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 + 4H_{2}O = Fe_{3}O_{4} + 8H.
On the other hand, when hydrogen is passed over hot iron oxide the
reverse reaction takes place:
Fe_{3}O_{4} +8H = 3Fe + 4H_{2}O.
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 + 4H_{2}O <--> Fe_{3}O_{4} + 8H.
In a similar way, the equation
N + 3H <--> NH_{3}
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 + 4H_{2}O <--> Fe_{3}O_{4} + 8H,
Zn + H_{2}O <--> 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 + 4H_{2}O <--> Fe_{3}O_{4} + 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 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^{+} + NO_{3}^{-} <--> HNO_{3}.
~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^{+}, NO_{3}^{-}, H^{+}, SO_{4}^{--}, are formed. These ions are
free to rearrange themselves in various combinations. For example, the
H^{+} and the NO_{3}^{-} ions will reach the equilibrium
H^{+} + NO_{3}^{-} <--> HNO_{3}.
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 NO_{3}^{-} ions have combined to form
undissociated HNO_{3}.
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 NO_{3}^{-} 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 (NaHSO_{4}),
and the reaction can therefore be expressed by the equation
NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.
2. _An insoluble solid may be formed._ When hydrochloric acid (HCl) and
silver nitrate (AgNO_{3}) are brought together in solution the
following ions will be present: H^{+}, Cl^{-}, Ag^{+}, NO_{3}^{-}. 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:
AgNO_{3} + HCl = AgCl + HNO_{3}.
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^{-} <--> H_{2}O.
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 = H_{2}O + 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.
EXERCISES
1. What would take place when solutions of silver nitrate and sodium
chloride are brought together? What other chlorides would act in the
same way?
2. Is the reaction expressed by the equation NH_{3} + H_{2}O = NH_{4}OH
reversible? If so, state the conditions under which it will go in each
direction.
3. Is the reaction expressed by the equation 2H + O = H_{2}O reversible?
If so, state the conditions under which it will go in each direction.
4. Suggest a method for the preparation of hydrochloric acid.