CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC. 115 



oxide reversely as fast as the iron reduces the water vapor ill the forward 

 direction. 



When the experiment is performed in the same manner as above, with iron 

 oxide and hydrogen sealed in the tube at the equilibrium point, the same kinds 

 of products exist as in the first case, thus : 



Fe 3 O 4 + 8H Til 3Fe + 4H 2 O. 



Evidently a necessary condition for maintaining a chemical equilibrium in 

 any reversible action is the keeping intact of all the factors taking part. If 

 one of the products of an action be removed from the field of action as fast as 

 it is formed, we might reasonably predict that the action would proceed to 

 completion in the direction made easiest by the removal of such product. This 

 is exactly what happens, as may be shown in the above instances. When steam 

 is passed over highly heated iron through an open tube, the action takes place 

 to completion, thus : 



Fe 3 + 4H 2 = Fe 3 4 + H. 



The hydrogen is swept out of the tube and away from contact with the iron 

 oxide by the current of steam. The action continues until all the iron is 

 exhausted. 



Conversely, when hydrogen gas is passed over heated iron oxide in an open 

 tube, the action runs to completion reversely thus : 



Fe 3 O 4 + 8H = 3Fe + 4H 2 O. 



The current of hydrogen sweeps the water vapor (steam) out of the tube as fast 

 as it is produced. The action continues until all the iron oxide is exhausted 

 by conversion to elementary iron. 



If one considered only the first action he would conclude that iron has a 

 greater affinity for oxygen than hydrogen has, whereas if he considered only 

 the second action, he would say that hydrogen has a greater affinity for oxygen 

 than iron has, which apparently is a contradiction. But both conclusions are 

 correct, depending on circumstances. In fact, in reversible actions affinity 

 plays a minor part in determining which direction a chemical change will 

 take, this being controlled in largest measure by the physical conditions of the 

 experiment, which have nothing to do with affinity. This is admirably shown 

 by the following example : When common salt (sodium chloride, NaCl) is dis- 

 solved in 20 per cent, aqueous solution of sulphuric acid (H 2 SO 4 ), nothing 

 apparently happens except solution of the salt. Yet a reversible action takes 

 place, thus : 



2NaCl + H 2 SO 4 7=1 Na 2 SO 4 -f 2HC1. 



Four products are present in solution in equilibrium. When, however, con- 

 centrated sulphuric acid, which is about 95 per cent., is -poured upon salt, a 

 brisk evolution of hydrochloric acid gas takes place, because of the fact that it 

 is nearly insoluble in concentrated sulphuric acid. One of the factors in the 

 equilibrium equation above is thus removed from the field of action, which 

 thus allows the action to go forward nearly to completion and leaves the'im- 

 pression that the sulphuric acid has a greater affinity for the metal sodium than 

 has the hydrochloric acid, or, as it is put sometimes in text-books, that sul- 

 phuric acid is a " stronger " acid than hydrochloric, which, in fact, is not true. 



