Ammonium hydrosulphide, dissociation

Numerical Example.—Dissociation of ammonium hydrosulphide NH4IIfc NH3+HaS. 

A mixture of ammonia and hydrogen sulphide does not unite if the press, is less than a certain critical value, which depends on the temp. If the press, is at or above this value crystals of ammonium hydrosulphide are formed if the press, be increased,more crystals will form and if the press, be reduced, crystals will decompose. When the two gases are present in eq. amounts this press, is called the dissociation pressure of the solid. In the present case, if the vapour phase has the same composition as the solid with which it is in equilibrium, the system is univariant, and there is a definite dissociation press, for each temp. F. Isambert found the dissociation press, of ammonium hydrosulphide, in mm. of mercury, increases rapidly with a rise of tomp. ... 

The melting point of ammonium hydrosulphide in a closed vessel was found by E. Briner to be 120° and, in the presence of an excess of hydrogen sulphide, the m.p. is a triple point NHiSHv HgS+NHg, and the equilibrium constant is K=0 04 at 22°. The heat of vaporization of the solid hydrosulphide, in consequence of dissociation, will be equal to the heat of formation of the solid from the component gases, viz., 22 4 Cals., as found by J. Thomsen. According to F. Isambert, the heat of vaporization between 77° and 132° is 23 Cals., and, according to J. H. van t Hoff, calculated between 9 5° and 25 1° at constant press., 22 7 Cals. J. Walker and J. S. Lumsden find that the value of this constant increases with a rise of temp., being 19 7 Cals, between 4 2° and 18°, and 22 0° between 30 9° and 44 4°. 

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