494 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 + I., = 2KI + CO., -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 (besides which the reaction is 

 here complicated by iodine being more easily oxidised than chlorine). 

 Aluminium iodide burns in a current of oxygen (Deville and Troost), 

 and a similar, although not so highly developed, relation exists for 

 aluminium choride, and shows that the halogens have a distinctly 

 smaller affinity for metals which only form feeble bases. This refers to 

 a still greater extent to non-metals, which form acids and evolve much 

 more heat with oxygen than the halogens. 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, 

 whilst the properties of the atoms of all the halogens are analogous. 

 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 

 strong heat is required to only partially decompose hydrogen chloride, 



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 83-68 90'74 91'70 95'49 



Consequently, the conversions which are accompanied by this evolution of heat proceed 

 with very much greater rapidity than the reverse conversions. If the rates at the 

 commencement of the first reactions be placed side by side with the amounts of heat 

 evolved by them, then a perfect conformity is seen to exist between the data. In the 

 reaction between AgCl and KBr 3'5 thousand heat units are evolved, and the rate of 

 this reaction in the first two hours is expressed by the formation of 79"8 p.c. of bromide of 

 silver; if NaBr act on AgCl, then 4*3 thousand heat units are evolved, and the rale 

 during the first two hours is 83'2 p.c. This conformity between the rate of reaction and 

 the thermal figures is also remarked for other compounds. This shows that the thennal 

 figures are not equivalent to the entire work of affinity, but are only proportional to the 

 first rates of reactions. This explains why it is possible upon their basis to foresee the 

 direction of the dominating reaction in a complex medium, but impossible to foretell in 

 what direction the reaction will proceed. 



