Page 97 - Chemical equilibria Volume 4

demonstrate that the slope of line PP’ (which is necessarily negative) gives
the temperature of inversion of the reaction: Molecular Chemical Equilibria 73
CO 2 + H 2 = CO + H 2O [3R.5]

This is the temperature for which, at equilibrium, we have the relation:

P CO = P H 2 O [3.46]
P P
CO 2 H 2
Similarly, let us now compare the pole diagrams of reduction of two
metal oxides, MO and M’O by hydrogen, as follows:

MO + H 2 = M + H 2O [3R.6]

M’O + H 2 = M’ + H 2O [3R.7]

The straight line joining the two poles P and P’ of those two reactions at
the same temperature gives the equilibrium temperature of the reaction
between solids:

MO + M’ = M + OM’ [3R.8]

3.3. Representation of the evolution of an equilibrium with the
temperature

As temperature is an important variable of chemical equilibrium, users
have attempted to represent the evolution of a chemical equilibrium with
changing temperature. Two methods are discussed below.

3.3.1. Diagram in van ’t Hoff coordinates

The first mode of representation is based on relation [3.28], applied in
convention (I). The method involves representing, in standard pressure
conditions (in practice at the pressure of 1 bar), the logarithm of the
equilibrium constant as a function of the inverse of temperature. This
representation gives us practically a straight line, because the standard
0
enthalpy Δ h and standard entropy Δ s of the reaction are practically
0
r
r
independent of the temperature. Hence, the slope of that line may be