Thermodynamic study of disorder in mercury(II) diamminodichloride and mercury(II) diamminodibromide

Abstract

In the salts mercury(II) diammino-dichloride and -dibromide the mercury atoms are positionally disordered at room temperature, the disorder being tantamount to that of dimers (the linear NH₃—Hg—NH²⁺₃ cations) on a cubic lattice. The heat capacity of the chloride has been measured from 3 to 300 K and that of the bromide from 10 to 300 K, giving the apparent (calorimetric) entropies of the salts at 298.15 K. The true (equilibrium) entropies at the same temperature have been determined by e.m.f. measurements on cells incorporating the complexes in the cell reaction, combined with previous measurements of the heat of solution of gaseous ammonia in the electrolytes used in the cells. A comparison of the entropy values shows that both salts retain entropy, and hence are still disordered, at the absolute zero. The value of the residual entropy for the chloride (considered to be the more accurate of the two) is 12.0 ± 3.0 J K^–1 mol^–1, and that for the bromide is 20.0 ± 7.5 J K^–1 mol^–1. These estimates of the residual entropy are briefly considered in the light of theoretical work on the disordered dimer problem. It seems likely that the residual entropy due to the disordered arrangement of the cations is supplemented by some degree of (possibly consequential) orientational disorder of the ammonia molecules.