WRITING EQUATIONS 



121 



/ 



p 





X 



only one molecule each of AgNO 3 and HC1 is necessary, 

 because all of these elements and radicals are monova- 

 lent. A simple exchange takes place in which the H of 

 the acid is replaced by the metal Ag. If the elements H 

 and Ag were to exchange places, they would occupy, 

 after the exchange, the positions shown on the right-hand 

 side of the equation. This exchange is represented 

 graphically in Fig. 52 ; A represents the order before, 

 and B after the reac- 

 tion. Blocks of 

 wood marked to rep- ** 

 resent the elements 

 and radicals can be 

 used, the block marked B 

 H being replaced by 



Fig. 52. Graphic illustration of a chemical 



the equivalent block reaction, 



marked Ag. When difficulty is experienced in writing 

 chemical equations, this method of illustration will be 

 found helpful. 

 In an equation as 



2NaCl + H 2 S0 4 = Na 2 SO, + 2HC1, 

 where both monovalent and bivalent elements and rad- 

 icals are present, it is necessary to take two molecules 

 of NaCl because there are two H atoms to be replaced. 

 H and Na have the same valence, viz. , i , and SO 4 has a 

 valence of 2. In order to obtain two atoms of Na, it is 

 necessary to take 2 NaCl ; then two atoms of Na replace 

 two of H. The products are Na 2 SO 4 and 2HC1. SO 4 is 

 a radical with a valence of 2 and requires 2Na atoms in 

 order to form a compound. A similar reaction is repre- 



