Le Chatelier’s theorem

Le Chatelier s theorem or principle When a constraint is applied to a system in equilibrium, the equilibrium will lend to move in such a way as to neutralize the efferi of Hie constraint. Thus, e.g., in a reaction such as... 

In the same way (18.4) expresses the fact that if a reaction is accompanied by an increase in volume ( >0), an increase in pressure (Sp>0) leads to reduction of the extent of change ( <0) correspondingly if PjT is negative an increase in pressure advances the equilibrium value of the extent of reaction Le Chatelier s theorem),... 

Le Chatelier has extended van t Hoff s law and enunciated the important generalization any change in the factors of equilibrium from outside, is followed by a reversed ohange within the system . This rule, known as Le Chatelier s theorem, enables the chemist to foresee the influence of pressure and other agents on physical and chemical equilibria. 

The effect of external pressure on the rates of liquid phase reactions is normally quite small and, unless one goes to pressures of several hundred atmospheres, the effect is difficult to observe. In terms of the transition state approach to reactions in solution, the equilibrium existing between reactants and activated complexes may be analyzed in terms of Le Chatelier s principle or other theorems of moderation. The concentration of activated complex species (and hence the reaction rate) will be increased by an increase in hydrostatic pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules. The rate of reaction will be decreased by an increase in external pressure if the volume of the activated complex molecules is greater than the sum of the volumes of the reactant molecules. For a decrease in external pressure, the opposite would be true. In most cases the rates of liquid phase reactions are enhanced by increased pressure, but there are also many cases where the converse situation prevails. 

In discussing the alterations which may take place at the triple point with change of temperature and pressure, we have considered only the triple point S—L— V. The same reasoning, however, applies mutatis mutandis to all other triple points, so that if the specific volumes of the phases are known, and the sign of the heat effects which accompany the transformation of one phase into the other, it is possible to predict (by means of the theorem of Le Chatelier) the changes which will be produced in the system by alteration of the pressure and temperature. 

The first experimental determinations of the effect of pressure on solubility were made by E. von Stackelberg, who obtained values which are in qualitative agreement with Braun s law and the theorem of Le Chatelier, as the following table shows —... 

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