of an enthalpy on the basis of a set of reaction
heats appropriately chosen and known
This method is based on the first law of thermodynamics, which enables
us to deduce that the enthalpy is a function of state. If we choose a certain
number of reactions such that – a linear combination of their balance
equations gives us the balance equation for the reaction under study; and we
know the enthalpies of the different reactions chosen then we know that,
between the enthalpy of the sought reaction Δ H and those of the chosen
r
reactions Δ H , there is the same linear combination as that which exists
m
between the balance equations in the form:

Δ r ∑ λ m Δ H = m H [4.18]
k

This method enables us to determine the enthalpy at a specific
temperature provided the enthalpies of the combined reactions are known at
that same temperature.

In fact, this method is Hess’s long-established method for determining
reaction heats.

4.2.5. Enthalpies of formation

One category of reaction enthalpies plays a very particular role in the
methods for determining the standard enthalpy associated with a reaction:
the enthalpies of formation.

4.2.5.1. Definition of the enthalpies of formation

The enthalpy of formation of substance is the enthalpy associated with
the reaction to synthesize a mole of that compound from its elements in
molecular form.

For example, the enthalpy of formation of methane is the enthalpy
associated with the reaction:

C(graphite) + 2 H 2 = CH 4 [4R.1]