THE CONVERSION OF ARGENTIC SULPHATE INTO CHLORIDE. 



83 



Again, in this case the law of concentration-effect is assisted by the rela- 

 tive affinity, indicated approximately by the large amount of heat evolved 

 by the reaction. It can be calculated from Thomsen's data that the reac- 

 tion gives out 40,400 grams calories, 1 or 170 kilojoules. Rarely, if ever, is 

 the difference between total-energy and free-energy changes in a reaction 

 of this kind as great as this, hence it is safe to infer that there is a con- 

 siderable preponderance of driving tendency in the desired direction, aris- 

 ing from the mutual affinities concerned. 



Thirdly, constancy of the weight on continued treatment indicated the 

 completion of the reaction. In seven of the experiments the argentic 

 chloride was fused a second time in hydrochloric acid for 15 minutes with 

 occasional agitation, followed by 5 minutes in air. The following table 

 gives the changes in weight found in this way: 



The Effect of Continued Treatment ivith Hydrochloric Acid. 



The constancy in weight was thus entirely satisfactory, the average loss 

 being only 0.0003 per cent of the weight of the chloride. 



This experiment, however, does not absolutely preclude the possibility 

 that a small but constant amount of sulphate may remain. In order to test 

 this question 4.90 grams of argentic chloride which had never been con- 

 taminated with sulphate was fused in the tube, and then 0.00479 gram of 

 pure argentic sulphate was added and thoroughly mixed with the chloride 

 by fusion. On cooling the appearance was very different from the pure 



1 Thomsen, Thermochemische Untersuchungen : 

 2Ag + C1 2 = 2AgCl -f 58760. (Vol. 3, p. 381.) 

 Ag 2 SC>4 = 2Ag+O 2 -f SO 2 96200. (Vol. 3, p. 382.) 

 SO + O -f- Ha = H 2 SO- -f 121840. (Vol. 2, p. 255.) 

 2HC1 = H 2 + Cl 44000. (Vol. 2, p. 114.) 

 Therefore, Ag 2 SO 4 + 2HC1 (gaseous) = 2AgCl 4- H 2 SO, (Liquid) -f 40,400. 



A different set of equations gave 40,200. The result, of course, only applies to 18. 



In this connection it is worth while to note that Ag 2 SO 4 + C1 2 = 2AgCl + SO 2 -{- 

 Oj 37440 cal. ; which indicates that chlorine would not be as suitable for our 

 purpose as hydrochloric acid. This expectation is confirmed by the experience of 

 Krutwig, Ber., 14, 306 (1881) : "Die Einwirkung ist hier (chlorine on silver sul- 

 phate) keine direkte; nur bei sehr holier Temperatur, nachdem das Salz geschmol- 

 zen ist und sich zersetz, giebt es schweflige Saiire, Chlorsilber und Sauerstoff ab." 



