Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/793

This page needs to be proofread.


CHEMISTRY 737 lytic dissociation of the fundamental molecules is con- new state of equilibrium is established in which the salts siderably retarded. A striking illustration of the influence AD and CB also exist along with AB, AC, A, B, C, and of composition on the extent to which dissociation takes D. The new salts are produced in quantities determined place is afforded by the substituted acetic acids, thus :— by the concentration, and if the solution be sufficiently i dilute, the salts AB, AD, CD, CB are present in negligibly Kxl05 (v=1000) small quantities; consequently the properties of the soluAcetic acid. 0-125 0-0018 tion are simply the properties of the dissolved ions, and Chloracetic acid . 0-692 0-155 there is no appreciable change on mixing AB with CD. Dichloracetic acid 0-981 5-1 Trichloracetic acid 0-999 121-0 But this is only true if the products are soluble. If the It is generally noticeable that the value of the dissocia- ions A and D are capable of uniting to form a substance tion constant K is subject to great variation as the struc- which is insoluble under the conditions of the experiment, ture changes; consequently this factor becomes of great the compound AD is removed from the system as rapidly as it is formed, until eventually, when equilibrium is importance in diagnosing the structure of electrolytes. Ethereal salts, cane sugar, and other substances which attained, only a very minute proportion of the ions A and undergo hydrolysis in presence of acids and alkalies are D remains in solution. The effect on each other which two acted upon at rates which differ not only as the conditions electrolytes having an ion in common often exercise by of temperature and concentration are varied, but also accord- the one diminishing the solubility of the other is easily ing as the change is conditioned by the one or the other understood from this point of view. In the case of a hydrolyst. Investigation shows that the relative activity saturated solution of a salt AB, the solid present is in of hydrolysts corresponds with the electric conductivity of equilibrium with the undissociated dissolved salt AB, their solutions; hence it is assumed that acids, Ac., do which, in turn, is in equilibrium with its ions A and B ; not act as such in these cases, but that the ions are the if now another salt containing either the ion A or the ion active agents. From this point of view, the constant K is B be brought into the solution, the extent to which AB the measure not only of the extent to which a substance is undergoes dissociation is reduced, because the proportion dissociated, but also—with certain limitations—of its in which the one or the other of its ions is present in the activity in chemical interchanges. The following results liquid is increased. Consequently AB is partially thrown obtained by Ostwald with cane sugar may be cited as out of solution. The precipitation of sodium chloride on proof that the rate at which inversion is effected by the saturating its solution with hydrogen chloride, and the acids referred to, in comparison with that deduced for fact that the solubility of barium sulphate in water is hydrogen chloride (in h normal solution at 25°), is almost reduced by the addition of either sulphuric acid or barium chloride, may be instanced as cases to which this explanaproportional to the degree of dissociation :— tion is applicable. The same principle is applicable in Rate of Degree of Inversion. Dissociation. explanation of the lowering of the activity of weak acids Acetic acid 0-0040 0-006 by their salts. The fact that chlorine in chlorates, iron in Formic acid 0-0153 0-0245 ferrocyanides, &c., are not to be detected by the “ ordinary ” Monochforacetic acid . 0-0484 0-0542 tests, is ascribable to the fact that they are not present in Dichloracetic acid 0-271 0-272 solution as simple ions, but as complex ions having altoAs the hydrolysis of cane sugar is effected only by gether different and peculiar properties. acids and not by alkalies or salts, it is customary to The considerable evolution of heat which occurs in regard the action as brought about solely by the hydrogen neutralizing an acid by an alkali is apparently a contraions, which are abundantly present in most acid solutions, vention of the rule that practically no change Water bllt but not in those of alkalies and salts. It should be noted, occurs on mixing solutions of highly dissociated slightly however, that although it is true, especially in the case of substances if there be no insoluble product, dissociweak acids, that acids act the more readily the more This, however, is held to be due to the fact ated' highly they are dissociated, there is only a roughly ap- that water itself is peculiar in being dissociated only to a proximate numerical proportionality between the inversion very minute extent. Consequently water is the one velocity and the quantity of hydrogen ions. It is also product formed in the solution, and the heat evolved is to noteworthy, and difficult to explain, that the catalytic be regarded as that liberated by the combination of its activity of the hydrogen ions is greatly stimulated by the constituent ions, thus : presence of other ions; thus on adding to an acid solution (H + X) + (Y + OH) = HOH + X + Y an equivalent proportion of the corresponding potassium Ions of acid. Ions of base. salt, the velocity of inversion is increased by some 10 per cent, in the case of strong acids, although it is decreased In the case of all strong (i.e., highly dissociated) acids and in the case of the weaker acids. bases, therefore, the character of the change is independent The ionic dissociation hypothesis is held to afford a simple of the nature of the acid and base. This conclusion is in explanation of the phenomena underlying the processes in striking accord with the well-known fact that the strong , . . use in analytical chemistry. Thus it is well acids and alkalies all have practically the same heat of thesis known that the tests applied in detecting neutralization (cp. Ency. Brit. vol. v. p. 487). Further applied to metallic and also non-metallic radicles are, with evidence of the same kind is afforded by the observation that chemical few exceptions, of general application—a metal, in dilute aqueous solution all the metallic salts of a strong ysts ’ for example, being detected by the same test optically-active acid, such as bromocamphorsulphonic acid, whatever the salt may be in which it is present as positive have the same molecular rotatory power, viz., that of the constituent. This must be the case, on the ionic hypothesis, acid ion. as the chemically active components in a solution are not If water be regarded as very slightly dissociated, it is the dissolved substances, but the ions into which they are also easy to understand that it should act as a weak acid resolved on entering into solution. The state of a salt towards salts of very feeble acids and bases, which, in AB is such that the undissociated salt and its ions A and solution, undergo dissociation to an extent comparable B are in equilibrium; a second salt CD also exists in with the degree of dissociation of water; in such cases, solution partly as CD and partly as the ions C and D. the ions A and B of the salt should combine with those of On mixing two such solutions interchanges occur, and a water, and recognizable quantities of the acid and base S. IL —93