500 PRINCIPLES OF CHEMISTRY 



sometimes behave with respect to metallic oxides in exactly the same 

 manner as chlorine. Gay-Lussac, by igniting potassium carbonate in 

 iodine vapour, obtained (as with chlorine) an evolution of oxygen and 

 carbonic anhydride, K 2 C0 3 4- I 2 == 2KI + CO 2 -f- O, only the reac- 

 tions between the halogens and oxygen are more easily reversible with 

 bromine and iodine than with chlorine. Thus, at a red heat oxygen 

 displaces iodine from barium iodide. Aluminium iodide burns in 

 a current of oxygen (Deville and Troost), and a similar, although 

 not so clearly marked, relation exists for aluminium chloride, and shows 

 that the halogens have a distinctly smaller affinity for those metals 

 which only form feeble bases. This is still more the case with 

 the non-metals, whfch form acids and evolve much more heat with 

 oxygen than with the halogens (Note 13). But in all these instances 

 the affinity (and amount of heat evolved) of iodine and bromine is less 

 than that of chlorine, probably because the atomic weights are greater. 



from observations on the chlorides of Li, K, Na, Ag (n = l), Ca, Sr, Ba, Co, Ni, Hg, Pb 

 (n = 2), Bi (n = 8), Sn (n = 4), and Fe 2 (n = 6). 



In these determinations of Potilitzin we see not only a brilliant confirmation of 

 Berthollet's doctrine, but also the first effort to directly determine the affinities of 

 elements by means of displacement. The chief object of these researches consisted in 

 proving whether a displacement occurs in those cases where heat is absorbed, and in 

 this instance it should be absorbed, because the formation of all metallic bromides is 

 attended with the evolution of less heat than that of the chlorides, as is seen by the 

 figures given in Note 55. 



If the mass of the bromine be .increased, then the amount of chlorine displaced also 

 increases. For example, if masses of bromine of 1 and 4 equivalents act on a molecule 

 of sodium chloride, then the percentages of the chlorine displaced will be 6-08 p.c. and 

 12-46 p.c. ; in the action of 1, 4, 25, and 100 molecules of bromine on a molecule of 

 barium chloride, there will be displaced 7'8, 17'6, 35-0, and 45 p.c. of chlorine. If 

 an equivalent quantity of hydrochloric acid act on metallic bromides in closed tubes, 

 and in the absence of water at a temperature of 800, then the percentages of the sub- 

 stitution of the bromine by the chlorine in the double decomposition taking place between 

 univalent metals are inversely proportional to their atomic weights. For example, 

 NaBr + SCl gives at the limit 21 p.c. of displacement, KC1 12 p.p. and AgCl 4$ p.c. 

 Essentially the same action takes place in an aqueous solution, although the phenomenon 

 is complicated by the participation of the water. The reactions proceed spontaneously in 

 one or the other direction at the ordinary temperature but at different rates. In the 

 action of a dilute solution (1 equivalent per 5 litres) of sodium chloride on silver bromide 

 at the ordinary temperature the amount of bromine replaced in six and a half days is 

 2'07 p.c., and with potassium chloride 1'5 p.c. With an excess of the chloride the mag- 

 nitude of the substitution increases. These conversions also proceed with the absorption 

 of heat. The reverse reactions evolving heat proceed incomparably more rapidly, but 

 also to a certain limit ; for example, in the reaction AgCl + RBr the following percentages 

 of silver bromide are formed in different times : 



hours 2 3 22 96 120 



K 79-82 87'4 88'22 94'21 



Na 88-68 90'74 91'70 95'49 



That is, the conversions which are accompanied by an evolution of heat proceed 

 with very much greater rapidity than the reverse conversions. 



