HOW CHEMICAL AFFINITY WORKS.
It is an interesting fact that elements and compounds manifest an
exceedingly great variety of tendencies to combination. Fragments
of matter, so small that no eye perceives them, have, wrapped up in
themselves, a multitude of determinate powers. A given atom as of lead,
for example, will very readily combine with oxygen and with some other
substances, but it seems to absolutely refuse to combine with nitrogen.
So hydrogen will combine very readily with chlorine and with many other
elements, but it refuses to form any union with silver and with many
other elementary substances. It cannot be called a whim that determines
the kind of element or its amount that a certain substance will combine
with, though the likes and dislikes of atoms are in this respect
exceedingly marked and even incomprehensible. But however impossible
it may be _to explain_ an element’s friendly or unfriendly deportment
toward another, it is possible in each case to learn the facts with
certainty, for each atom possesses its true individuality and is always
constant and consistent in its affinities and hates.
It is the purpose of this chapter to present in an orderly manner some
of the peculiarities of this mysterious power of chemical affinity.
First. Each Kind of Atom Has Its Peculiar Chemical Affinities.
Chemical affinity seems to reside within the atom as a permanent, ever
present and guiding energy. Thus while iron oxidizes readily—that is
manifests under a multitude of common conditions a willingness to
combine with oxygen and form a new compound called oxide of iron, and
well-known under the name of iron-rust—gold on the other hand oxidizes
unwillingly; indeed in order to get it to combine with oxygen it must
be coaxed by means of circuitous and carefully planned devices. But
these atoms are always consistent in their action, for iron under any
and every condition oxidizes more readily than gold does.
Second. Chemical Affinity Acts Only Under Favorable Conditions.
While chemical action often works with most intense energy it does so
only when certain outside and incidental conditions are favorable.
Thus carbon has under certain conditions an affinity for oxygen and
manifests its tendency to combination, with an intensity that is
scarcely surpassed. In order however to awaken and vivify the dormant
inclination it must be stimulated by certain definite and favorable
conditions; the most important of these conditions is a certain
amount of warmth. The stores of fuel in our cellars—the coal and
the wood and all other combustible things—are surrounded by great
quantities of oxygen which winds its way, with every slightest stir
of the mobile air, in and out through all the crevices that the fuel
affords, passing continually in the immediate neighborhood of ample
quantities of atoms of carbon. But it does not ordinarily unite with
them. Subject the whole or any portion of these combustible things to
a slight rise in temperature—then the atoms of oxygen and the atoms of
carbon seem to arouse themselves from repose: they unite in friendly
and firm grasp, and what is called chemical union takes place. To the
ordinary observer the heat that is produced is the most notable sign of
this kind of combination. The chemist, however, discovers a yet more
conclusive evidence, for he finds that several kinds of new molecules
have been produced; one of these kinds, for example, is expressible by
the name carbon dioxide and by the formula CO₂. Evidently this formula
means that each atom of carbon has united with two atoms of oxygen. In
this familiar example heat is the agency that stimulates the atoms to
a display of the chemical force that previously was slumbering within
them.
Light and the electric current and the vital forces of animals and
plants, though acting in a manner less familiar to us, are energizers
of chemical affinity and all have their proper influence to make atoms
join in union; indeed in some cases they make atoms burst from each
others bonds and fly away to more congenial conditions.
Third. Each Atom Has a Certain Equivalence or Atom-Fixing Power.
The chemist also recognizes each atom as possessing certain peculiar
numerical preferences in its combinations; a manifestation of chemical
affinity called equivalence. Thus when carbon burns in a stove, by
reason of the air passing by it on its way to the chimney, it seizes
upon some of the oxygen atoms and _binds a definite number of them_
to itself. If there is much air, each atom of carbon of the millions
present, picks out two atoms of oxygen from the air; if there is but
little air, each atom of carbon has to be satisfied with one atom of
oxygen. Now in these two cases of course different substances are
formed. The first, whose composition is represented by the formula
CO₂, has already been spoken of as carbon dioxide. To the other, whose
composition is represented by the formula CO, is applied the name
carbon monoxide. Here then we see that _the same atom may sometimes
combine with two atoms of oxygen and sometimes with only one_.
Further the chemist knows four simple and familiar compounds whose
molecules illustrate very strikingly the difference of equivalence of
_different atoms_. These compounds are the following:
Chlorohydric acid (Hydric chloride), HCl or H—Cl
H— }
Water (Hydric oxide), H₂O or } O
H— }
H— }
Ammonia gas (Hydric nitride), H₃N or H— } N
H— }
H— }
Marsh gas (Hydric carbide), H₄C or H— } C
H— }
H— }
It has been found advisable to adopt the atom of hydrogen as the
standard of equivalence or atom-fixing power. It is plain that by
this method of comparison the atom chlorine may be said to have the
equivalence _one_, since it combines with one atom of hydrogen. And so
the atom oxygen may be said to have the equivalence _two_, since it
combines with two atoms of hydrogen. And the atom nitrogen may be said
to have the equivalence _three_, since it combines with three atoms
of hydrogen. And the atom carbon may be said to have the equivalence
_four_, since it combines with four atoms of hydrogen.
The language of chemistry sometimes presents the same observed facts
in a slightly different form, somewhat as follows: Chlorine is said
to have one point of attraction and is called a monad (a term derived
from the Greek word μονάς, _monas_, a unit). Oxygen is said to have
two points of attraction and is called a dyad (a term derived from the
Greek root δυάς, _dyas_, two). Nitrogen is said to have three points of
attraction and is called a triad (a term derived from the Greek word
τριάς, _trias_, a group of three). Carbon is said to have four points
of attraction and is called a tetrad (a term derived from the Greek
word τετράς, _tetras_, four).
While hydrogen as the basis of the system has the uniform equivalence
one, and is always a monad, and oxygen its close friend and ally has
always the equivalence two, and is always a dyad, most other elements
have some variety of equivalence. Thus chlorine has at different
times different equivalences, sometimes _one_, sometimes _three_, or
_five_, or _seven_. So nitrogen has at different times the different
equivalences, _one_, _three_, _five_. So carbon has sometimes an
equivalence _two_, sometimes _four_.
Fourth. Chemical Changes Neither Create nor Destroy Matter.
When chemical changes are produced by reason of the action of chemical
affinity, there is never either gain in weight or loss in weight. In
other words there is no creation of matter and no destruction of it.
In former times, people who observed the disappearance of solid matter
when charcoal burns, thought that the substance was destroyed—partly
if not wholly. The modern chemist finds, however, that the carbon is
only turned into the form of an invisible gas, and that by the use of
appropriate appliances he can find the weight of this gas, and compare
it with that of the carbon producing it. In the combustion of carbon
the chemical change is represented by the following equation:
=C= + =O₂= = =CO₂=
One atom of Two atoms of One molecule of
Carbon Oxygen Carbon dioxide
12 32 44
parts by weight. parts by weight. parts by weight.
\________________________________/ \_______________/
| |
44 44
This equation means that the chemist has discovered, by careful
experiments, that when any twelve parts by weight of carbon—say twelve
pounds—are completely burned, they always unite with thirty-two
corresponding parts of oxygen (in this case thirty-two pounds), and
they produce forty-four parts by weight of carbon dioxide (in this case
forty-four pounds).
And so in all chemical changes the substances taking part—whether
solid, liquid, or gaseous—may be weighed, and the sum of the weights
of all the matters finally produced is just equal to the sum of the
weights of the original factors.
Fifth. Chemical Changes are Often Attended by Displays of Force.
In many chemical changes the union of the atoms is attended with the
_production_ of heat, or electricity, or some other form of energy. Now
it is a law derived from modern discoveries that the amount of energy
given out by any chemical union is fixed and invariable, and that it is
just the same in amount as the quantity of that kind of energy that is
absorbed when such chemical action is reversed.
Sixth. Chemical Changes Produce Striking Results.
Each of the atoms of matter is in itself fixed and unchangeable and it
possesses through all its varied combinations an inherent character
which belongs to it and which no human being can permanently alter.
But when atoms unite to build up either simple or complex molecules,
the various original atomic characters are so blended and balanced and
reinforced as to afford in the molecular product an entirely new and
unexpected set of properties. An example of these principles is found
in the union of copper, sulphur, oxygen and hydrogen. These substances
may combine to form a new molecule which is called cupric sulphate, and
which has the composition expressed by the formula
CuSO₄ + 5H₂O.
Of the constituents of this molecule, copper is red, sulphur is yellow,
oxygen is colorless, hydrogen is colorless; but when they unite the
cupric sulphate formed is blue, that is its color is not that of
either of its constituents, nor is it intermediate between them. There
is simply a new and unexpected result, and one which in the present
state of knowledge cannot be explained; it can merely be recorded.
And this example is only one of a myriad. Throughout nature chemical
changes most marked—and to the human thought unexpected—arise from the
union of familiar elementary substances.
Seventh. Chemical Atoms Unite in Obedience to Definite Law.
Careful chemical study of the way in which atoms combine has developed
the following as a fundamental law of nature. The same chemical
compound always contains the same kind and number of elementary atoms,
and these atoms are united in the same proportions by weight. This law
is a formal statement of facts similar to those already referred to in
paragraphs _third_ and _fourth_ of this chapter. It does not therefore
seem to call for further explanation at this point.
The Modern Atomic Theory.
The same chemical study which has developed the truth of the law just
stated has also given rise to the modern atomic theory. The chemist is
constrained to believe that matter is composed of ultimate indivisible
particles called atoms. While these atoms are invisible to mortal eye
even with the help of the finest known optical appliances, yet when
their existence is once admitted this admission affords an explanation
that is a satisfactory one, and indeed the only one that harmonizes
with the multitude of observed chemical and physical laws.
This atomic theory, in its essential particulars, was suggested in
the early part of this century by Dr. John Dalton, who was a teacher
of mathematics in Manchester, England, and who died as recently as
in 1844. Dalton found recreation in chemical experiments, and the
mathematical turn of his mind led him to express the results of his
chemical analyses in a new numerical form. Thus previous to his time it
had been customary to express the composition of all substances in the
ordinary percentage form. Now Dalton found that _if some special weight
was adopted as the unit_, a variety of new and previously concealed
facts was revealed. The idea to be here conveyed is partially but
perhaps sufficiently expressed by the following examples derived from
the two compounds of carbon already referred to:
[Illustration: JOHN DALTON:
Born at Eaglesfield. (England.) Sept. 5th, 1766; died July 27th, 1844.]
Composition of the Two Compounds of Carbon and Oxygen.
+------------------------------------------------------+
| EXPRESSED IN PER CENTS. |
+------------------------------------------------------+
|_Carbon Monoxide (CO)._ _Carbon Dioxide (CO₂)._ |
|Carbon, 43 - per cent. 27 + per cent. |
|Oxygen, 57 + per cent. 73 - per cent. |
| ———— ———— |
| 100 100 |
+------------------------------------------------------+
+------------------------------------------------------+
| EXPRESSED IN DALTON’S FORM. |
+------------------------------------------------------+
|_Carbon Monoxide (CO)._ _Carbon Dioxide (CO₂)._ |
|Carbon, 12 parts by weight, 12 parts by weight. |
|Oxygen, 16 parts by weight. 32 parts by weight. |
| ——— ——— |
| 28 44 |
+------------------------------------------------------+
In Dalton’s expression it is at once evident that, as compared with
the weight of carbon, the amount of oxygen in carbon dioxide is
exactly twice what it is in carbon monoxide: but to the ordinary
unmathematical mind this fact is buried in the percentage statement.
Dalton’s experiments with still other compounds gave him results
showing a simplicity of relationships similar to that obtained from the
carbon compounds just referred to. To his mind these facts suggested
immediately the idea that an elementary substance is made up of atoms
each of a determinate weight, and that these atoms combine by wholes
and not by fractional parts, and that although it is impossible to
weigh any atom separately, yet _the weight ratios of a multitude of
them that combine as wholes_ express at once the weight ratios of
the atoms themselves. He thus got the idea of atomic weights and
constructed the first table of them. Since Dalton’s first declaration
of his atomic theory, the combining numbers of the different atoms
have been studied by chemists with the most thoughtful care and the
most painstaking methods known to modern science; and tables have been
constructed showing the combining numbers which are believed also
to be the true atomic weights for all the various elements thus far
recognized.
READING REFERENCES.
Atomic Constitution of Bodies.
=Saint-Venant.=—Jour. of Chem. Soc. of London. xxx,
pt. II, 472.
Atomic Philosophy.
———— Amer. Chemist, iii, 326.
Atomic Theory.
=Williamson=, A. W. (and others.)—Jour. of Chem. Soc.
of London. xxii, 328, 433.
=Wurtz=, Ad.—The Atomic Theory. New York. 1881.
Atomic Volumes, Etc.
=Avogadro.=—Annales de Chimie et de Physique.
3 Sér. xiv, 330; xxix, 248.
Atoms, Vortex Theory of
=Thomson=, Sir Wm.—Phil. Mag. 1867.
=Thompson=, J. J.—Science. iii, 289.
=Tait=, P. G.—Recent Advances in Physical Science.
London. 1876. p. 283.
Atomic Weights, Dalton’s First Table of
=Roscoe=, H. E.—Chem. News. xxx, 266.
Chemical Operations, Calculus of
=Brodie=, B. C.—Jour. of Chem. Soc. of London. xxi, 367.
Dalton, John
=Henry=, W. C.—Life of Dalton. London. 1854.
Definite Proportions, Variability in Law of
=Boutlerow.=—Silliman’s Journal. 3d Ser. xxvi, 63.
=Cooke=, J. P.—loc. cit. 310.
Equivalents of the Elements.
=Dumas=, J.—Annales de Chimie et de Physique. 3 Sér. lv, 129.
Energy.
=Stewart=, Balfour.—The Conservation of Energy. New York.
1874.
=Tait=, P. G.—Recent Advances in Physical Science. London.
1876. Encyclopædia Britannica. vol. viii.
Gaseous and Liquid States of Matter.
=Andrews=, T.—Jour. of Chem. Soc. of London. xxiii, 74; xxx,
pt. II, 159.
=Ramsey=, W.—Jour. of Chem. Soc. of London. xlii, 136.
Matter, Constitution of
=Ditte=, A.—Annales de Chimie et de Physique. 5 Sér. x, 145.
Nomenclature of Salts.
=Madan=, H. G.—Jour. of Chem. Soc. of London. xxiii, 22.