THE HALOGENS 601 



The smaller store of energy in iodine and bromine is seen still more 

 clearly in the relation of the halogens to hydrogen. In a gaseous state 

 they all enter, with more or less ease, into direct combination with 

 gaseous hydrogen for example, in the presence of spongy platinum, 

 forming halogen acids, HX but the latter are far from being equally 

 stable ; hydrogen chloride is the most stable, hydrogen iodide the least 

 so, and hydrogen bromide occupies an intermediate position A very 

 high temperature is required to decompose hydrogen chloride.. even par- 

 tially, whilst hydrogen iodide is decomposed by light even at the 

 ordinary temperature and very easily by a red heat. Hence the reaction 

 I 2 + H 2 = HI + HI is very easily reversible, and consequently has a 

 limit, and hydrogen iodide easily dissociates. 67 Judging by the direct 

 measurement of the heat evolved (22,000 heat units) in the formation 

 of HC1, the conversion of 2HC1 into H 2 + C1 2 requires the expenditure 



67 The dissociation of hydriodic acid has been studied in detail by Hautefeuille and 

 Lemoine, from whose researches we extract the following information. The decom- 

 position of hydriodic acid is decided, but proceeds slowly at 180 ; the rate and limit of 

 decomposition increase with a rise of temperature. The reverse action that is, I^ + H^ 

 = 2HI proceeds not only under the influence of spongy platinum (Corenwinder), 

 which also accelerates the decomposition of hydriodic acid, but also by itself, although 

 slowly. The limit of the reverse reaction remains the same with or without spongy 

 platinum. An increase of pressure has a very powerful accelerative effect on the 

 rate of formation of hydriodic acid, and therefore spongy platinum by condensing 

 gases has the same effect as increase of pressure. At the atmospheric pressure the 

 decompcsit.on of hydriodic acid reaches the limit at 250 in several months, and at 

 440 in several hours. The limit at 250 is about 18 p.c. of decomposition that is, 

 out of 100 parts of hydrogen previously combined in hydriodic acid, about 18 p.c. may 

 be disengaged .at this temperature (this hydrogen may be easily measured, and the 

 measure of dissociation determined), but not more ; the limit at 440 is about 26 p.o. 

 If the pressure under which 2HI passes into H 3 + I 2 be 4 atmospheres, then the limit is 

 24 p.c. ; under a pressure of atmosphere the limit is 29 p.c. The small influence of 

 pressure on the dissociation of hydriodic acid (compared with N 2 O 4 , Chapter VI. Note 46) 

 is due to the fact that the reaction 2HI = I 2 + H 2 is not accompanied by a change of 

 volume. In order to show the influence of time, we will cite the following figures 

 referring to 350: (1) Reaction H 2 + I 2 ; after 3 hours, 88 p.c. of hydrogen remained free; 

 8 hours, 69 p.c.; 84 hours, 48 p.c.; 76 hours, 29 p.c. ; and 327 hours, 18-5 p.c. (2) The 

 reverse decomposition of 2HI ; after 9 hours, 3 p.c. of hydrogen was set free, and after 

 250 hours 18*6 p.c.- that is, the limit was reached. The addition of extraneous 

 hydrogen diminishes the limit of the reaction of decomposition, or increases the 

 formation of hydriodic acid from iodine and hydrogen, as would be expected from 

 Berthollet's doctrine (Chapter X.). Thus at 440 26 p.c. of hydriodic acid is decomposed 

 if there be no admixture of hydrogen, while if H 2 be added, then at the limit only half 

 as large a mass of HI is decomposed. Therefore, if an infinite mass of hydrogen b* 

 added there will be no decomposition of the hydriodic acid. Light aids the decomposition 

 of hydriodic acid very powerfully. At the ordinary temperature 80 p.c. is decomposed 

 under the influence of light, whilst under the influence of heat alone this, limit corre- 

 sponds with a very high temperature. The distinct action of light, spongy platinum, and 

 of impurities in glass (especially of sodium sulphate, which decomposes hydriodic acid), not 

 only render the investigations difficult, but also show that in reactions like 2HI = I 2 +H 3 , 

 which are accompanied by slight heat effects, all foreign and feeble influences may strongly 

 affect the progress of the action (Note 47). 



