NITRIC ACID 317 



The valence of N in HN0 3 , from its anhydride N 2 O 5 , is N T . 

 In Cu(N0 3 ) 2 the valence of N is unchanged; all that has happened 

 here is the displacement of 2H 1 by Cu 11 . The valence of N in 

 NO, however, has been reduced to N 11 . 



Let us see how we can utilize these facts to balance our equation. 

 The formation of one molecule of Cu(N0 3 )2 involves the displace- 

 ment of 2 atoms of hydrogen. The reduction of one molecule 

 of N 2 5 to 2NO involves the liberation of 3 atoms of oxygen. 

 Both reactions are occuring simultaneously, so that neither hydro- 

 gen nor oxygen is actually obtained in the free state, they will of 

 course unite to form water. But 3 atoms of oxygen are equivalent 

 to 6 atoms of hydrogen, therefore 3 molecules of Cu(N0 3 ) 2 must 

 be formed for every molecule of N 2 5 that is reduced. To obtain 

 3Cu(NO 3 ) 2 , we must use 3Cu and 6HN0 3 . To reduce 1N 2 6 , 

 we must use 2 more HN0 3 , that is, 8HN0 3 in all. The balanced 

 equation therefore becomes: 

 Balanced: 3Cu + 8HN0 3 - 3Cu(N0 3 ) 2 + 2NO + 4H 2 0. 



1'hese operations should be practiced on the following skeleton 

 equations : 



HN0 3 + C -> N0 2 + C0 2 + H 2 (p. 309). 

 H 2 S0 4 + HI -* H 2 S + H 2 O + I 2 (p. 204). 

 Zn + H 2 S0 4 -> ZnS0 4 + H 2 O + H 2 S (p. 271). 



These are typical examples of rections which it is very difficult 

 to write correctly without some such guide as is given above. 



Exercises. 1. What is the valence of carbon in carbonic 

 acid H 2 C0 3 ? 



2. What is the anhydride of nitrous acid HN0 2 , and what the 

 valence of nitrogen in this compound? 



3. When nitric acid acts upon copper, which substance is oxi- 

 dized and which reduced? 



4. How could you show experimentally that both nitrogen tetr- 

 oxide and oxygen are formed when cupric nitrate is heated? 



