Chapter VII: 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:

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.

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=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.
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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.