324 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



this last value being only approximate, on account of the 

 physical changes experienced by the silver oxide and cyanide. 

 It is less by a third than the heat, 33*2, given off in the 

 analogous formation of silver chloride. These values explain 

 why hydrocyanic acid displaces nitric acid from its combina- 

 tion with silver oxide, and why silver cyanide resists the action 

 of dilute nitric acid. 



2. Formation from the elements. 

 1. Ag -f C (diamond) + N + AgCN absorbs - 13'6. 



The calculation of this value is as follows : 

 Initial system : 



[Ag 2 + C 2 + N 2 + H 2 + 0], 

 Final system : 



i[2AgCN (solid) + H 2 (liquid)]. 



FIRST STEP. 



J[Ag 2 + = Ag 2 0] disengages ......... + 3-5 



H + C + N = HCN (diluted) absorbs ...... - 23'4 



|[Ag 2 + 2HCN (diluted)] disengages ...... + 20'9 



Sum ... + 0-8 

 SECOND STEP. 



H 2 0] ............... +34-5 



Sum ... + 34-5 + x 



whence, x = 337. 

 2. Again, we have 



Ag + ON (gas) = AgGN gives off + 3*6. 



Let us compare the differences observed between the heats 

 of formation of the chloride and cyanide, formed by the same 

 metal or system of elements. For potassium, the difference is 



105-67-6= +374; 

 for ammonium 



76-7 - 40-5 = + 36-2. 



The value here is almost the same. But the figures become 

 very unequal for metallic salts, such as those from silver : 



For silver and mercury we observe that the value is only 

 half that relating to alkaline salts. 



