Nitric acid hydrates have received much attention as possible catalysts for heterogeneous reactions that bring about the Antarctic ozone hole. Worsnop et al. investigated the thermodynamic stability of these hydrates under conditions typical of the polar winter stratosphere (D.R. Worsnop, 1oE.
Fox, M.S. Zahniser, and S. C. Wofsy, Science 259,71 (1993)). Standard reaction Gibbs energies can be computed for the following reactions at 190 K from their data:
(i) H20 (g) → t H20 (s) ∆rGΘ= -23.6 k] mol-1
(ii) H2O (g) + HN03 (g) → HNO3 H20 (s) ∆rGΘ= -57.2 kJ mol-1
(iii) 2 H2O (g) + HN03 (g) → HN03•2H2O (s) ∆rGΘ=-85.6 k] mol-1
(iv) 3 H2O (g) + HN03 (g) → HNOf3H2O (s) ∆rGΘ = -112.8 k] mol-1
Which solid is thermodynamically most stable at 190 K if PH2O = l.3x 10-7 bar and PHNO, = 4.1xlO-IO bar? Hint. Try computing L/P for each reaction under the prevailing conditions; if more than one solid forms spontaneously, examine ∆rG for the conversion of one solid to another.