374 Gooch and McClenahan — Typical Hydrous Chlorides. 



This symbol brings out the experimental fact that water cannot 

 be eliminated without simultaneous liberation of hydrogen 

 chloride, at least in the first reaction, and shows that from a 

 molecule thus constituted the formation of aluminum hydrox- 

 ide or oxide with evolution of water and hydrogen chloride 

 would seem to be natural. 



A little consideration makes it obvious, however, that the 

 decomposition of a molecule containing the group 



H H 



i i 

 _ <j— O— CI 

 I I 

 H H 



need not in every case result finally in the elimination of all 

 chlorine as hydrochloric acid and the formation of the metallic 

 hydroxide, oxyhydroxide, or oxide. When conditions favor, 

 there would seem to be nothing to prevent more or less secon- 

 dary reaction of the liberated hydrogen chloride upon the 

 metallic hydroxide first produced. Whether such secondary 

 reaction will take place in any given case will depend largely 

 upon the relative strengths of affinities of which we have, at 

 least in many cases, some sort of measure in the heats of 

 reaction. 



It is not likely, for example, that the reaction of hydrogen 

 chloride upon aluminum oxide with formation of aluminum 

 chloride, and gaseous water will take place to any considerable 

 degree when the heat of such reaction would be negative to 

 the extent of approximately 27,000 cal. for every gram-molecule 

 of aluminum oxide converted. On the other hand, it is not 

 unreasonable to look for some convertion of magnesium oxide 

 to magnesium chloride by a reaction which involves the libera- 

 tion of approximately 19,000 calories to the gram-molecule of 

 magnesium oxide attacked. So it appears that an inner link- 

 age such as is suggested by Cushman might possibly exist in 

 the case of such a salt as hydrous magnesium chloride without 

 coming very much into evidence by reason of the evolution of 

 hydrogen chloride and formation of magnesium hydroxide or 

 oxide when the salt is heated. 



Turning now to the consideration of what is likely to occur 

 when a molecule possessing the linking 



II H 



i i 

 _0 — O— CI 

 I I 

 H H 



is heated in presence of hydrogen chloride, it is obvious that 

 the first effect of increasing the concentration of hydrogen 





