« PreviousContinue »
composition and the property of interacting in a certain
But why is it that acids and alkalis interact to produce salts? Does a specified acid always produce the same mass of a salt when it interacts with a specified alkali? The formula of an acid tells us the composition and something about the reactions of the acid, it expresses the composition of the reacting weight of the acid; but would it be possible to find a number for each acid which should express how much of a specified chemical change—say interaction with alkali to form a salt—this acid is capable of performing? Or are the conditions of chemical action so complex, and chemical actions themselves so much modified by slight changes in the conditions under which they occur, as to render hopeless all attempts to determine constants of chemical change? We must now attempt to find answers to such questions as these.
That property of elements and compounds by virtue of 242 which they interact to produce new combinations is called chemical affinity.
A chemical change may always be regarded as one of the 243 results of the mutual action and reaction between two or more substances or systems of substances. There is always a change in the configuration of each part of the chemical system. One member, or one part, of the system may be said to exert force on the other members, or on the other parts, of the system. Thus, when solutions of potash and sulphuric acid interact to produce potassium sulphate and water, the potash may be said to exert force on the acid, and the acid may be said to exert force on the alkali.
Of chemical forces, that is the forces which come into play in chemical changes, we know very little. It is probably better, at any rate for the present, to make no attempt to consider chemical change from the strictly dynamical point of view.
Let us rather look to the manifestations of that property 244 of bodies which is called chemical affinity.
If mercury is placed in a solution of silver in sulphuric acid, the silver is slowly precipitated and some of the mercury is dissolved; if a piece of copper is placed in a solution of mercury in sulphuric acid, the mercury is slowly precipitated and some of the copper is dissolved ; if a piece of iron is brought into a solution of copper in sulphuric acid the copper is slowly precipitated and some of the iron is dissolved. The older chemists said, mercury has a greater affinity for sulphuric acid than silver has, but the affinity for sulphuric
acid of copper is greater than that of mercury, and the affinity for the same acid of iron is greater than the affinity of copper.
Similarly they said that the affinity for potash of hydrochloric acid is greater than that of phosphoric acid because potassium phosphate is decomposed by hydrochloric acid, that the affinity of nitric acid for potash is greater than that of hydrochloric acid, and the affinity of sulphuric acid for potash is greater than that of nitric acid, because potassium chloride is decomposed by nitric acid, and potassium nitrate is de. composed by sulphuric acid.
Chemical affinity was regarded by the older chemists as analogous to a mechanical force. As two mechanical forces opposite in direction and unequal in magnitude produce motion in the direction of the greater, so it was supposed that a stronger affinity always overcame a weaker and produced chemical change. The affinity of nitric acid for potash, for instance, was supposed to be stronger under all circumstances than that of hydrochloric acid for potash. According to this conception, chemical affinity acts exclusively in one direction.
The facts that potassium chloride is decomposed by heating with a considerable quantity of nitric acid, but that potassium nitrate is decomposed by heating with much hydrochloric acid, shew that a complete account of chemical change cannot be given by regarding only the affinities of the interacting substances. It is necessary to pay attention also to the relative masses of these substances.
In the early years of this century Berthollet formulated the statement " Every substance which tends to enter into chemical combination with others reacts by reason of its affinity and its mass*.” Berthollet taught that a chemical change between substances in solution, wherein neither solids nor gases are formed, results in the production of a system in equilibrium; that each member of the complete system interacts with each other in proportion to its affinity and its mass; and that therefore the equilibrium of the system may be overthrown by changes in the relative masses of one or more of the members of the system.
Berthollet further taught that changes in which solid or gaseous substances are formed are not suitable for the study of chemical affinity, because in these changes all the members of the chemical system are not free to interact, inasmuch as some
“ Toute substance qui tend à entrer en combinaison, agit en raison de son affinité et de sa quantité."
of them are removed from the sphere of interaction almost as quickly as they are formed. The typical normal case of chemical change, according to Berthollet's view, is one wherein every member of the system is free to interact with all the other members throughout the whole of the change; those changes wherein a final distribution of the interacting substances is quickly established by the formation of solid precipitates or the evolution of gases are special limiting cases.
Berthollet's law of mass has been developed in recent years 247 chiefly by the researches of Guldberg and Waage and of Ostwald. Guldberg and Waage formulate the law of mass thus chemical action is proportional to the active mass of each substance taking part in the change. By active mass is meant that quantity of a substance measured in equivalent weights which is present in unit volume of the chemical system.
The expression equivalent weights will be explained more fully hereafter (8. Chap. XVII.). We know that the amounts of potash and soda which severally neutralise 36.5 parts by weight of hydrochloric acid (HCl = 36.5) are those expressed by the formulae KOH (56) and NaOH (40), respectively. We also know that to neutralise a reacting weight of sulphuric acid (H SO, = 98) 112 parts by weight of potash (2KOH) or 80 of soda (2NaOH) are required. So far as neutralising by alkali is concerned, the quantities expressed by the formulae 2HCl (or HCl) and H So, are equivalent; so far as neutralising by acid is concerned the quantities KOH and NaOH (or 2KOH and 2NaOH) are equivalent.
Suppose that a solution of 112 parts by weight of potash (2KOH), 73 parts by weight of hydrochloric acid (2HCl), and 98 parts by weight of sulphuric acid (H SO,), is diluted with water to a specified volume; then the active masses of potash, hydrochloric acid, and sulphuric acid, respectively, in this solution are one equivalent of each, provided that by one equivalent is meant the quantity expressed by the formulae 2KOH (or K,0,H,), 2NaOH (or Na,0,H,), and H 30, respectively.
The law of mass-action has been experimentally proved in many different reactions; it probably holds good in all chemical changes.
The principle of the coexistence of reactions states that 248 when several reactions occur simultaneously, each proceeds as if it alone took place. No direct experimental investigation of this principle has been made; but it has been largely M. E. C.
applied in work on chemical affinity, and numerous results have been obtained in keeping with the principle.
Assuming the law of mass-action, and the principle of the coexistence of reactions, let us briefly examine a fairly simple chemical change. Let equivalent quantities of the alkali caustic potash, and the acids hydrochloric and sulphuric, be mixed in dilute aqueous solution ; let the substances be present in the ratio K,0,H, : H,C1, : H,80. The possible
HCl products of the interactions are potassium sulphate (K,SO.), potassium-hydrogen sulphate (KHSO.), potassium chloride (KCI), and water (HO). But these substances may interact to reproduce the original substances. We have then certain direct changes and certain reverse changes possible. Chemical equilibrium will result when the velocities of the opposite reactions have become equal, that is, when the quantities of the substances formed in the direct change are equal to the quantities of the substances formed in the reverse change, in unit of time. But we say that each change, the direct and the reverse, is proportional to the affinities, and the masses, of the reacting substances. Now we can measure the mass of each substance present at the beginning of the change, and we can also measure the mass of each substance present when equilibrium is established; hence we can deduce numerical values for the affinities of the reacting substances. Guldberg and Waage, Ostwald, van 't Hoff, and others, have deduced the necessary equations from the fundamental statements already made.
But there is another method by which values for the relative affinities of the substances taking part in a chemical change may be deduced from experimental data. The change may be allowed to proceed to a certain extent only, but not until the system has settled down into equilibrium; the quantity of each substance present in the system may then be measured, and the velocity of the change may thus be determined. Then, assuming that the change which has occurred is proportional to the affinities and the masses of the interacting substances, we may deduce relative values for these affinities from our measurements of the masses. The necessary equations have been deduced by Guldberg and Waage, Ostwald, and others.
One of the great difficulties in applying these methods is to find reactions which are sufficiently simple. Very many chemical changes which appear to be simple are complicated