338 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



The heat of formation of liquid cyanogen chloride from 

 chlorine and cyanogen, or + 9*9, is quite comparable to the 

 heat of formation of iodine chloride and iodine bromide, under 

 the same form 



I (gas) + 01 = IC1 (liquid) + 9-8 ; IC1 (solid) + 12-1. 

 I (gas) + Br (gas) = IBr (solid) + 11-9. 



This is another point of resemblance between cyanogen and 

 the halogens. In the formation of cyanogen chloride, as well 

 as in the combinations of the halogen elements with one another, 

 there is hardly any other heat given off than that which corre- 

 sponds to the change of state of the compound, i.e. to the con- 

 version of the gaseous body into liquid or solid. 



6. Substitution of chlorine for the cyanogen. From the previous 

 results we conclude that the simple substitution of chlorine for 

 hydrogen in gaseous hydrocyanic acid 



Cl + HCN (gas) = CNC1 (gas) + H, 



would absorb 6'2. Such a reaction is therefore impossible, 

 unless accompanied by the formation of secondary products, 

 furnishing supplementary energy. On the contrary, the simple 

 substitution of chlorine for the cyanogen 



Cl + HCN (gas) = HC1 (gas) + CN (gas), 



would give off + 14*2. 



Lastly, the simultaneous formation of cyanogen chloride and 

 gaseous hydrochloric acid 



C1 2 + HCN (gas) = CNC1 (gas) + HC1 (gas), would give off 



+ 15-8. 



We see that this last formation answers to the maximum of 

 heat disengaged. This reaction, in fact, is produced in prefer- 

 ence to any other. It is all the more readily effected as the 

 combination between the cyanogen chloride and hydrochloric 

 acid gives off a fresh quantity of heat, at least if the substances 

 are in the liquid state, which again acts in the same way. 

 However, the effects of these direct reactions between chlorine 

 and free hydrocyanic aeid are complicated by various secondary 

 reactions which are imperfectly understood. The direct reactions 

 become more obvious, if we are operating on cyanides, the 

 corresponding bodies being always considered in comparable 

 forms : 



KCN (solid) + da (gas) = KC1 (solid) + CNC1 (gas) would 



give off + 39-0. 

 i[Hg(CN) 2 (solid) + 2C1 2 (gas) = HgCl 2 (solid) + 2CNC1 (gas)] 



gives off + 21-3. 



All these quantities of heat are positive. The formation of 



