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Isomorphous nitrate. . . . NaNO 3 .

I Calcium carbonate . . . CaCO 3 .

Isomorphous I Sodium sul P hate (anhydrous) . Na 2 SO 4 .

( Barium permanganate . . BaMn 2 O 8 .



52 Introductory Outlines

Isomorphism of this order, where little or no chemical relations
exist between the compounds, is sometimes distinguished as
isogonism. It must not be supposed, that because two chemically
analogous compounds contain the same number of atoms, they will
necessarily crystallise in the same form : there are indeed a large
number of similarly constituted analogous compounds that do not
exhibit isomorphism.

No simple definition of isomorphism is possible, but the following
test is generally accepted as a criterion, namely, the power to form
either mixed crystals or layer crystals. Thus, when two substances
are mixed in a state of liquidity, and allowed to crystallise, if the
crystals are perfectly homogeneous, they are known as mixed
crystals, and the substances are regarded as isomorphous.

Or when a crystal of one compound is placed in a solution of
another compound, and the crystal continues to grow regularly
in the liquid, the compounds are isomorphous. Thus, if a crystal
of potassium alum (white) be placed in a solution of manganese
alum, the crystal continues to grow without change of form, and
a layer of amethyst-coloured manganese alum is deposited upon it.

In making use of the law of isomorphism in the determination of
atomic weights, it is assumed that the weights of different atoms
that can mutually replace each other without altering the crystal-
line form are proportional to their atomic weights.*

Thus, if we suppose that, in the case of the sulphates of zinc
and magnesium, the atomic weight of zinc is known, viz., 65, and
that of magnesium is doubtful ; from the fact of the isomorphism
of the sulphates it may be premised that the elements are present in
proportions relative to their atomic weights. Analysis shows that
the proportion is 24 of magnesium to 65 of zinc, therefore 24 is pre-
sumably the atomic weight of magnesium.

In this way Berzelius corrected many of the atomic weights
which in his day had been assigned to the elements.

* The group (NH 4 ) may be regarded as an atom, having the relative weight 18.



CHAPTER VII
QUANTITATIVE CHEMICAL NOTATION

THE use of chemical symbols and formulae, as a convenient means
of representing concisely the qualitative nature of chemical changes,
has been explained in chapter iv. We are now in a position to
read into these symbols a quantitative significance, which at that
stage it would have been premature to explain.

The symbol of an element stands for an atom ; but, as we have
now learnt, the atoms of the various elements have different relative
weights, hence these symbols represent relative weights of matter.
The symbol Na signifies 23 relative parts by weight of sodium, O
stands for 16 relative parts by weight of oxygen, H for I part of
hydrogen ; in other words, the weight of sodium represented by
the symbol Na is 23 times as heavy as that which is conveyed
by a symbol H. A chemical equation, therefore, is a strictly
quantitative expression, in which certain definite weights of matter
are present in the form of the reacting substances, and which
reappear without loss or gain in the compounds resulting from the
change. In this sense a chemical equation is a mathematical
expression. Thus, the equation

Na + Cl = NaCl,

not only means that an atom of sodium combines with an atom of
chlorine and forms I molecule of sodium chloride, but it also means

23 + 35-5 = 58.5
Na Cl NaCl.

In other words, that sodium and chlorine unite in the relative pro-
portion of 23 parts of the former and 35.5 parts of chlorine, and
produce 58.5 parts of sodium chloride.

In the same way, into the equation which expresses the action of

53



54 Introductory Outlines

sulphuric acid upon sodium carbonate, we read the quantitative
meaning of the symbols

H 2 SO 4 + Na 2 CO 3 = Na 2 S0 4 + CO 2 + H 2 O.

2 46 46

2 12 32 12 2

48 64 32 16

98* + 1 06 = 142 4- 44 + 1 8

That is to say, 98 parts by weight of sulphuric acid act upon
106 parts of sodium carbonate, producing 142 parts of sodium
sulphate, 44 parts of carbon dioxide, and 18 parts of water. It will
be evident that it becomes a matter of the simplest arithmetic to
calculate the weight of any product that can be obtained from a
given weight of the reacting substances ; or vice versa, to find
the weight of any reacting substance which would be required to
produce a given weight of the product of the action.

Not only is information respecting the quantitative relations
by 'weight embodied in a chemical equation, but when gaseous
substances are reacting, the equation also represents the volu-
metric relation between the gases. In order that the volumetric
relations may be more manifest, the equations expressing the re-
actions are written in such a manner as to represent the molecules
of the substances.

H + C1 = HC1

is an atomic equation, but as the molecule is the smallest particle
which can exist alone, a more exact statement of the chemical
change is made, by representing the action as taking place between
molecules, thus

H 2 + C1 2 = 2HC1.

From such an equation we see that I molecule of hydrogen, or
2 unit volumes, unites with I molecule or 2 unit volumes of chlorine,
and forms 2 molecules or 4 unit volumes of hydrochloric acid :
or again

O 2 + 2H 2 = 2H 2 O.

One molecule, or 2 unit volumes of oxygen, unite with 2 mole-
cules, or 4 unit volumes of hydrogen, and produce 2 molecules of

* The number obtained by adding together the weights of the atoms in a
formula is known as a " formula weight," thus 98 is the formula weight of
sulphuric acid.



Quantitative Notation 55

water, which when vaporised, and measured under the same con-
ditions of temperature and pressure, occupy 4 unit volumes. In
other words, the number of molecules, in all cases * where gases
and vapours are concerned, represent exactly the volumetric
relations. In the cases quoted, it will be observed, the same ratio
also subsists between the number of atoms of the reacting gases
and the molecules of the compound, but this is not always the
case, for example

Atomic equation, Hg + 2C1 = HgClg.

In this equation 3 atoms unite to produce i molecule, but the
ratio between the volumes is not represented by the statement,

1 volume of mercury vapour and 2 volumes of chlorine produce

2 volumes of vapour of mercury chloride.

Molecular equation, Hg -f- C1 2 = HgCl 2 .

By this we see that i molecule t (2 unit volumes) of mercury-
vapour and i molecule (2 unit volumes) of chlorine give i mole-
cule (2 unit volumes) of vapour of mercury chloride.

Again,

P + 3C1 = PC1 3

is an atomic equation, showing that i atom of phosphorus unites
with 3 atoms of chlorine ; but it is not true that the ratio between
the volumes is represented by the statement, I volume of phos-
phorus vapour combines with 3 volumes of chlorine and gives 2
volumes of the vapour of phosphorus trichloride, as will be seen
by comparison with the molecular formulae

P 4 + 6C1 2 = 4PC1 3 .

This equation tells us that i molecule J (2 unit volumes) of phos-
phorus vapour combines with 6 molecules (12 unit volumes) of
chlorine, producing 4 molecules (8 unit volumes) of phosphorus
trichloride vapour.

Knowing the relative densities of gases compared with hydro-
gen, it is obviously possible, by ascertaining the actual weight in
grammes of some definite volume of hydrogen, to calculate the
actual weight of any given volume of any other gas.

Two units are in common use, namely

* See Dissociation, where apparent exceptions are explained.

f The atomic volume of mercury vapour being equal to 2 unit volumes (p. 44).

t The atomic volume of phosphorus is .5 of a unit volume (p. 44).



56 Introductory Outlines

(i.) The weight of I litre of hydrogen, measured at a temperature
of o C., and under a pressure of 760 mm. of mercury.*

(2.) The volume occupied by i gramme of hydrogen, measured
under the same conditions.

I. One litre of hydrogen, measured at the standard temperature
and pressure, weighs .0896 grammes.t This number is known as
the crith;\ and by means of it the weight of i litre, and therefore
any given volume, of any gas can be deduced : thus, the relative
densities of oxygen, nitrogen, and chlorine are 16, 14, and 35.5
respectively, therefore i litre of these gases (measured always at
the standard temperature and pressure) weighs 16 criths, 14 criths,
and 35.5 criths respectively, or

I litre of oxygen weighs 16 x. 0896= 1.4336 grammes,
i nitrogen 14 x. 0896= 1.2544
I chlorine 35.5 X. 0896 = 3. 1808

So also with reference to compound gases, where in each case
the density is represented by the half of the molecular weight.
Thus, the relative densities of hydrochloric acid, ammonia, and
carbon dioxide are




and the weights of I litre of these gases are therefore

i litre of hydrochloric acid= 18.25 x -0896= 1.6352 gramme,
i ammonia = 8.5 x. 0896 = 0.7610

i carbon dioxide =22.0 x. 0896= 1.9712

II. The volume occupied by I gramme of hydrogen at the
standard temperature and pressure is 11.127 litres. As the rela-
tive density of oxygen is 16, it obviously follows that 16 grammes

* This temperature and pressure is chosen as the standard at which volumes
of gases are compared. See General Properties of Gases, chapter ix.

f From time to time slightly different values have been given for this
constant. The most recent determinations give the number .089873.

J From the Greek, signifying a barley-corn, and used symbolically to denote
a little weight.



Quantitative Notation 57

of this gas will also occupy 11.127 litres; in other words, this
number 11.127 represents the volume in litres of any gas, which
will be occupied by the number of grammes corresponding to its
relative density, thus

14 grammes of nitrogen . . occupy 11.127 litres.
35-5 chlorine . . 11.127
18.25 hydrochloric acid 11.127
22.0 carbon dioxide . 11.127

The number of grammes of a substance, equal to the number
which represents its molecular weight, is spoken of as the gramme-
molecule. The molecular weight of hydrogen = 2, therefore the
gramme-molecule of hydrogen (that is, 2 grammes of hydrogen)
will occupy 11.127x2 = 22.25 litres. The molecular weight of
oxygen = 32, therefore 32 grammes of oxygen will occupy 22.25
litres ; in other words, 22.25 litres is the volume which will be
occupied by the gramme-molecule of any gas.

By means of this important constant, 22.25, the volume of any,
or all, of the gaseous products of a chemical change (when
measured at the standard temperature and pressure) can be de-
duced directly from the equation representing the change, thus

Zn + H 2 SO 4 = ZnSO 4 +H 2

expresses the reaction taking place when zinc is dissolved in
sulphuric acid. Just as in the former illustrations it carries the
information that 65 grammes of zinc + 98 grammes of sulphuric
acid produce 161 grammes of zinc sulphate and 2 grammes of
hydrogen. But 2 grammes of hydrogen occupy 22.25 litres, there-
fore by the solution of 65 grammes of zinc, the volume of hydrogen
obtained will be 22.25 litres.

So also in the following equation, which represents the formation
of carbon dioxide from chalk (calcium carbonate) by the action
upon it of hydrochloric acid

CaCO 3 + 2HC1 = CaCl 2 + H 2 O + CO 2 .
40+12 + 48 2(1+35.5) 40 + 71 2 + 16 12 + 32

100 + 73 = in + 18 + 44

100 grammes of chalk, when acted upon by 73 grammes of hydro-
chloric acid, yield ill grammes of calcium chloride and 18
grammes of water, and 44 grammes of carbon dioxide.

Carbon dioxide is gaseous, therefore 44 grammes (the gramme-



58 Introductory Outlines

molecule) will occupy, at the standard temperature and pressure,
22.25 litres ; hence, by the decomposition of 100 grammes of
chalk, 22.25 litres of carbon dioxide are produced.

This chapter may be concluded with one illustration of the
methods employed in the exact determination of atomic weights
which depends essentially upon the quantitative character of
chemical reactions. By the three following processes the atomic
weights of chlorine, potassium, and silver may be deduced.

1. By heating a known weight of potassium chlorate, the formula
weight of potassium chloride is found

KC1O 3 = KC1 + 3O.

50 grammes of potassium chlorate when heated left a residue
of potassium chloride weighing 30.395 grammes. 50 - 30.395 =
19.605 = grammes of oxygen evolved.

As potassium chlorate contains in its formula weight 3 atoms
of oxygen (16 X 3 = 48), we get the expression

19.605 : 30.395 = 48 : 74.4o=formula weight of potassium chloride.

2. By dissolving a known weight of potassium chloride, and
adding to it excess of silver nitrate, silver chloride is precipitated,
which can be washed and dried and weighed, and from which the
formula weight of silver chloride is obtained

KC1 + AgNO 3 = AgCl + KNO 3 .

10 grammes of potassium chloride were found to yield 19.225
grammes of silver chloride ; therefore,

10 : 19.225 = 74.40 : 143.03 = formula weight of silver chloride.

3. By the direct combination of silver and chlorine, by heating
the metal in a stream of the gas, the ratio of chlorine to silver in
silver chloride is found :

10 grammes of silver so treated yielded 13.285 grammes of silver
chloride ; therefore,

13.285 : 10 = 143.03 : 107.66 = atomic weight of silver.
Since the formula weight of silver chloride, AgCl = 143.03,

therefore, 143.03- 107.66 = 35.37 = atomic weight of chlorine.
And since the formula weight of potassium chloride, KC1 = 74.40,

therefore, 74.40 35.37 = 39.03 = atomic weight of potassium.



CHAPTER VIII
VALENCY OF THE ELEMENTS

WHEN chlorine unites with hydrogen, the combination takes place
between one atom of chlorine (relative weight = 35.5) and one
atom of hydrogen (relative weight = i); but when oxygen com-
bines with hydrogen, one atom of oxygen unites with two atoms
of hydrogen. The compound ammonia consists of one atom of
nitrogen, combined with three atoms of hydrogen ; while one atom
of carbon, on the other hand, can unite with four atoms of
hydrogen.

One atom of chlorine never combines with more than one atom
of hydrogen ; its affinity for that element is satisfied, or saturated,
by union with one atom.

The affinity of one atom of oxygen for hydrogen, however, is
not satisfied by one atom of that element, but requires two atoms
for its saturation ; while nitrogen requires three, and carbon four
hydrogen atoms, in order to satisfy their respective affinities for
this element.

This varying power of combining with hydrogen is seen in a
number of other instances : thus, the elements fluorine, bromine,
and iodine, resemble chlorine in being only able to unite with one
atom of hydrogen. Sulphur, like oxygen, has its affinity for
hydrogen saturated by two atoms of that element. Phosphorus
and arsenic require three atoms of hydrogen in order to saturate
their combining capacity, while silicon resembles carbon in com-
bining with four hydrogen atoms. This combining capacity of
an element is termed its valency. Elements like chlorine,
fluorine, bromine, and iodine, whose atoms are only capable
of uniting with one atom of hydrogen, are called monovalent
(or sometimes monad) elements ; while those whose atoms com-
bine with two, three, or four hydrogen atoms, are distinguished
as di-valent (or dyad), tri-valent (or triad), and tetra-valent (or
tetrad) elements. All elements, however, are not capable of

59



60 Introductory Outlines

entering into combination with hydrogen ; in which case, their
valency is measured by the number of atoms of some other
monovalent element which is capable of satisfying their com-
bining capacity. Thus :

i atom of sodium combines with i atom of chlorine, forming NaCl.
i ,, calcium ,, ,, 2 atoms ., , CaCl 2 .

i , , boron , , . , 3 , , . .. , BCls.

i ,, tin ,, ,, 4 ,, ,, , SnCl 4 .

i phosphorus* ,, 5 ,, , PC1 5 .

i ,, tungsten ,, ,, 6 ,, ,, , WC1 6 .

In the combinations of elements with hydrogen alone, no in-
stances are known in which a higher valency is exhibited than
that of four ; but with chlorine, as here seen, cases are known in
which elements exhibit pentavalent and hexavalent characters.

Measured by their combining capacity for hydrogen and chlorine,
elements do not, however, always exhibit the same valency : thus,
the affinity of phosphorus for hydrogen is satisfied by three hydrogen
atoms, whereas one atom of this element can unite with five atoms
of chlorine.

As measured by hydrogen, the valency of sulphur is two, the
compound that it forms with hydrogen being expressed by the
formula SH 2 , while, as estimated by its capacity for chlorine, it
becomes tetravalent, as seen in the compound SC1 4 . As a general
rule, however, the highest number of monovalent atoms with which
one atom of an element is capable of combining is accepted as
representing the valency of that element. Thus, one atom of
phosphorus not only combines with five atoms of chlorine, but
also with five atoms of fluorine ; phosphorus is therefore a penta-
valent element.

As measured by hydrogen alone, or by chlorine alone, nitrogen
is a trivalent element, for the largest number of these atoms with
which one atom of nitrogen can unite is three, as seen in the
compounds having the composition NH 3 and NC1 3 ; neverthe-
less, one atom of nitrogen is capable of combining with four
atoms of hydrogen and one of chlorine, forming the compound
NH 4 C1, ammonium chloride, in which the nitrogen atom is penta-
valent.

This rule, however, is not always followed ; for example, one
atom of iodine will unite with three atoms of chlorine, forming the

* Phosphorus also combines with hydrogen.



Valency of the Elements 6 1

compound IC1 3 , but iodine is not generally regarded as a trivalent
element.*

In symbolic notation, this power possessed by an atom, of uniting
to itself monovalent atoms, is often represented by lines, each line
signifying the power of combination with one monovalent atom.
Thus, in the symbol H Cl, the line is intended to give a concrete
expression to the fact that both hydrogen and chlorine are mono-
valent elements, and that the affinity of each element for the
other is satisfied when one atom of the one unites with one atom of
the other. The symbol H O H, in like manner, signifies that
the oxygen atom is divalent, that its affinity for hydrogen is satisfied
only when it has united with two monad atoms. In the same way
we may express the facts that nitrogen and carbon, in their com-
binations with hydrogen, are respectively trivalent and tetravalent,

H
I
by the symbols H N H, and H C H. These lines are merely

H H

a convenient symbolic expression for the operation of the force of
chemical affinity ; their length and direction bear no meaning.t
The power to combine with one monovalent atom is sometimes
spoken of simply as one affinity : thus it is said that in the com-
pound having the composition PH 3 , or H P H, three of the

H

affinities of the phosphorus atom are saturated, and that two
affinities still remain unsatisfied, phosphorus, as already stated,
being a pentavalent element.

* See Iodine, Compounds.

f The student cannot be too often warned against attaching any materialistic
significance to these lines. The use of this convention is always attended with
the danger that the beginner is liable to fall into the error of regarding these
lines as representing in some manner fixed points of attachment, or links,
between the atoms. It must be remembered, therefore, that these lines not only
have no materialistic signification, but they must not even be regarded as convey-
ing any statical meaning. The atoms are undergoing rapid movements with
respect to each other, which movements are in some way governed by the
chemically attractive force exerted by the individual atoms upon one another;
and the molecule will be more correctly considered, if we regard its atoms as
being held together in a manner resembling that by which the numbers of a
cosmical system are bound together. The lines simply denote that the atoms
are held to each other by the attractive force which we call chemical affinity.



62 Introductory Outlines

Compounds of this order, in which one of the elements has still
unsatisfied affinities, are called unsaturated compounds.

In its power to satisfy the affinities of an element, a divalent
atom is equal to two monovalent atoms : thus, when the affinities
of the tetravalent carbon atom are saturated with oxygen, the mole-
cule contains two atoms of oxygen, which may be symbolically
expressed thus, O = C = O, in which the four affinities of the
carbon (represented by the four lines) are satisfied by the two
divalent atoms of oxygen. Carbon, however, combines with a
smaller proportion of oxygen, forming the compound carbon mon-
oxide, CO. The carbon atom in this case is divalent, as expressed
by the formula C = O, and' this substance is also an unsaturated
compound.

The number of divalent atoms with which an element can unite
cannot, however, be taken as a safe criterion or measure of the
valency of that element in cases where that number is greater
than i ; for example, in such a compound as calcium oxide, CaO,
we regard the two affinities of the divalent atom of oxygen as being
satisfied by two affinities possessed by the calcium, and express this
belief in the formula Ca = O, and regard the calcium as divalent.
In the same way, in carbon monoxide, CO, the carbon being united
with one atom of the divalent element oxygen is itself divalent in
this compound ; but in the case of carbon dioxide, where the carbon
atom is united with two atoms of divalent oxygen, we are not
justified in asserting that the atoms are united, as represented by
the formula O = C = O, in which the four affinities of carbon
are represented as saturated with oxygen. There exists no posi-
tive proof that the carbon is not divalent in this compound, and
that the molecule does not consist of three divalent atoms united,

C

as shown in the formula /\. From the fact, however, that car-
O O

bon forms a compound with four atoms of hydrogen, and another
with four atoms of chlorine, we know that this element is tetra-
valent, and therefore we believe that in carbon dioxide it is also
tetravalent.

Again, as measured by its compound with hydrogen, sulphur is
divalent ; while with chlorine it forms SC1 4 . But sulphur unites
with oxygen, forming the two compounds sulphur dioxide, SO 2 , and
sulphur trioxide, SO 3 . If it be assumed that in these molecules the



Valency of the Elements 63

whole of the oxygen affinities are satisfied with sulphur, then the
symbolic representation of these oxides will be O = S = O, and
O = S = O, the sulphur being in one case tetravalent and in the

II
O

other hexavalent. There is, however, no positive proof that the
affinities of one oxygen atom are not partially satisfied by union
with another oxygen atom, and that the valency of the sulphur is
higher than either two or four, as seen in the alternative formulae,

S \ S

S0. 2 /\ SO, \ S = 0; or / \

O O O O O



Chemists believe, however, that in these two oxides the
sulphur functions in the one case as a tetravalent, and in
the other as a hexavalent element; and this belief is strengthened
by the recent discovery (Moissan) of a fluoride having the com-
position SF G , in which the hexavalent character of sulphur is
unquestionable.

It will be evident from these considerations, that in many cases
the valency of an element is a variable quantity, depending partly
upon the particular atoms with which it unites. It is also found
that it is dependent in many instances upon temperature and
upon pressure. Thus, between a certain limited range of
temperature, one atom of phosphorus combines with five atoms
of chlorine in the compound PC1 5 , but above that limit two atoms
of chlorine leave the molecule, and the phosphorus becomes tri-
valent. Again, if hydrogen phosphide, PH 3 , be mixed with hydro-
chloric acid, HC1, and the mixed gases be subjected to increased
pressure, the gases combine and form a solid crystalline com-
pound known as phosphonium chloride, PH 4 C1, in which the
phosphorus 'atom, being united with five monovalent atoms, is
pentavalent. When the pressure is released an atom of hydrogen
and an atom of chlorine leave the molecule, and the phosphorus
returns to its trivalent condition.

A compound, in whose molecules there is an atom which for the



Online LibraryGeorge S NewthA text-book of inorganic chemistry → online text (page 6 of 67)