502 PRINCIPLES OF 'CHEMISTRY 



of 44,000 heat units. The decomposition of 2HBr into H 2 + Br 2 

 only requires, if the bromine be obtained in a gaseous state, a con- 

 sumption of about 24,000 units, whilst in the decomposition of 2HI 

 into H 2 4- 12 as vapour about, 3,000 heat units are evolved ; 68 

 these facts, without doubt, stand in causal connection with the great 

 stability of hydrogen chloride, the easy decomposability of hydrogen 

 iodide, and the intermediate properties of hydrogen bromide. From 

 this it would be expected that chlorine is capable of decomposing water 

 with the evolution of oxygen, whilst iodine has not the energy to 

 produce this disengagement, 69 although it is able to liberate the oxygen 

 from the oxides of potassium and sodium, the affinity of these metals for 

 the halogens being very considerable. For this reason oxygen, especially 

 in compounds from which it can be evolved readily (for instance, C1HO, 

 CrO 3 , <tc.), easily decomposes hydrogen iodide. A mixture of hydrogen 

 iodide and oxygen burns in the presence of an ignited substance, forming 

 water and iodine. Drops of nitric acid in an atmosphere, of hydrogen 

 iodide cause the disengagement of violet fumes of iodine and brown 

 fumes of nitric peroxide. In the presence of alkalis and an excess of 

 water, however, iodine is able to effect oxidation like chlorine that is, 



68 The thermal determinations of Thomsen (at 18) gave in thousands of calories, 

 C1 + H= + 22, HCl + Aq (that is, on dissolving HC1 in a large amount of water)' = + 17'8, 

 and therefore H + Cl + Aq = + 39'3. In taking molecules, all these figures must be 

 doubled. Br + H=+8'4; HBr + Aq = 19'9; H + Br + Aq= +28'8. According to Ber- 

 thelot 7'2 are required for the vaporisation of Br 2 , hence Br 2 + H 3 -I6 i 8 + 7 < 2= +24, 

 if Br 2 be taken as vapour for comparison with C1 2 . H + I= 6'0, HI + Aq=19'2; 

 H + I + Aq= +18'2, and, according to Berthelot, the heat of fusion of I 2 =3'0, and of 

 vaporisation 6'0 thousand heat units, and therefore I 2 + H 2 = 2(6'0) + 8 + 6= 3'0, if the 

 iodine be taken as vapour. Berthelot, on the basis of his determinations, gives, however, 

 + 0'8 thousand heat units. Similar contradictory results are often met with in thermo- 

 chemistry owing to the imperfection of the existing methods, and particularly the 

 necessity of depending on indirect methods for obtaining the fundamental figures. Thus 

 Thomsen decomposed a dilute solution of potassium iodide by gaseous chlorine ; the 

 reaction gave + 26'2, whence, having first determined the heat effects 6f the reactions 

 KHO + HC1, KHO + HI and Cl + H in aqueous solutions, it was possible to find H + 1 + Aq ; 

 then, knowing HI + Aq, to find I + H. It is evident that unavoidable errors may 

 accumulate. 



69 One can believe, however, on the basis of Berthollet's doctrine, and the obser- 

 vations of Potililzin (Note 66), that a certain slow decomposition of water by iodine 

 takes place. On this view the observations of Dossios and Weith on the fact that the 

 solubility of iodine in water increases after the lapse of several months will be comprehen- 

 sible. Hydriodic acid is then formed, and, it increases the solubility. If the iodine be 

 extracted from such a solution by carbon bisulphide, then, as the authors showed, after 

 the action of nitrous anhydride iodine maybe again detected in the solution by means of 

 starch. It can easily be understood that a number of similar reactions, requiring much 

 time and taking place in small quantities, have up to now eluded the attention of inves* 

 tigators, who even still doubt the universal application of Berthollet's doctrine, or only 

 see the thermochemical side of reactions, or else neglect to pay attention to the element 

 of time and the influence of mass. 



