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An Elementary Study of Chemistry

W >> William McPherson >> An Elementary Study of Chemistry




DEFINITION: _A radical is a group of elements forming part of a
molecule, and acting as a unit in chemical reactions._

~Names of acids, bases, and salts.~ Since acids, bases, and salts are so
intimately related to each other, it is very advantageous to give names
to the three classes in accordance with some fixed system. The system
universally adopted is as follows:

~Naming of bases.~ All bases are called _hydroxides_. They are
distinguished from each other by prefixing the name of the element which
is in combination with the hydroxyl group. Examples: sodium hydroxide
(NaOH); calcium hydroxide (Ca(OH)_{2}); copper hydroxide (Cu(OH)_{2}).

~Naming of acids.~ The method of naming acids depends upon whether the
acid consists of two elements or three.

1. _Binary acids._ Acids containing only one element in addition to
hydrogen are called _binary acids_. They are given names consisting of
the prefix _hydro-_, the name of the second element present, and the
termination _-ic_. Examples: hydrochloric acid (HCl); hydrosulphuric
acid (H_{2}S).

2. _Ternary acids._ In addition to the two elements present in binary
acids, the great majority of acids also contain oxygen. They therefore
consist of three elements and are called _ternary acids_. It usually
happens that the same three elements can unite in different proportions
to make several different acids. The most familiar one of these is given
a name ending in the suffix _-ic_, while the one with less oxygen is
given a similar name, but ending in the suffix _-ous_. Examples: nitric
acid (HNO_{3}); nitrous acid (HNO_{2}). In cases where more than two
acids are known, use is made of prefixes in addition to the two suffixes
_-ic_ and _-ous_. Thus the prefix _per-_ signifies an acid still richer
in oxygen; the prefix _hypo-_ signifies one with less oxygen.

~Naming of salts.~ A salt derived from a binary acid is given a name
consisting of the names of the two elements composing it, with the
termination _-ide_. Example: sodium chloride (NaCl). All other binary
compounds are named in the same way.

A salt of a ternary acid is named in accordance with the acid from which
it is derived. A ternary acid with the termination _-ic_ gives a salt
with the name ending in _-ate_, while an acid with termination _-ous_
gives a salt with the name ending in _-ite_. The following table will
make the application of these principles clear:

ACIDS SYMBOL SALTS SYMBOL

Hydrochloric HCl Sodium chloride NaCl
Hypochlorous HClO Sodium hypochlorite NaClO
Chlorous HClO_{2} Sodium chlorite NaClO_{2}
Chloric HClO_{3} Sodium chlorate NaClO_{3}
Perchloric HClO_{4} Sodium perchlorate NaClO_{4}


EXERCISES

1. 25 cc. of a solution containing 40 g. of sodium hydroxide per liter
was found to neutralize 25 cc. of a solution of hydrochloric acid. What
was the strength of the acid solution?

2. After neutralizing a solution of sodium hydroxide with nitric acid,
there remained after evaporation 100 g. of sodium nitrate. How much of
each substance had been used?

3. A solution contains 18 g. of hydrochloric acid per 100 cc. It
required 25 cc. of this solution to neutralize 30 cc. of a solution of
sodium hydroxide. What was the strength of the sodium hydroxide solution
in parts per hundred?

4. When perfectly dry sulphuric acid is treated with perfectly dry
sodium hydroxide, no chemical change takes place. Explain.

5. When cold, concentrated sulphuric acid is added to zinc, no change
takes place. Recall the action of dilute sulphuric acid on the same
metal. How do you account for the difference?

6. A solution of hydrochloric acid in benzene does not conduct the
electric current. When this solution is treated with zinc, will hydrogen
be evolved? Explain.

7. (a) Write equation for preparation of hydrogen from zinc and dilute
sulphuric acid. (b) Rewrite the same equation from the standpoint of
the theory of electrolytic dissociation, (c) Subtract the common
SO_{4} ion from both members of the equation, (d) From the resulting
equation, explain in what the preparation of hydrogen consists when
examined from the standpoint of this theory.

8. In the same manner as in the preceding exercise, explain in what the
action of sodium on water to give hydrogen consists.




CHAPTER XI

VALENCE


~Definition of valence.~ A study of the formulas of various binary
compounds shows that the elements differ between themselves in the
number of atoms of other elements which they are able to hold in
combination. This is illustrated in the formulas

HCl, H_{2}O, H_{3}N, H_{4}C.
(hydrochloric acid) (water) (ammonia) (marsh gas)

It will be noticed that while one atom of chlorine combines with one
atom of hydrogen, an atom of oxygen combines with two, an atom of
nitrogen with three, one of carbon with four. The number which expresses
this combining ratio between atoms is a definite property of each
element and is called its _valence_.

DEFINITION: _The valence of an element is that property which determines
the number of the atoms of another element which its atom can hold in
combination._

~Valence a numerical property.~ Valence is therefore merely a numerical
relation and does not convey any information in regard to the intensity
of the affinity between atoms. Judging by the heat liberated in their
union, oxygen has a far stronger affinity for hydrogen than does
nitrogen, but an atom of oxygen can combine with two atoms only of
hydrogen, while an atom of nitrogen can combine with three.

~Measure of valence.~ In expressing the valence of an element we must
select some standard for comparison, just as in the measurement of any
other numerical quantity. It has been found that an atom of hydrogen is
never able to hold in combination more than one atom of any other
element. Hydrogen is therefore taken as the standard, and other elements
are compared with it in determining their valence. A number of other
elements are like hydrogen in being able to combine with at most one
atom of other elements, and such elements are called _univalent_. Among
these are chlorine, iodine, and sodium. Elements such as oxygen,
calcium, and zinc, which can combine with two atoms of hydrogen or other
univalent elements, are said to be _divalent_. Similarly, we have
_trivalent, tetravalent, pentavalent_ elements. None have a valence of
more than 8.

~Indirect measure of valence.~ Many elements, especially among the metals,
do not readily form compounds with hydrogen, and their valence is not
easy to determine by direct comparison with the standard element. These
elements, however, combine with other univalent elements, such as
chlorine, and their valence can be determined from the compounds so
formed.

~Variable valence.~ Many elements are able to exert different valences
under differing circumstances. Thus we have the compounds Cu_{2}O and
CuO, CO and CO_{2}, FeCl_{2} and FeCl_{3}. It is not always possible to
assign a fixed valence to an element. Nevertheless each element tends to
exert some normal valence, and the compounds in which it has a valence
different from this are apt to be unstable and easily changed into
compounds in which the valence of the element is normal. The valences of
the various elements will become familiar as the elements are studied in
detail.

~Valence and combining ratios.~ When elements combine to form compounds,
the ratio in which they combine will be determined by their valences. In
those compounds which consist of two elements directly combined, the
union is between such numbers of the two atoms as have equal valences.
Elements of the same valence will therefore combine atom for atom.
Designating the valence of the atoms by Roman numerals placed above
their symbols, we have the formulas

II II II III III IV IV
HCl, ZnO, BN, CSi.

A divalent element, on the other hand, will combine with two atoms of a
univalent element. Thus we have

II II II II
ZnCl_{2} and H_{2}O

(the numerals above each symbol representing the sum of the valences of
the atoms of the element present). A trivalent atom will combine with
three atoms of a univalent element, as in the compound

III III
H_{3}N.

If a trivalent element combines with a divalent element, the union will
be between two atoms of the trivalent element and three of the divalent
element, since these numbers are the smallest which have equal valences.
Thus the oxide of the trivalent metal aluminium has the formula
Al_{2}O_{3}. Finally one atom of a tetravalent element such as carbon
will combine with four atoms of a univalent element, as in the compound
CH_{4}, or with two atoms of a divalent element, as in the compound
CO_{2}.

We have no knowledge as to why elements differ in their combining power,
and there is no way to determine their valences save by experiment.

~Valence and the structure of compounds.~ Compounds will be met
from time to time which are apparent exceptions to the general
statements just made in regard to valence. Thus, from the
formula for hydrogen dioxide (H_{2}O_{2}), it might be
supposed that the oxygen is univalent; yet it is certainly
divalent in water (H_{2}O). That it may also be divalent in
H_{2}O_{2} may be made clear as follows: The unit valence of
each element may be represented graphically by a line attached
to its symbol. Univalent hydrogen and divalent oxygen will then
have the symbols H- and -O-. When atoms combine, each unit
valence of one atom combines with a unit valence of another
atom. Thus the composition of water may be expressed by the
formula H-O-H, which is meant to show that each of the unit
valences of oxygen is satisfied with the unit valence of a
single hydrogen atom.

The chemical conduct of hydrogen dioxide leads to the
conclusion that the two oxygen atoms of its molecule are in
direct combination with each other, and in addition each is in
combination with a hydrogen atom. This may be expressed by the
formula H-O-O-H. The oxygen in the compound is therefore
divalent, just as it is in water. It will thus be seen that the
structure of a compound must be known before the valences of
the atoms making up the compound can be definitely decided
upon.

Such formulas as H-O-H and H-O-O-H are known as _structural
formulas_, because they are intended to show what is known in
regard to the arrangement of the atoms in the molecules.

~Valence and the replacing power of atoms.~ Just as elements having the
same valence combine with each other atom for atom, so if they replace
each other in a chemical reaction they will do so in the same ratio.
This is seen in the following equations, in which a univalent hydrogen
atom is replaced by a univalent sodium atom:

NaOH + HCl = NaCl + H_{2}O.

2NaOH + H_{2}SO_{4} = Na_{2}SO_{4} + 2H_{2}O.

Na + H_{2}O = NaOH + H.

Similarly, one atom of divalent calcium will replace two atoms of
univalent hydrogen or one of divalent zinc:

Ca(OH)_{2} + 2 HCl = CaCl_{2} + 2H_{2}O.

CaCl_{2} + ZnSO_{4} = CaSO_{4} + ZnCl_{2}.

In like manner, one atom of a trivalent element will replace three of a
univalent element, or two atoms will replace three atoms of a divalent
element.

~Valence and its applications to formulas of salts.~ While the true nature
of valence is not understood and many questions connected with the
subject remain unanswered, yet many of the main facts are of much help
to the student. Thus the formula of a salt, differs from that of the
acid from which it is derived in that the hydrogen of the acid has been
replaced by a metal. If, then, it is known that a given metal forms a
normal salt with a certain acid, the formula of the salt can at once be
determined if the valence of the metal is known. Since sodium is
univalent, the sodium salts of the acids HCl and H_{2}SO_{4} will be
respectively NaCl and Na_{2}SO_{4}. One atom of divalent zinc will
replace 2 hydrogen atoms, so that the corresponding zinc salts will be
ZnCl_{2} and ZnSO_{4}.

The formula for aluminium sulphate is somewhat more difficult to
determine. Aluminium is trivalent, and the simplest ratio in which the
aluminium atom can replace the hydrogen in sulphuric acid is 2 atoms of
aluminium (6 valences) to 3 molecules of sulphuric acid (6 hydrogen
atoms). The formula of the sulphate will then be Al_{2}(SO_{4})_{3}.

~Valence and its application to equation writing.~ It will be readily seen
that a knowledge of valence is also of very great assistance in writing
the equations for reactions of double decomposition. Thus, in the
general reaction between an acid and a base, the essential action is
between the univalent hydrogen ion and the univalent hydroxyl ion. The
base and the acid must always be taken in such proportions as to secure
an equal number of each of these ions. Thus, in the reaction between
ferric hydroxide (Fe(OH)_{3}) and sulphuric acid (H_{2}SO_{4}), it will
be necessary to take 2 molecules of the former and 3 of the latter in
order to have an equal number of the two ions, namely, 6. The equation
will then be

2Fe(OH)_{3} + 3H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + 6H_{2}O.

Under certain conditions the salts Al_{2}(SO_{4})_{3} and CaCl_{2}
undergo double decomposition, the two metals, aluminium and calcium,
exchanging places. The simplest ratio of exchange in this case is 2
atoms of aluminium (6 valences) and 3 atoms of calcium (6 valences).
The reaction will therefore take place between 1 molecule of
Al_{2}(SO_{4})_{3} and 3 of CaCl_{2}, and the equation is as follows:

Al_{2}(SO_{4})_{3} + 3 CaCl_{2} = 3CaSO_{4} + 2AlCl_{3}.


EXERCISES

1. Sodium, calcium, and aluminium have valences of 1, 2, and 3
respectively; write the formulas of their chlorides, sulphates, and
phosphates (phosphoric acid = H_{3}PO_{4}), on the supposition that they
form salts having the normal composition.

2. Iron forms one series of salts in which it has a valence of 2, and
another series in which it has a valence of 3; write the formulas for
the two chlorides of iron, also for the two sulphates, on the
supposition that these have the normal composition.

3. Write the equation representing the neutralization of each of the
following bases by each of the acids whose formulas are given:

NaOH HCl
Ba(OH)_{2} H_{2}SO_{4}
Al(OH)_{3} H_{3}PO_{4}

4. Silver acts as a univalent element and calcium as a divalent element
in the formation of their respective nitrates and chlorides. (a) Write
the formula for silver nitrate; for calcium chloride. (b) When
solutions of these two salts are mixed, the two metals, silver and
calcium, exchange places; write the equation for the reaction.

_5._ Antimony acts as a trivalent element in the formation of a
chloride. (a) What is the formula for antimony chloride? (b) When
hydrosulphuric acid (H_{2}S) is passed into a solution of this chloride
the hydrogen and antimony exchange places; write the equation for the
reaction.

6. Lead has a valence of 2 and iron of 3 in the compounds known
respectively as lead nitrate and ferric sulphate. (a) Write the
formulas for these two compounds. (b) When their solutions are mixed
the two metals exchange places; write the equation for the reaction.




CHAPTER XII

COMPOUNDS OF NITROGEN


~Occurrence.~ As has been stated in a former chapter, nitrogen constitutes
a large fraction of the atmosphere. The compounds of nitrogen, however,
cannot readily be obtained from this source, since at any ordinary
temperature nitrogen is able to combine directly with very few of the
elements.

In certain forms of combination nitrogen occurs in the soil from which
it is taken up by plants and built into complex substances composed
chiefly of carbon, hydrogen, oxygen, and nitrogen. Animals feeding on
these plants assimilate the nitrogenous matter, so that this element is
an essential constituent of both plants and animals.

~Decomposition of organic matter by bacteria.~ When living matter dies and
undergoes decay complicated chemical reactions take place, one result of
which is that the nitrogen of the organic matter is set free either as
the element nitrogen, or in the form of simple compounds, such as
ammonia (NH_{3}) or oxides of nitrogen. Experiment has shown that all
such processes of decay are due to the action of different kinds of
bacteria, each particular kind effecting a different change.

~Decomposition of organic matter by heat.~ When organic matter is strongly
heated decomposition into simpler substances takes place in much the
same way as in the case of bacterial decomposition. Coal is a complex
substance of vegetable origin, consisting largely of carbon, but also
containing hydrogen, oxygen, and nitrogen. When this is heated in a
closed vessel so that air is excluded, about one seventh of the nitrogen
is converted into ammonia, and this is the chief source from which
ammonia and its compounds are obtained.


COMPOUNDS OF NITROGEN WITH HYDROGEN

~Ammonia~ (NH_{3}). Several compounds consisting exclusively of nitrogen
and hydrogen are known, but only one, ammonia, need be considered here.

~Preparation of ammonia.~ Ammonia is prepared in the laboratory by a
different method from the one which is used commercially.

1. _Laboratory method._ In the laboratory ammonia is prepared from
ammonium chloride, a compound having the formula NH_{4}Cl, and obtained
in the manufacture of coal gas. As will be shown later in the chapter,
the group NH_{4} in this compound acts as a univalent radical and is
known as _ammonium_. When ammonium chloride is warmed with sodium
hydroxide, the ammonium and sodium change places, the reaction being
expressed in the following equation.

NH_{4}Cl + NaOH = NaCl + NH_{4}OH.

The ammonium hydroxide (NH_{4}OH) so formed is unstable and breaks down
into water and ammonia.

NH_{4}OH = NH_{3} + H_{2}O.

Calcium hydroxide (Ca(OH)_{2}) is frequently used in place of the more
expensive sodium hydroxide, the equations being

2NH_{4}Cl + Ca(OH)_{2} = CaCl_{2} + 2NH_{4}OH,

2NH_{4}OH = 2H_{2}O + 2NH_{3}.

In the preparation, the ammonium chloride and calcium hydroxide
are mixed together and placed in a flask arranged as shown in
Fig. 35. The mixture is gently warmed, when ammonia is evolved
as a gas and is collected by displacement of air.

[Illustration: Fig. 35]

2. _Commercial method._ Nearly all the ammonia of commerce comes from
the gasworks. Ordinary illuminating gas is made by distilling coal, as
will be explained later, and among the products of this distillation a
solution of ammonia in water is obtained. This solution, known as _gas
liquor_, contains not only ammonia but other soluble substances. Most of
these combine chemically with lime, while ammonia does not; if then lime
is added to the gas liquor and the liquor is heated, the ammonia is
driven out from the mixture. It may be dissolved again in pure, cold
water, forming _aqua ammonia_, or the ammonia water of commerce.

~Preparation from hydrogen and nitrogen.~ When electric sparks
are passed for some time through a mixture of hydrogen and
nitrogen, a small percentage of the two elements in the mixture
is changed into ammonia. The action soon ceases, however, for
the reason that ammonia is decomposed by the electric
discharge. The reaction expressed in the equation

N + 3H = NH_{3}

can therefore go in either direction depending upon the
relative quantities of the substances present. This recalls the
similar change from oxygen into ozone, which soon ceases
because the ozone is in turn decomposed into oxygen.

~Physical properties.~ Under ordinary conditions ammonia is a gas whose
density is 0.59. It is therefore little more than half as heavy as air.
It is easily condensed into a colorless liquid, and can now be purchased
in liquid form in steel cylinders. The gas is colorless and has a
strong, suffocating odor. It is extremely soluble in water, 1 l. of
water at 0 deg. and 760 mm. pressure dissolving 1148 l. of the gas. In
dissolving this large volume of gas the water expands considerably, so
that the density of the solution is less than that of water, the
strongest solutions having a density of 0.88.

~Chemical properties.~ Ammonia will not support combustion, nor will it
burn under ordinary conditions. In an atmosphere of oxygen it burns with
a feeble, yellowish flame. When quite dry it is not a very active
substance, but when moist it combines with a great many substances,
particularly with acids.

~Uses.~ It has been stated that ammonia can be condensed to a liquid by
the application of pressure. If the pressure is removed from the liquid
so obtained, it rapidly passes again into the gaseous state and in so
doing absorbs a large amount of heat. Advantage is taken of this fact in
the preparation of artificial ice. Large quantities of ammonia are also
used in the preparation of ammonium compounds.

~The manufacture of artificial ice.~ Fig. 36 illustrates the
method of preparing artificial ice. The ammonia gas is
liquefied in the pipes X by means of the pump Y. The heat
generated is absorbed by water flowing over the pipes. The
pipes lead into a large brine tank, a cross section of which is
shown in the figure. Into the brine (concentrated solution of
common salt) contained in this tank are dipped the vessels A,
B, C, filled with pure water. The pressure is removed from
the liquid ammonia as it passes into the pipes immersed in the
brine, and the heat absorbed by the rapid evaporation of the
liquid lowers the temperature of the brine below zero. The
water in A, B, C is thereby frozen into cakes of ice. The
gaseous ammonia resulting from the evaporation of the liquid
ammonia is again condensed, so that the process is continuous.

[Illustration Fig. 36]

~Ammonium hydroxide~ (NH_{4}OH). The solution of ammonia in water is found
to have strong basic properties and therefore contains hydroxyl ions. It
turns red litmus blue; it has a soapy feel; it neutralizes acids,
forming salts with them. It seems probable, therefore, that when ammonia
dissolves in water it combines chemically with it according to the
equation

NH_{3} + H_{2}O = NH_{4}OH,

and that it is the substance NH_{4}OH, called ammonium hydroxide, which
has the basic properties, dissociating into the ions NH_{4} and OH.
Ammonium hydroxide has never been obtained in a pure state. At every
attempt to isolate it the substance breaks up into water and ammonia,--

NH_{4}OH = NH_{3} + H_{2}O.

~The ammonium radical.~ The radical NH_{4} plays the part of a metal in
many chemical reactions and is called ammonium. The ending _-ium_ is
given to the name to indicate the metallic properties of the substance,
since the names of the metals in general have that ending. The salts
formed by the action of the base ammonium hydroxide on acids are called
ammonium salts. Thus, with hydrochloric acid, ammonium chloride is
formed in accordance with the equation

NH_{4}OH + HCl = NH_{4}Cl + H_{2}O.

Similarly, with nitric acid, ammonium nitrate (NH_{4}NO_{3}) is formed,
and with sulphuric acid, ammonium sulphate ((NH_{4})_{2}S0_{4}).

It will be noticed that in the neutralization of ammonium hydroxide by
acids the group NH_{4} replaces one hydrogen atom of the acid, just as
sodium does. The group therefore acts as a univalent metal.

~Combination of nitrogen with hydrogen by volume.~ Under suitable
conditions ammonia can be decomposed into nitrogen and hydrogen by
passing electric sparks through the gas. Accurate measurement has shown
that when ammonia is decomposed, two volumes of the gas yield one volume
of nitrogen and three volumes of hydrogen. Consequently, if the two
elements were to combine directly, one volume of nitrogen would combine
with three volumes of hydrogen to form two volumes of ammonia. Here, as
in the formation of steam from hydrogen and oxygen, small whole numbers
serve to indicate the relation between the volumes of combining gases
and that of the gaseous product.


COMPOUNDS OF NITROGEN WITH OXYGEN AND HYDROGEN

In addition to ammonium hydroxide, nitrogen forms several compounds with
hydrogen and oxygen, of which nitric acid (HNO_{3}) and nitrous acid
(HNO_{2}) are the most familiar.

~Nitric acid~ (HNO_{3}). Nitric acid is not found to any extent in nature,
but some of its salts, especially sodium nitrate (NaNO_{3}) and
potassium nitrate (KNO_{3}) are found in large quantities. From these
salts nitric acid can be obtained.

[Illustration Fig. 37]

~Preparation of nitric acid.~ When sodium nitrate is treated with
concentrated cold sulphuric acid, no chemical action seems to take
place. If, however, the mixture is heated in a retort, nitric acid is
given off as a vapor and may be easily condensed to a liquid by passing
the vapor into a tube surrounded by cold water, as shown in Fig. 37. An
examination of the liquid left in the retort shows that it contains
sodium acid sulphate (NaHSO_{4}), so that the reaction may be
represented by the equation

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