If aqueous solutions of potassium sulphocyanide (KCNS) and ferric chloride are mixed, ferric sulphocyanide [Fe(CNS),] and potassium chloride are formed. In this case the four substances all remain in solution. The change may be represented thus;

6KCNSAq+KCNSAq + FeCl Aq = 6KClAq + Fe ̧(CNS) ̧Aq + xKCNSAq.

If the salts are allowed to react in the ratio 6KCNS : FeCl very little of either is decomposed; in order to change the mass of ferric chloride represented by the formula FeCl almost completely into ferric sulphocyanide the salts must be mixed in about the ratio 500KCNS: Fe,Cl. But at the close of the reaction only 6KCNS has been decomposed for every FeCl changed; the rest of the potassium sulphocyanide remains chemically unchanged.

When hydrogen and iodine are mixed at 440° and under a pressure of 340 mm. a portion of the elements combine to form hydrogen iodide; the amount of hydrogen iodide formed is increased by increasing the mass of either hydrogen or iodine relatively to the other gas. The following numbers shew the influence of increasing the mass of hydrogen :

:

5

2

239 The influence of the relative masses of the interacting substances on the course of a chemical change is most marked when all the interacting substances and all the products of the change are in solution, or are liquids. Ethylic alcohol and acetic acid for instance react to produce ethylic acetate and water, thus C.H.OH + CH2O.OH= C2H2O.OČÍ ̧ + H ̧0; but much more acetic acid than is represented in this equation must be used to complete the change. Such changes as this, or as that which occurs between solutions of potassium sulphocyanide and ferric chloride (v. ante), may be divided into two parts; the direct, and the reverse, change. The direct change in the case of alcohol and acetic acid is that shewn in the equation, the reverse change is that from ethylic acetate and water to ethylic alcohol and acetic acid (C,H2O.OCH + H2O = C ̧H2O.OH + CH.OH). The direct and reverse

3

2

changes in the case of potassium sulphocyanide and ferric chloride solution may be represented thus,

(1) direct 6KCNSAq + Fe,Cl,Aq= Fe, (CNS), Aq + 6KClAq ; (2) reverse Fe, (CNS),Aq + 6KClAq = FeCl ̧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 tempera

ture.

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,SO. The change CaCO, (solid) + 2HCIAq = CaCl, + H2O+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.

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2

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, especially where gases are produced, on the pressure.

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 decomposed 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."