GAS-PHASE REACTION OF MERCURY WITH AMMONIUM IODIDE AND HYDROGEN IODIDE

The gas-phase reaction of mercury with ammonium iodide is the basis of at least one scheme for the thermochemical production of hydrogen. We have studied the reactions Hg + 2HI ..-->.. HgI/sub 2/ + H/sub 2/ and Hg + 2NH/sub 4/I ..-->.. HgI/sub 2/ + H/sub 2/ + 2NH/sub 3/ in the gas phase for 623 to 693 K. Under our conditions NH/sub 4/I is completely dissociated to NH/sub 3/ and HI. The reactions were followed by spectrophotometrically monitoring the buildup of the product, HgI/sub 2/. In the absence of NH/sub 3/ and with approximately 20-fold excess of HI (10/sup -3/ M equal to or less than (HI) equal to or less than 10/sup -2/ M), the rate of appearance of HgI/sub 2/ is given by d(HgI/sub 2/)/dt = k/sub 4/(Hg)(HI). With excess Hg, however, the rate follows the expression d(HgI/sub 2/)/dt = k/sub 5/(Hg)(HI)/sup 2/. Introduction of NH/sub 3/ into the system increases the rate substantially. In the presence of ammonia at concentrations comparable to those of the Hg and HI, the rate law assumes the form d(HgI/sub 2/)/dt = k/sub 3/(Hg)(HI)/sup 2/(NH/sub 3/). At 663 K k/sub 4/, k/sub 5/, and k/sub 3/ have the values 3.7 Lmore » mol/sup -1/ min/sup -1/, 590 L/sup 2/ mol/sup -2/ min/sup -1/, and 2.2 x 10/sup 6/ L/sup 3/ mol/sup -3/ min/sup -1/, respectively. Increasing the surface-to-volume ratio has no discernible effect on the reaction rate. The constant k/sub 3/ has a negative temperature dependence, which corresponds to an activation energy of -34 kJ/mol. The reaction in the presence of ammonia is interpreted in terms of a preequilibrium to form a complex HI . Hg . NH/sub 3/. 4 tables, 5 figures.« less