318 HEATS OF FOBMATION OF THE CYANOGEN SERIES. 



This is only half the heat disengaged in the similar formations 

 of chloride ( + 42'5), bromide, and iodide of ammonium ; l the 

 acetate comes nearer (+ 28*2), and the hydrosulphide nearer 

 still (+23). 



3. Starting from the elements, we should get 



N 2 + C + 2H 2 = NH 4 CN (solid) gives off + 40-5 Gal. 



The analogous formation of ammonium chloride gives off 

 + 767. 



4. Lastly, the heat of formation of ammonium chloride from 

 the elements is less than that of potassium chloride by 28'3 

 Cal. ; whereas, between the formation of ammonium cyanide 

 and that of potassium cyanide, the thermal difference is 27'1. 

 The difference in the two cases is therefore almost the same; 

 i.e. this state does not depend on the halogen generator. 



6. METALLIC CYANIDES. 



1. It was found that gaseous cyanogen combines directly, not 

 only with potassium, but also with certain true metals, such 

 as iron, zinc, cadmium, lead, and even copper ; but this tendency 

 towards direct combination does not extend to mercury and silver. 



2. The reactions are effected by heating cyanogen and the 

 metals in sealed tubes, to 100 for the first-named metals, and 

 to about 300 for the two last. 



3. Such combinations are always attended by a disengage- 

 ment of heat. In particular, according to M. Joannis 



J[Zn -f (CN) a = Zn(CN) 2 ] gives off + 28'5. 

 J[Cd + (ON), = Cd(CN) 2 ] gives off + 19-8. 

 From the elements, on the contrary 



J[Zn + C 2 + N 2 = Zn(CN) 2 ] absorbs - 8-8. 

 [Cd + C 2 - N 2 = Cd(CN) 2 ] absorbs - 17-5. 



7. MERCURIC CYANIDE. 



1. Formation from the acid and the oxide. It was found, by 

 experiment, that dilute hydrocyanic acid and mercuric oxide 



i[2HCN (1 equiv. = 2 litres) + HgO (precipitated and diluted 

 with 10 litres of water)], gives off -f 15-48. 



An excess of hydrocyanic acid does not cause any alteration 

 in this value, which is considerable, exceeding even the heat 

 given off in the action of dissolved hydrochloric acid on potash. 



It is owing to this difference in values that potash, combined 

 with hydrocyanic acid, with which, moreover, it gives off much 



1 See p. 127. 



