Non-ideality and calculation of extents of dissociation of liquid compounds

Abstract

The consequences are examined of including activity coefficients in the formulae for calculating extents of dissociation from the rounding of freezing-point curves and similar thermodynamic data. Application of a simple regular solution theory to a single dissociative equilibrium suggests that the error in calculating α_o(the extent of dissociation of the pure compound) without any allowance for non-ideality would be negligible if α_o= 0.01, and should amount to no more than 4-5 % even when the compound is 10% dissociated. This favourable result does, however, depend upon the assumption that the two ends of a compound molecule interact with their neighbours in similar ways to the free species into which the compound dissociates, thereby giving rise to some cancellation of activity coefficient effects. For ionic dissociations, the rate of change of ionic strength with composition at the maximum freezing-point is expected to introduce an additional factor, but in certain types of dissociation errors from this source may also be small.