250 ELECTRO-MUTIVE SERIES OF THE METALS. 



than a concentrated acid. Thus it is that some nTetals on treat- 

 ment with dihite acid give chietly ammonia, whereas with a more 

 concentrated acid N2(_) or X^ may be the chief product of re- 

 duction. 



Moreover, the metal as it dissolves in the acid does so in 

 the ionic form. According to the Iqw of mass action the greater 

 tihe concentration of metal ions in solution the more difficult does 

 it become for more of the same ions to enter the solution. Hence 

 an accumulation of metallic nitrate in the system makes the metal 

 appear to have a less tendency to enter the ionic condition^ that 

 is. the metal then acts as if it were lower in the Electro-motive 

 Series than it really is. The reducing power of the nascent 

 hydrogen developed at its surface thus becomes less, and conse- 

 quently products of reduction containing more oxygen are formed. 

 Tlius from a metal which gives chiefly N(3 at first, the accumu- 

 lation oif metallic nitrate nicay give rise to increasing amounts of 

 NO2. This tendency, however, may be balanced more or less 

 by another complication, z'iz., that as the reaction proceeds the 

 acid is becoming more dilute. Not only is this because the acid 

 is used up in oxidising nascent hydrogen, ,and in forming metallic 

 nitrate, Ibut water is being produced which continually dilutes the 

 acid. Hence the acid becomes very rapidly more dilute and, as 

 we have seen above, this means in general the production of 

 products of a lower state of oxidation. 



This latter consideration may out-balance the former, and 

 so some metals which at first pro^cluce chiefly NO, when acted 

 on by nitric acid may give chiefly N2O as tlie reaction proceeds. 



This production of water may be of much greater importance 

 in the action of acids on metals than is generally thought to be 

 the case. To put the matter in the form of a question, let us 

 ask. " Why does not moderately concentrated nitric or sulphuric 

 acid act upon certain metals, wdiereas a more dilute aid reacts 

 immediately?" That such is often the case is very quickly ob- 

 served by students during the preparation of nitrates from metals. 

 Lead forms a very good example. One explanation, and the one 

 generally given, is that the metallic nitrate formed is insoluble 

 in concentrated acid and forms a protective covering over the 

 surface of the metal. Tliere may be. however, much more in the 

 matter than this. T should like to suggest an explanation which 

 I have not ])reviously heard expressed. Although not perhaps 

 suitable as an explanation to a young student, yet it may possess 

 a certain amount of interest. 



There are many facts in connection with the strong acids 

 which point to the conclusion that they undergo chemical com- 

 bination with water. The heat evolution and the contraction on 

 mixing with water are ' arguments in favour oif this conclusion, 

 and moreover in certain cases definite hydrates have been actually 

 isolated. 



If such combination does really take place, and if one 

 hydrate largely predominates in the solution, as is very likely, 

 then the law of mass action demands that at some definite con- 



