Chemical affinity

“ When, however, the nearness becomes sufficient, then chemical affinity begins to operate. We have, in fact, the familiar act of combustion, and, as its consequence, the chemical union of the [Pg 54] carbon or coal with the oxygen of the air, carbonic acid being the result. Here, then, we have a very powerful force acting only at a very small distance, which we name chemical affinity, inasmuch as it represents the attraction exerted between atoms of different bodies in contradistinction to cohesion, which denotes the attraction between molecules of the same body. ”

“ Oxygen and nitrogen, nitrogen and hydrogen, hydrogen and carbon, which show no affinity for each other under ordinary circumstances, readily unite when one or both are in a nascent condition. The reason seems to be that, when the elements of a compound are torn asunder, the chemical affinity which previously bound them together is set free, ready and eager to unite the nascent elements with whatever they come in contact with. ”

“ The peculiar manner in which hydrogen gas appears to dissolve solid substances,—as iron, potassium, sodium, sulphur, phosphorus, selenium, and arsenic, may be explained by regarding the results as a manifestation of the powers of chemical affinity over the forms of bodies. In like manner, the solution of salt in water, or the mixture of alcohol in that fluid, may be viewed as chemical phenomena, although usually considered as simple cases of solution or mixture: alterations of temperature and other physical changes taking place in either. ”

“ Every chemical combination is an illustration of this rule. But let us proceed—
The 6th law is, that the force of chemical affinity between the constituents of a body is estimated by that which is required for their separation. This force is not always proportional to the facility with which bodies unite; for manganese, for instance, which, you know, is so much disposed to unite with oxygen that it is never found in a metallic state, yields it more easily than any other metal. ”

“ Chemical affinity depends entirely upon the energy with which particles of different kinds attract each other. Oxygen and hydrogen are particles of different kinds, and it is their attraction to each other which makes them chemically combine and produce water. ”

“ In organic as in inorganic chemistry, atoms are bound together by chemical affinity, though it was formerly supposed that an additional or vital force was instrumental in forming organic compounds. For this reason none of these substances, it was thought, could be built up in the laboratory, although many had been analyzed. In 1828 the first organic compound, urea, was artificially prepared, and since then thousands have been synthesized. They are not necessarily manufactured from organic products, but can be made from mineral matter. ”

“ Thus we have traced out the distinction between mechanical aggregation and chemical affinity, taking for an example the difference between gunpowder as a whole (in which the ingredients are so nicely balanced that it is almost a chemical combination) , and its constituents, sulphur, charcoal, and nitre, when they are chemically combined; or, in briefer language, we have noticed the difference between the mechanical mixture, and some of the chemical combinations, of three important elements. ”

“ Where two elements combine to form a compound, heat is almost always evolved, and the amount evolved serves as a measure of the affinity. In order that chemical affinity may come into play it is necessary that the substances should be in contact, and usually one of them at least is a fluid or a gas. The results produced by chemical combination are endlessly varied. Colour, taste, and smell are changed, destroyed, or created; harmless constituents produce strong poisons, strong poisons produce harmless compounds. ”

“ But as soon as the solution is rendered stronger, or the solvent and the dissolved body are endowed with a mutual chemical affinity, chemical reactions set in. Part of the molecules of the dissolved body dissociate, and the atoms of which they were composed, on being set free, combine with the atoms of the solvent. Chemical forces, much more energetic than the physical forces, enter into play, and most complicated chemical reactions—the intensity of which may be judged of from the changes of temperature—begin. ”

“ This explains why salts are usually inactive and neutral in character, while acids and bases are active agents. Thus we see why the most positive or the most negative elements in general have the strongest affinities, while those intermediate in the list are inactive, and have weak affinities; why alloys of the metals are weak compounds; why a neutral substance, like water, has such a weak affinity for the salts which it holds in solution; and why an aqueous solution is regarded as a mechanical mixture rather than a chemical compound. ”

“ But we need not go beyond the laboratory to see abundant examples of this so-called resistance to chemical affinity, when the conditions are altered. The decomposition of carbonates by tartaric acid is a chemical process which is wholly resisted if alcohol19 instead of water be the solvent employed. The union of sulphur with lead is said to be due to the affinity of the one for the other ”

“ Hitherto I have spoken of chemical decomposition only as produced by motion, and heat, and light, but now I must give a familiar instance in which you can produce great chemical change and movement amongst atoms by simply mixing two chemicals together. Add tartaric acid to a solution of carbonate of soda and a great commotion ensues, owing to the superior affinity of the tartaric acid for the soda; and which acid displaces the carbonic acid in previous union with the soda, and the latter acid is turned out and escapes by violent effervescence. ”

“ The view long maintained—until late in the nineteenth century, in fact—that organic and inorganic chemistry are separated from each other by a wide gap, is no longer held. Hundreds of organic substances, some of them of great complexity, have been made in the chemist’s laboratory, and can now be classed as properly with inorganic as with organic substances. The gap has been closed, and there is now but one chemistry. Only the most intricate chemical compounds still lie beyond the chemist’s grasp, and the isolation of these may be at any time overthrown. ”

“ Thus we learn that chalk is a body consisting of two compound molecules,—carbonic acid, which is formed by the combination of an atom of carbon with two atoms of oxygen,—and lime, which results from the union of an atom of calcium with one of oxygen.
The condition requisite to the production of chemical action between bodies is that they should be dissimilar. Two elementary atoms are placed within the spheres of each other’s influences, and a compound molecule results. Oxygen and hydrogen form water; oxygen and carbon give rise to carbonic acid; nitrogen and hydrogen unite to form ammonia ”

“ When hydrogen chloride and ammonia enter into combination to form ammonium chloride, for example, do they combine in some special manner, molecularly, so that each molecule retains its individuality as a radicle in the new compound; or is a redistribution effected, so that the several atoms become arranged around the one which exercises the dominant influence much as they are in the parent compound ammonia? In the former case, two orders of affinity would come into operation; in the latter, only one. The general opinion has always been in favour of the latter view. ”

“ It must not be understood that this force, which I finally came to control, annihilated matter; it merely annihilated form. Nor, as I soon discovered, had it any effect on inorganic structure; but to all organic form it was absolutely fatal. This partiality puzzled me at first, though had I stopped to think deeper I would have seen through it. Since the number of atoms in organic molecules is far greater than in the most complex mineral molecules, organic compounds are characterized by their instability and the ease with which they are split up by physical forces and chemical reagents. ”

“ They absorb oxygen from the surrounding medium, air or water, and, in addition to certain inorganic substances, take into their bodies, as their proper nutritive material, complicated protein compounds, which they derive either directly or indirectly from plants. Through the oxidation of these compounds they form substances of a simpler chemical structure, such as carbonic acid, water, and ammonia, and discharge these through their bodies as waste. ”

“ It was formerly supposed that the substances found in plants and animals, mostly complex compounds of carbon, hydrogen and oxygen, could only be produced by "vital forces." If this were true it meant that chemistry was limited to the mineral kingdom and to the extraction of such carbon compounds as happened to exist ready formed in the vegetable and animal kingdoms. But fortunately this barrier to human achievement proved purely illusory. The organic field, once man had broken into it, proved easier to work in than the inorganic. ”

“ In chemical stability these triatomic compounds, considered as a group, are in a marked degree below the diatomic ones. The various sugars and kindred bodies, decompose at no very high temperatures. The oils and fats also are readily carbonized by heat. Resinous and gummy substances are easily made to render up some of their constituents. And the alcohols, with their allies, have no great power of resisting decomposition. These bodies, formed by the union of oxygen, hydrogen, and carbon, are also, as a class, chemically inactive. Formic and acetic are doubtless energetic acids ”

“ The putrefaction of animal and other azotized bodies is a chemical process, by which they are gradually dissipated in a gaseous form, chiefly in that of carbonic acid and ammonia; now to convert the carbon of the animal substance into carbonic acid requires oxygen, and to convert the azote into ammonia requires hydrogen, which are the elements of water. ”

“ When oxygen combines with a single element, what is the compound called? We have found that O makes up a certain portion of the air; later, we shall see how large the proportion is. Its tendency to combine with almost everything is a reason for the decay, rust, and oxidation of so many substances, and for conflagrations, great and small. New compounds are thusformed, of which O constitutes one factor. Water, H2O, is only a chemical union of O and H. Iron rust, Fe2O3 and H2O, is composed of O, Fe, and water. The burning of wood or of coal gives rise to carbon dioxide, CO2, and water. ”

“ Such an arrangement, if erected upon a scale sufficiently extensive, with the medium of a neutro-saline solution, would, in his opinion, produce large quantities of acids and alkalies with very little trouble or expense.
249 Alterations in chemical equilibrium are constantly taking place in Nature, and he thinks it probable that the electric influence, in its faculties of decomposition and transference, may considerably interfere with the chemical changes occurring in different parts of our system. ”

“ The comparatively fixed class, as regards diffusion, is represented by a different order of chemical substances (marked out by the absence of the power to crystallise) , which are slow in the extreme. Among the latter are hydrated silicic acid, hydrated alumina, and other metallic peroxides of the aluminous class, when they exist in the soluble form; with starch, dextrine, and the gums, caramel, tannin, albumen, gelatin, vegetable and animal extractive matters. Low diffusibility is not the only property which the bodies last enumerated possess in common. ”

“ When this element, nitrogen, is combined with the wonderful variety of compounds formed by carbon, we have not only a great many intimately related yet distinct [100] substances, but compounds which readily change from one form to another. These are the distinctive qualities or conditions necessary, from a chemical standpoint, to make the processes of life possible. Protoplasm, which is the basis of all life, is formed by an intimate mixture of a number of complex chemical compounds, the chief elements of which are carbon, hydrogen, oxygen, and nitrogen. ”

“ ACID-AMIDES, chemical compounds which may be considered as derived from ammonia by replacement of its hydrogen with acidyl residues, the substances produced being known as primary, secondary or tertiary amides, according to the number of hydrogen atoms replaced. ”

“ Since normal solutions of various reagents are all referred to a common standard, they have an advantage not possessed by empirical solutions, namely, that they are exactly equivalent to each other. Thus, a liter of a normal solution of an acid will exactly neutralize a liter of a normal alkali solution, and a liter of a normal oxidizing solution will exactly react with a liter of a normal reducing solution, and so on. ”

“ It brings home to me what an unstable chemical compound man is. Here are the stage accessories as good as ever, while the players have all split up into hydrogen and oxygen and nitrogen and carbon, with traces of iron and silica and phosphorus. ”

“ Chemical Composition. When pure it consists of oxygen, hydrogen, and carbon; the rectified æther however still contains some water and alcohol, for Lovitz obtained an æther of 632. Solubility. One part requires for its solution ten of water; with alcohol and ammonia it unites in every proportion. Solvent Powers. It is one of 268the most powerful solvents known in vegetable chemistry, as it dissolves balsams, resins, gum-resins, wax, camphor, extractive, &c.; it takes up about a twentieth of its weight of sulphur, but it exerts no solvent power upon the fixed alkalies. Forms of Exhibition. ”