An Elementary Study of Chemistry

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NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.

If a smaller quantity of sulphuric acid is taken and the
mixture is heated to a high temperature, normal sodium sulphate
is formed:

2NaNO_{3} + H_{2}SO_{4} = Na_{2}SO_{4} + 2HNO_{3}.

In this case, however, the higher temperature required
decomposes a part of the nitric acid.

~The commercial preparation of nitric acid.~ Fig. 38 illustrates
a form of apparatus used in the preparation of nitric acid on a
large scale. Sodium nitrate and sulphuric acid are heated in
the iron retort A. The resulting acid vapors pass in the
direction indicated by the arrows, and are condensed in the
glass tubes B, which are covered with cloth kept cool by
streams of water. These tubes are inclined so that the liquid
resulting from the condensation of the vapors runs back into
C and is drawn off into large vessels (D).

[Illustration Fig. 38]

~Physical properties of nitric acid.~ Pure nitric acid is a colorless
liquid, which boils at about 86 deg. and has a density of 1.56. The
concentrated acid of commerce contains about 68% of the acid, the
remainder being water. Such a mixture has a density of 1.4. The
concentrated acid fumes somewhat in moist air, and has a sharp choking
odor.

~Chemical properties.~ The most important chemical properties of nitric
acid are the following.

1. _Acid properties._ As the name indicates, this substance is an acid,
and has all the properties of that class of substances. It changes blue
litmus red and has a sour taste in dilute solutions. It forms hydrogen
ions in solution and neutralizes bases forming salts. It also acts upon
the oxides of most metals, forming a salt and water. It is one of the
strongest acids.

2. _Decomposition on heating._ When boiled, or exposed for some time to
sunlight, it suffers a partial decomposition according to the equation

2HNO_{3} = H_{2}O + 2NO_{2} + O.

The substance NO_{2}, called nitrogen peroxide, is a brownish gas, which
is readily soluble in water and in nitric acid. It therefore dissolves
in the undecomposed acid, and imparts a yellowish or reddish color to
it. Concentrated nitric acid highly charged with this substance is
called _fuming nitric acid_.

3. _Oxidizing action._ According to its formula, nitric acid contains a
large percentage of oxygen, and the reaction just mentioned shows that
the compound is not a very stable one, easily undergoing decomposition.
These properties should make it a good oxidizing agent, and we find that
this is the case. Under ordinary circumstances, when acting as an
oxidizing agent, it is decomposed according to the equation

2HNO_{3} = H_{2}O + 2NO + 3O.

The oxygen is taken up by the substance oxidized, and not set free, as
is indicated in the equation. Thus, if carbon is oxidized by nitric
acid, the oxygen combines with carbon, forming carbon dioxide (CO_{2}):

C + 2O = CO_{2}.

4. _Action on metals._ We have seen that when an acid acts upon a metal
hydrogen is set free. Accordingly, when nitric acid acts upon a metal,
such as copper, we should expect the reaction to take place which is
expressed in the equation

Cu + 2HNO_{3} = Cu(NO_{3})_{2} + 2H.

This reaction does take place, but the hydrogen set free is immediately
oxidized to water by another portion of the nitric acid according to the
equation

HNO_{3} + 3H = 2H_{2}O + NO.

As these two equations are written, two atoms of hydrogen are given off
in the first equation, while three are used up in the second. In order
that the hydrogen may be equal in the two equations, we must multiply
the first by 3 and the second by 2. We shall then have

3Cu + 6HNO_{3} = 3Cu(NO_{3})_{2} + 6H,

2HNO_{3} + 6H = 4H_{2}O + 2NO.

The two equations may now be combined into one by adding the quantities
on each side of the equality sign, canceling the hydrogen which is given
off in the one reaction and used up in the other. We shall then have the
equation

3Cu + 8HNO_{3} = 3Cu(NO_{3})_{2} + 2NO + 4H_{2}O.

A number of other reactions may take place when nitric acid acts upon
metals, resulting in the formation of other oxides of nitrogen, free
nitrogen, or even ammonia. The reaction just given is, however, the
usual one.

~Importance of steps in a reaction.~ This complete equation has
the advantage of making it possible to calculate very easily
the proportions in which the various substances enter into the
reaction or are formed in it. It is unsatisfactory in that it
does not give full information about the way in which the
reaction takes place. For example, it does not suggest that
hydrogen is at first formed, and subsequently transformed into
water. It is always much more important to remember the steps
in a chemical reaction than to remember the equation expressing
the complete action; for if these steps in the reaction are
understood, the complete equation is easily obtained in the
manner just described.

~Salts of nitric acid,--nitrates.~ The salts of nitric acid are called
nitrates. Many of these salts will be described in the study of the
metals. They are all soluble in water, and when heated to a high
temperature undergo decomposition. In a few cases a nitrate on being
heated evolves oxygen, forming a nitrite:

NaNO_{3} = NaNO_{2} + O.

In other cases the decomposition goes further, and the metal is left as
oxide:

Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.

~Nitrous acid~ (HNO_{2}). It is an easy matter to obtain sodium nitrite
(NaNO_{2}), as the reaction given on the previous page indicates.
Instead of merely heating the nitrate, it is better to heat it together
with a mild reducing agent, such as lead, when the reaction takes place
which is expressed by the equation

NaNO_{3} + Pb = PbO + NaNO_{2}.

When sodium nitrite is treated with an acid, such as sulphuric acid, it
is decomposed and nitrous acid is set free:

NaNO_{2} + H_{2}SO_{4} = NaHSO_{4} + HNO_{2}.

The acid is very unstable, however, and decomposes readily into water
and nitrogen trioxide (N_{2}O_{3}):

2HNO_{2} = H_{2}O + N_{2}O_{3}.

Dilute solutions of the acid, however, can be obtained.

COMPOUNDS OF NITROGEN WITH OXYGEN

Nitrogen combines with oxygen to form five different oxides. The
formulas and names of these are as follows:

N_{2}O nitrous oxide.
NO nitric oxide.
NO_{2} nitrogen peroxide.
N_{2}O_{3} nitrogen trioxide, or nitrous anhydride.
N_{2}O_{5} nitrogen pentoxide, or nitric anhydride.

These will now be briefly discussed.

~Nitrous oxide~ (_laughing gas_) (N_{2}O). Ammonium nitrate, like all
nitrates, undergoes decomposition when heated; and owing to the fact
that it contains no metal, but does contain both oxygen and hydrogen,
the reaction is a peculiar one. It is represented by the equation

NH_{4}NO_{3} = 2H_{2}O + N_{2}O.

The oxide of nitrogen so formed is called nitrous oxide or laughing gas.
It is a colorless gas having a slight odor. It is somewhat soluble in
water, and in solution has a slightly sweetish taste. It is easily
converted into a liquid and can be purchased in this form. When inhaled
it produces a kind of hysteria (hence the name "laughing gas"), and even
unconsciousness and insensibility to pain if taken in large amounts. It
has long been used as an anaesthetic for minor surgical operations, such
as those of dentistry, but owing to its unpleasant after effects it is
not so much in use now as formerly.

Chemically, nitrous oxide is remarkable for the fact that it is a very
energetic oxidizing agent. Substances such as carbon, sulphur, iron, and
phosphorus burn in it almost as brilliantly as in oxygen, forming oxides
and setting free nitrogen. Evidently the oxygen in nitrous oxide cannot
be held in very firm combination by the nitrogen.

[Illustration Fig. 39]

~Nitric oxide~ (NO). We have seen that when nitric acid acts upon metals,
such as copper, the reaction represented by the following equation takes
place:

3Cu + 8HNO_{3} = 3Cu(NO_{3})_{3} + 2NO + 4H_{2}O.

Nitric oxide is most conveniently prepared in this way. The metal is
placed in the flask A (Fig. 39) and the acid added slowly through the
funnel tube B. The gas escapes through C and is collected over
water.

Pure nitric oxide is a colorless gas, slightly heavier than air, and is
practically insoluble in water. It is a difficult gas to liquefy. Unlike
nitrous oxide, nitric oxide does not part with its oxygen easily, and
burning substances introduced into this gas are usually extinguished. A
few substances like phosphorus, which have a very strong affinity for
oxygen and which are burning energetically in the air, will continue to
burn in an atmosphere of nitric oxide. In this case the nitric oxide
loses all of its oxygen and the nitrogen is set free as gas.

~Action of nitric oxide with oxygen.~ When nitric oxide comes into contact
with oxygen or with the air, it at once combines with the oxygen even at
ordinary temperatures, forming a reddish-yellow gas of the formula
NO_{2}, which is called nitrogen peroxide. This action is not energetic
enough to produce a flame, though considerable heat is set free.

~Nitrogen peroxide~ (NO_{2}). This gas, as we have just seen, is formed by
allowing nitric oxide to come into contact with oxygen. It can also be
made by heating certain nitrates, such as lead nitrate:

Pb(NO_{3})_{2} = PbO + 2NO_{2} + O.

It is a reddish-yellow gas of unpleasant odor, which is quite poisonous
when inhaled. It is heavier than air and is easily condensed to a
liquid. It dissolves in water, but this solution is not a mere physical
solution; the nitrogen peroxide is decomposed, forming a mixture of
nitric and nitrous acids:

2NO_{2} + H_{2}O = HNO_{2} + HNO_{3}.

Nitrogen peroxide will not combine with more oxygen; it will, however,
give up a part of its oxygen to burning substances, acting as an
oxidizing agent:

NO_{2} = NO + O.

~Acid anhydrides.~ The oxides N_{2}O_{3} (nitrogen trioxide) and
N_{2}O_{5} (nitrogen pentoxide) are rarely prepared and need not be
separately described. They bear a very interesting relation to the acids
of nitrogen. When dissolved in water they combine with the water,
forming acids:

N_{2}O_{3} + H_{2}O = 2HNO_{2},

N_{2}O_{5} + H_{2}O = 2HNO_{3}.

On the other hand, nitrous acid very easily decomposes, yielding water
and nitrogen trioxide, and by suitable means nitric acid likewise may be
decomposed into water and nitrogen pentoxide:

2HNO_{2} = H_{2}O + N_{2}O_{3},

2HNO_{3} = H_{2}O + N_{2}O_{5}.

In view of the close relation between these oxides and the corresponding
acids, they are called _anhydrides_ of the acids, N_{2}O_{3} being
nitrous anhydride and N_{2}O_{5} nitric anhydride.

DEFINITION: _Any oxide which will combine with water to form an acid, or
which together with water is formed by the decomposition of an acid, is
called an anhydride of that acid._

EXERCISES

1. Perfectly dry ammonia does not affect litmus paper. Explain.

2. Can ammonia be dried by passing the gas through concentrated
sulphuric acid? Explain.

3. Ammonium hydroxide is a weak base, i.e. it is not highly dissociated.
When it is neutralized by strong acids the heat of reaction is less than
when strong bases are so neutralized. Suggest some possible cause for
this.

4. Why is brine used in the manufacture of artificial ice?

5. Discuss the energy changes which take place in the manufacture of
artificial ice.

6. What weight of ammonium chloride is necessary to furnish enough
ammonia to saturate 1 l. of water at 0 deg. and 760 mm.?

7. What weight of sodium nitrate is necessary to prepare 100 cc. of
commercial nitric acid? What weight of potassium nitrate is necessary to
furnish the same weight of acid?

8. 100 l. of nitrogen peroxide were dissolved in water and neutralized
with sodium hydroxide. What substances were formed and how much of
each?(1 l. nitrogen peroxide weighs 2.05 grams.)

9. How many liters of nitrous oxide, measured under standard conditions,
can be prepared from 10 g. of ammonium nitrate?

10. What weight of copper is necessary to prepare 50 l. of nitric oxide
under standard conditions?

11. (a) Calculate the percentage composition of the oxides of
nitrogen. (b) What important law does this series of substances
illustrate?

12. Write the equations representing the reactions between ammonium
hydroxide, and sulphuric acid and nitric acid respectively, in
accordance with the theory of electrolytic dissociation.

13. In the same way, write the equations representing the reactions
between nitric acid and each of the following bases: NaOH, KOH,
NH_{4}OH, Ca(OH)_{2}.

CHAPTER XIII

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 deg.), 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.

CHAPTER XIV

SULPHUR AND ITS COMPOUNDS

~Occurrence.~ The element sulphur has been known from the earliest times,
since it is widely distributed in nature and occurs in large quantities
in the uncombined form, especially in the neighborhood of volcanoes.
Sicily has long been famous for its sulphur mines, and smaller deposits
are found in Italy, Iceland, Mexico, and especially in Louisiana, where
it is mined extensively. In combination, sulphur occurs abundantly in
the form of sulphides and sulphates. In smaller amounts it is found in a
great variety of minerals, and it is a constituent of many animal and
vegetable substances.

~Extraction of sulphur.~ Sulphur is prepared from the native substance,
the separation of crude sulphur from the rock and earthy materials with
which it is mixed being a very simple process. The ore from the mines is
merely heated until the sulphur melts and drains away from the earthy
impurities. The crude sulphur obtained in this way is distilled in a
retort-shaped vessel made of iron, the exit tube of which opens into a
cooling chamber of brickwork. When the sulphur vapor first enters the
cooling chamber it condenses as a fine crystalline powder called
_flowers of sulphur_. As the condensing chamber becomes warm, the
sulphur collects as a liquid in it, and is drawn off into cylindrical
molds, the product being called _roll sulphur_ or _brimstone_.

~Physical properties.~ Roll sulphur is a pale yellow, crystalline solid,
without marked taste and with but a faint odor. It is insoluble in
water, but is freely soluble in a few liquids, notably in carbon
disulphide. Roll sulphur melts at 114.8 deg.. Just above the melting point
it forms a rather thin, straw-colored liquid. As the temperature is
raised, this liquid turns darker in color and becomes thicker, until at
about 235 deg. it is almost black and is so thick that the vessel containing
it can be inverted without danger of the liquid running out. At higher
temperatures it becomes thin once more, and boils at 448 deg., forming a
yellowish vapor. On cooling the same changes take place in reverse
order.

~Varieties of sulphur.~ Sulphur is known in two general forms, crystalline
and amorphous. Each of these forms exists in definite modifications.

~Crystalline sulphur.~ Sulphur occurs in two crystalline forms, namely,
rhombic sulphur and monoclinic sulphur.

1. _Rhombic sulphur._ When sulphur crystallizes from its solution in
carbon disulphide it separates in crystals which have the same color and
melting point as roll sulphur, and are rhombic in shape. Roll sulphur is
made up of minute rhombic crystals.

2. _Monoclinic sulphur._ When melted sulphur is allowed to cool until a
part of the liquid has solidified, and the remaining liquid is then
poured off, it is found that the solid sulphur remaining in the vessel
has assumed the form of fine needle-shaped crystals. These differ much
in appearance from the rhombic crystals obtained by crystallizing
sulphur from its solution in carbon disulphide. The needle-shaped form
is called _monoclinic sulphur_. The two varieties differ also in density
and in melting point, the monoclinic sulphur melting at 120 deg..

Monoclinic and rhombic sulphur remain unchanged in contact with each
other at 96 deg.. Above this temperature the rhombic changes into
monoclinic; at lower temperatures the monoclinic changes into rhombic.
The temperature 96 deg. is therefore called the transition point of sulphur.
Heat is set free when monoclinic sulphur changes into rhombic.

~Amorphous sulphur.~ Two varieties of amorphous sulphur can be readily
obtained. These are white sulphur and plastic sulphur.

1. _White sulphur._ Flowers of sulphur, the preparation of which has
been described, consists of a mixture of rhombic crystals and amorphous
particles. When treated with carbon disulphide, the crystals dissolve,
leaving the amorphous particles as a white residue.

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