114 PRINCIPLES OF CHEMISTRY. 



Reversible actions and chemical equilibrium. Experimental study 

 has shown that in many instances a chemical action, when once started, runs 

 to completion, that is, continues until the substance, or one of two substances, 

 undergoing change is used up. For example, when a piece of magnesium is 

 ignited the action continues until all the metal is used up, or the oxygen in 

 the supply of air is exhausted. Moreover, this action cannot be reversed, that 

 is, made to proceed in the opposite manner, no matter how much heat, or what 

 degree of heat, available in the laboratory, we apply to the magnesium oxide. 

 In other words, we cannot decompose the latter into magnesium element and. 

 oxygen by heat alone. 



On the other hand, there are many instances in which chemical action, 

 under a given set of conditions, is not complete, but proceeds to a certain point 

 beyond which the products formed tend to act in a reverse manner, and repro- 

 duce the original substance or substances. Such changes are known as revers- 

 ible actions, and evidently, while the conditions are maintained, the whole 

 chemical process comes apparently to a standstill. But in the light of the 

 kinetic-molecular theory of matter it is believed that action is constantly going 

 on, although there is no progress made in either direction. The forward action 

 of the system is counterbalanced by the reverse action which proceeds at the 

 same speed, and thus is produced a condition of seeming rest, or chemical equi- 

 librium. An example of equilibrium as a result of two equal and opposite 

 actions is the case of a liquid in a closed container. At a definite temperature, 

 the space above the liquid is saturated with its vapor which exerts a constant 

 pressure. Although there is apparent rest, molecules of the liquid are passing 

 off into the space above it, while vapor molecules are flying back into the 

 liquid. These opposite actions finally balance each other, and then the system 

 is in equilibrium. If the conditions are changed, for example, by a rise in 

 temperature, the equilibrium is disturbed, a readjustment and new equilibrium 

 follow, in which more vapor molecules exist in the space above the liquid, 

 and a higher vapor pressure is produced. 



Reversible chemical actions are represented by equations which differ from 

 the ordinary chemical equations, in that the equality sign is replaced by two 

 oppositely directed arrows, thus : 

 * AB + CD 7=1 AD -f CB. 



Such an equation indicates that the action takes place in two directions, for- 

 ward and backward, and when equilibrium has been reached as much material 

 continues to be transformed in one direction as in the reverse direction. 



While in many cases the action is reversible, yet it runs far toward comple- 

 tion in one direction. This condition may be represented by making one of 

 the arrows heavier than the other, thus : 



MN -f PR ;i MR + PN. 



The following is a good example of a reversible chemical change. If finely 

 divided iron and water vapor be heated in a sealed glass tube so that none of 

 the products can escape, a state of equilibrium will result, in which four prod- 

 ucts exist in the tube namely, iron, iron oxide, water vapor, and hydrogen. 



3Fe + 4H 2 O 7 Fe 3 O 4 -f 8H. 

 This means that at the equilibrium stage the hydrogen reduces the iron 



