This Demonstration shows the displacement of a chemical equilibrium following pressure variation, a consequence of Le Chatelier's principle. In order for this shift to take place it is necessary that
, namely that the sums of the stoichiometric coefficients for the products and for the reactants are not equal.
Two specific reactions were considered, the synthesis of ammonia, with
, and the dissociation of phosphorus pentachloride, with
Ammonia synthesis takes place at 450 °C (in the Haber–Bosch process), associated with
. The equilibrium was studied starting from initial concentrations
; the same initial value was used for
in the dissociation reaction, at the temperature of 811.15 °C. Under these experimental conditions for the dissociation reaction
The various equilibrium constants are related by :
Plots for the dissociation and synthesis are shown with varying the moles of the reactants and products in equilibrium at various pressures. In the diagram below, the numbers of moles inside a piston are schematized, neglecting thermal agitation. Although the displacement of the equilibrium in the two cases is opposite (shifting toward the side of reactants →
; shifting toward the side of products →
), the general trend is the same: the decrease in the number of molecules inside the piston as the pressure increases. This can be visualized using "show/hide," which represents the molecules in both reactions as indistinguishable particles.
In "equilibrium constant and pressure," the trend of the molar fractions
is represented in a generic dimerization
and in a generic dissociation
as functions of pressure, according to the different values of
The different values of
can be selected with the
". In the dissociation reaction, these coincide with the values shown on the button; for the dimerization reaction, these vary instead with an appropriate scale factor (
) and are shown alongside the plot.