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changes in the case of potassium sulphocyanide and ferric
chloride solution may be represented thus,
(1) direct 6KCNSAq + Fe,ci. Aq = Fe, (CNS), Aq +6KCIAq;
(2) reverse Fe, (CNS)Aq+6KCIAq=Fe,cl, Aq+6KCNSAq.

Neither change is completed under ordinary conditions ; the changing system reaches a state of equilibrium, which may be overthrown by altering the relative masses of the members of the system, or by raising or lowering the temperature.

When one or more of the products of a chemical change 240 between liquids or bodies in solution is a solid or a gas, that product is removed from the sphere of interaction of the members of the changing system as quickly, or almost as quickly, as it is formed; hence the change proceeds more or less rapidly to a conclusion, and the direct change is only slightly retarded by interactions among the products tending to reproduce the original substances. Thus the change BaCl Aq + Na SO Aq = BaSO, (solid) + 2NaClAq is realised when barium chloride and sodium sulphate are mixed in solution in the ratio BaCl, : Na Soc. The change CaCO, (solid) + 2HClAq = CaCl,+H,O+ CO(gas) is completed by using the masses of the interacting compounds shewn in the equation. But comparatively few chemical changes are completed unless an excess of one of the interacting substances is used. It is certain that in many cases, and it is probable that in most cases, the products of the direct change interact to produce the substances originally present; the whole system swings in two directions, and the result is a state of equilibrium, which is generally more or less easily overthrown by adding more of one of the interacting substances, or by removing one of the products of the direct change, or by altering the temperature.

Whether a chemical change will or will not occur, and if it 241 occurs, what will be the extent and the products of the change, depend chiefly (1) on the chemical characters of the reacting bodies, (2) on the relative masses of these bodies allowed to interact, and (3) on the temperature, and (4) in some cases,

We have already learnt something of the meaning of the expression chemical character of a substance; we know, for instance, that certain compounds are acids, others are alkalis, and others salts; these names imply, in each case, a certain

composition and the property of interacting in a certain definite way.

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.

CHAPTER XIII.

CHEMICAL AFFINITY,

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

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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.”

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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 (s. 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 (2HCI), 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, O, H,), and H,80, 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.

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