Cupric tetramminosulphate. II. Dissociation pressures

Abstract

T h e dissociation pressure of cupric tetramminosulphate was measured a t different temperatures. Log,, P is plotted against I F , gave a curve which could be divided into two intersecting lines of different slopes, indicating that the two molecules of ammonia li'berated in the decomposition of the tetrammine to give the diammine are set free in two stages: a t temperatures lower than 50" the tetrammine decomposes giving the triammine; a t higher temperatures, it gives the diammine directly. The heat content change of the system ''A H" was calculated for each step and found to be 6.1 and 25.5 kcals per molecule for athe liberation of the first and second molecules ammonia respectively a t low temperatures.The heat of formation of CuS0,.3NH3 was calculated as 285.3 kcals per molecule.Sidgwick 1 states 'that cupric tetramminosulphate has an ammonia tension of one atmosphere at 90'. The present author, however, in previous work 2 on the dehydration of moist cupric tetramminosulphate observed that ammonia pressures up to 130' were so low that this value of one atmosphtere at 90' seemed doubtful. It was therefore decided to measure the dissociation pressure for the reaction: CU(NH,)~SO, = Cu(NH,),SO, + 2 NH, and calculate the change in heat content. E x p e r i m e n t a 1.T h e dissociation pressures of cupric tetramminosulphate at different temperatures were measured using the apparatus described previously z, which was used for the preparation of an'hydrous samples of the compound: the total volume of the apparatus was about 150 ml. Four experiments were carried out: