Remarks on dependence on temperature ‘at constant volume’

Abstract

It is sought to extend recent treatments of the dependence of equilibrium constants on temperature ‘at constant volume’, with explicit attention to the behaviour of equilibrium constants based on various different types of specification of concentration. It is shown that for reactions in solution, if ‘at constant volume’ means ‘at constant molar volume of the pure solvent,’ then the dependence on temperature is indeed related to a certain type of ΔU in precisely the way that has for a long time been taken to be the case. If, however, ‘at constant volume’ means ‘at constant total volume of an actual reacting system’, then the dependence on temperature may be incompletely specified except in the limit of infinite dilution, when it becomes the same as at constant molar volume of the pure solvent. In contrast to the situation which obtains with the dependence on temperature at constant pressure, the use of amounts of solute per unit volume involves no complications in the dependence at constant molar volume of the pure solvent.