REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM

Reversible reactions. The reactions so far considered have been represented as continuing, when once started, until one or the other substance taking part in the reaction has been used up. In some reactions this is not the case. For example, we have seen that when steam is passed over hot iron the reaction is represented by the equation

3Fe + 4H₂O = Fe₃O₄ + 8H. 

On the other hand, when hydrogen is passed over hot iron oxide the reverse reaction takes place:

Fe₃O₄ +8H = 3Fe + 4H₂O. 

The reaction can therefore go in either direction, depending upon the conditions of the experiment. Such a reaction is called a reversible reaction. It is represented by an equation with double arrows in place of the equality sign, thus:

3Fe + 4H₂O <--> Fe₃O₄ + 8H.

In a similar way, the equation

N + 3H <--> NH₃

 

expresses the fact that under some conditions nitrogen may unite with hydrogen to form ammonia, while under other conditions ammonia decomposes into nitrogen and hydrogen.

The conversion of oxygen into ozone is also reversible and may be represented thus:

oxygen <--> ozone.

Chemical equilibrium. Reversible reactions do not usually go on to completion in one direction unless the conditions under which the reaction takes place are very carefully chosen. Thus, if iron and steam are confined in a heated tube, the steam acts upon the iron, producing iron oxide and hydrogen. But these substances in turn act upon each other to form iron and steam once more. When these two opposite reactions go on at such rates that the weight of the iron changed into iron oxide is just balanced by the weight of the iron oxide changed into iron, there will be no further change in the relative weights of the four substances present in the tube. The reaction is then said to have reached an equilibrium.

Factors which determine the point of equilibrium. There are two factors which have a great deal of influence in determining the point at which a given reaction will reach equilibrium.

1. Influence of the chemical nature of the substances. If two reversible reactions of the same general kind are selected, it has been found that the point of equilibrium is different in the two cases. For example, in the reactions represented by the equations

3Fe + 4H₂O <--> Fe₃O₄ + 8H,

Zn + H₂O <--> ZnO + 2H, 

the equilibrium will be reached when very different quantities of the iron and zinc have been changed into oxides. The individual chemical properties of the iron and zinc have therefore marked influence upon the point at which equilibrium will be reached.

2. Influence of relative mass. If the tube in which the reaction

3Fe + 4H₂O <--> Fe₃O₄ + 8H 

has come to an equilibrium is opened and more steam is admitted, an additional quantity of the iron will be changed into iron oxide. If more hydrogen is admitted, some of the oxide will be reduced to metal. The point of equilibrium is therefore dependent upon the relative masses of the substances taking part in the reaction. When one of the substances is a solid, however, its mass has little influence, since it is only the extent of its surface which can affect the reaction.

Conditions under which reversible reactions are complete. If, when the equilibrium between iron and steam has been reached, the tube is opened and a current of steam is passed in, the hydrogen is swept away as fast as it is formed. The opposing reaction of hydrogen upon iron oxide must therefore cease, and the action of steam on the iron will go on until all of the iron has been transformed into iron oxide.

On the other hand, if a current of hydrogen is admitted into the tube, the steam will be swept away by the hydrogen, and all of the iron oxide will be reduced to iron. A reversible reaction can therefore be completed in either direction when one of the products of the reaction is removed as fast as it is formed.

Equilibrium in solution. When reactions take place in solution in water the same general principles hold good. The matter is not so simple, however, as in the case just described, owing to the fact that many of the reactions in solution are due to the presence of ions. The substances most commonly employed in solution are acids, bases, or salts, and all of these undergo dissociation. Any equilibrium which may be reached in solutions of these substances must take place between the various ions formed, on the[Pg 140] one hand, and the undissociated molecules, on the other. Thus, when nitric acid is dissolved in water, equilibrium is reached in accordance with the equation

H⁺ + NO₃^- <--> HNO₃. 

Conditions under which reversible reactions in solution are complete. The equilibrium between substances in solution may be disturbed and the reaction caused to go on in one direction to completion in either of three ways.

1. A gas may be formed which escapes from the solution. When sodium nitrate and sulphuric acid are brought together in solution all four ions, Na⁺, NO₃^-, H⁺, SO₄^-, are formed. These ions are free to rearrange themselves in various combinations. For example, the H⁺ and the NO₃^- ions will reach the equilibrium

H⁺ + NO₃^- <--> HNO₃. 

If the experiment is performed with very little water present, as is the case in the preparation of nitric acid, the equilibrium will be reached when most of the H⁺ and the NO₃^- ions have combined to form undissociated HNO₃.

Finally, if the mixture is now heated above the boiling point of nitric acid, the acid distills away as fast as it is formed. More and more H⁺ and NO₃^- ions will then combine, and the process will continue until one or the other of them has all been removed from the solution. The substance remaining is sodium acid sulphate (NaHSO₄), and the reaction can therefore be expressed by the equation

NaNO₃ + H₂SO₄ = NaHSO₄ + HNO₃. 

2. An insoluble solid may be formed. When hydrochloric acid (HCl) and [Pg 141]silver nitrate (AgNO₃) are brought together in solution the following ions will be present: H⁺, Cl^-, Ag⁺, NO₃^-. The ions Ag⁺ and Cl^- will then set up the equilibrium

Ag⁺ + Cl^- <--> AgCl. 

But silver chloride (AgCl) is almost completely insoluble in water, and as soon as a very little of it has formed the solution becomes supersaturated, and the excess of the salt precipitates. More silver and chlorine ions then unite, and this continues until practically all of the silver or the chlorine ions have been removed from the solution. We then say that the following reaction is complete:

AgNO₃ + HCl = AgCl + HNO₃. 

3. Two different ions may form undissociated molecules. In the neutralization of sodium hydroxide by hydrochloric acid the ions H⁺ and OH^- come to the equilibrium

H⁺ + OH^- <--> H₂O. 

But since water is almost entirely undissociated, equilibrium can only be reached when there are very few hydroxyl or hydrogen ions present. Consequently the two ions keep uniting until one or the other of them is practically removed from the solution. When this occurs the neutralization expressed in the following equation is complete:

NaOH + HCl = H₂O + NaCl. 

Preparation of acids. The principle of reversible reactions finds practical application in the preparation of most of the common acids. An acid is usually prepared by treating the most common of its salts with some other acid of high boiling point. The mixture is then heated until the lower boiling acid desired distills out. Owing to its high boiling point (338°), sulphuric acid is usually employed for this purpose, most other acids boiling below that temperature.