CHAPTER XX 

 CHEMICAL EQUILIBRIUM 



IN spite of its formidable title, this chapter will introduce noth- 

 ing novel. Its purpose is to collect together and organize more 

 definitely a number of scattered facts and ideas which have already 

 come up in various connections. On this account, however, it will 

 be all the more necessary for the reader to refresh his remembrance 

 of these facts and ideas by re-reading all pages to which reference 

 is made. 



Reversible Actions. In discussing Deacon's process (p. 140), 

 it was stated that the action 4HC1 + 2 -^2H 2 O + 2C1 2 comes to 

 rest although a large amount of both of the interacting substances 

 (20 per cent at 345) still remains available. Now the materials 

 thus left unused are presumably no less capable of interacting 

 than were the parts which have already reacted. The solution 

 of this mystery lies in the fact that the products themselves interact 

 to reproduce the initial substances (read the equation backwards). 

 Thus two changes, one of which undoes the work of the other, are 

 going on simultaneously. In consequence of this, neither action 

 can reach completion. As we should expect, experiment shows 

 that it makes no difference whether we start with pure chlorine 

 and steam, or with hydrogen chloride and oxygen ; the proportions 

 of the four substances found in the tube, after it has been kept at 

 345 for a sufficient time, are in both cases the same. A general 

 statement may be founded on facts like this, to the effect that a 

 chemical action must remain more or less incomplete when the 

 reverse action also takes place under the same conditions. Two 

 arrows pointing in opposite directions are used in equations 

 representing reversible changes. 



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