245 246 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." 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 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 H2SO, respectively. 2 The law of mass-action has been experimentally proved in many different reactions; it probably holds good in all chemical changes. 247 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. 12 249 250 applied in work on chemical affinity, and numerous results have been obtained in keeping with the principle. 2 2 2 : 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,O,H,: H2CI, H2SO. The possible products of the interactions are potassium sulphate (K2SO), potassium-hydrogen sulphate (KHSO), potassium chloride (KCl), 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 by the occurrence of secondary reactions among the products of the primary change. 2 2 Another difficulty is to measure the masses of the substances present when equilibrium is established. Suppose, for instance, that potash and sulphuric and nitric acids have been mixed in the ratio K,O,H,: H.NO.: H2SO,, in dilute aqueous solution; how are we to determine how much potassium nitrate, and how much potassium sulphate, is actually present in the solution? The ordinary methods of analysis are useless here, because they are based on the use of reagents other than the substances in the solution; but the addition of any reagent is forbidden because the equilibrium of the system would thereby be destroyed. We cannot here go into the methods adopted; many of them consist in measuring some definite physical change and using this as an index of the chemical change which has occurred. : In the case of the two acids, nitric and sulphuric, reacting 251 with potash in equivalent quantities it has been shewn, with a very high degree of probability, that about 10 parts of potash combine with the nitric acid to form potassium nitrate, for each 7 parts which combine with the sulphuric acid to form potassium sulphate, when the system is in equilibrium. Hence it is concluded that the ratio of the affinities for potash of nitric and sulphuric acids is approximately 10 7. When the acids are nitric and hydrochloric, and the base is potash, the potash divides itself almost equally between the two acids; that is one half of the potash reacts with the nitric acid to produce potassium nitrate, and one half with the hydrochloric acid to produce potassium chloride. Hence the affinities of hydrochloric and nitric acids for potash are approximately equal. When the acids are hydrochloric and acetic almost the whole of the potash reacts with the hydrochloric acid; only about 4 parts of potash react to produce potassium acetate for each 100 parts which react to produce potassium chloride. Hence the ratio of the affinities for potash of hydrochloric and acetic acids is approximately 100 4. It is very important to observe what is the exact meaning we are now giving to such a statement as this 'the relative affinity for potash of nitric acid is to that of hydrochloric acid as 1: 1,' or as this 'the ratio of the affinities for potash of hydrochloric and acetic acids is 100 : ·4.' When these statements are amplified they assert, (1) that if equivalent masses of caustic potash (KOH), hydrochloric acid (HCl), and nitric 252 253 acid (HNO), are allowed to interact freely in dilute aqueous solution, one half of the potash combines with each acid to produce potassium chloride and potassium nitrate, respectively; (2) that if equivalent masses of caustic potash, hydrochloric acid, and acetic acid (CHO), are allowed to interact freely in dilute aqueous solution, then for every 100 parts of potash which are changed to potassium chloride only 4 parts of potash are changed to potassium acetate. If these statements are correct, it is evident that when equilibrium results the solution contains in the first case, potassium chloride and nitrate and also hydrochloric and nitric acids; and in the second case, much potassium chloride, a little potassium acetate, a little hydrochloric acid, and much acetic acid. The experiments of Ostwald and Thomsen have shewn that the relative affinities of acids are almost, if not quite, independent of the nature of the base; in other words that if equivalent masses of, say, hydrochloric and nitric acids, are mixed in dilute aqueous solution with an equivalent mass of caustic potash (KOH), or soda (NaOH), or ammonia (NH,OH), or caustic lime (CaO,H,), or caustic baryta (BaOH2), &c. one half of the base combines with each acid. Ostwald's experiments have also rendered it very probable that the ratio of the affinities for bases of any two acids is independent of the temperature, at least within such a range as 0° to 60°. There are many chemical changes brought about by acids other than those which take place between acids and bases. Some of these changes have been examined with the object of determining whether they are quantitatively conditioned by the same values as have been found to condition the reactions between acids and bases. Thus Ostwald made a number of measurements of the effects of different acids on the velocity of the change of acetamide into ammonium acetate. This change may be represented thus The change occurs more or less quickly in the presence of acids; each acid increases the amount of change in unit time to a certain definite extent. The equations deduced from the fundamental statement, that chemical action is proportional to the relative affinities and the active masses of the interacting |