736
CHEMISTRY
they met with others of the opposite kind and united with them to form new molecules. On the assumption that the electromotive force acts on the molecular fragments during the intervals of freedom but does not produce disruption of the molecules, it is possible to understand that any electromotive force, however small, can produce sensible electrolysis. Kekule, in 1858, in discussing the manner in which interchanges are effected, took the very different view that in the first instance the molecules attract each other and become associated, and that atomic affinities then come into operation and cause the redistribution. He pictured the state of affairs in the following manner:— a b a b ab a' I b' a' V ~db' Before. During. After. He went so far as to speak of this conception as the simplest and as being universally applicable. For many years practically no attention was paid to the dissociation hypothesis, and it was not until 1884 that interest was revived in it by Arrhenius, who gave a quantitative form to the hypothesis of Clausius by treating electrolytic conductivity as a measure of the extent to which dissociation may be supposed to have taken place in a solution. A still greater advance was made in 1887 by van’t Hoff, who fully discussed the properties of solutions, and showed that the equations for the ordinary gaseous laws are equally applicable to solutions, provided that in the case of conducting solutions the increase in the number of molecules due to the “ dissociation ” be taken into account. In the modern form of the hypothesis, substances which afford conducting solutions (acids, bases and salts) are ^ assumed to undergo disruption or dissociation Ionl c d'Sm separate ions vjhen dissolved in water, the soc a on. int0 extent to which the ionic dissociation is effected
water together with an undissociated substance and its ions, the ordinary laws of equilibrium govern the relation between the degree of dilution and the extent to which the substance undergoes dissociation; therefore, in the simplest case, that of a binary electrolyte E, the two ions of which are A and B, if e, a, and b are the molecular concentrations of the undissociated material and of its two ions respectively, ab = eK, and a — b where K is a constant. If the total quantity of dissolved material be taken as unity and the dissociated fraction as i, a = b = i/v and (1 — 7)/-v = e. The relation between the degree of dissociation and the dilution is therefore that expressed by the Effector formula dilution on
- •2/(1
_i) = Kv. degree of i' ' dlssociation% The case of dichloracetic acid may be cited as one in which, as van’t Hoff points out, the agreement between the calculated and observed results is especially close.
20 205 408 2,060 4,080 10,100 20,700
51-6 132 170 251 274 295 300 311
TOv/Moo Obs.
Mv/Woo Calc.
0-166 0-423 0-547 0-806 0-881 0-948 0-963 1
0-163 0-43 0-543 0-801 0-88 0-944 0-971 1
Whereas carboxylic acids generally are but slightly dissociated in moderately dilute solutions, strong mineral acids, the alkalies, and most metallic salts—especially those of alkali metals—are highly dissociated. This is depending not only on the substance, but also on the well shown in the following table of results originally degree of dilution. Neutral substances, on the other hand, published by Ostwald in 1885, in which the molecular such as alcohol, sugar, &c., which do not behave as elec- conductivities are given in arbitrary units :— trolytes in solution, are assumed to be undissociated. Nitric Sulphuric HCl. HBr. HI. V. But ionic or electrolytic dissociation differs from ordinary Acid. Acid. dissociation — in which a compound is resolved into 92-7 77-9 2 77-9 80-4 80-4 mutually independent simple molecules—in that the two 96-4 80-4 64 80-9 83-4 83ions into which the electrolyte is resolved are associated 82-8 100-6 7848 83-6 85with opposite equal electric charges. These electric 107-4 84-9 10-00 8616 85-4 86116-3 13-14 868787charges, like matter, are, in a sense, to be regarded as 32 8787127-3 17-38 888864 88“ atomic,” a unit charge corresponding to a unit valency, 139-2 888923-11 89128 88so that each ion is capable of conveying only a certain 88-4 150-6 89-6 89-7 30-30 256 89charge; consequently the electric conductivity of a dissolved 160-9 88-8 512 89-6 89-7 89-7 39-11 169-1 88-9 electrolyte is quantitatively proportional to the degree of 1,024 89-5 89-5 89-3 49-49 174-4 88-2 2,048 89-5 88-9 89-0 59-56 ionization. The dissolved substance in its dissociated 86-6 177-1 4,096 88-6 87-6 87-8 69-42 state—not the solution as a whole—is therefore the effecJ tive conductor. To deduce m, its molecular conductivity, It will be noticed that in the case of the three halhydrides, ( the specific* conductivity, ~k, of the solution is multiplied by for example, a steady value is very soon reached, dissociav, the number of litres containing a gram molecular pro- tion being practically complete at very moderate dilutions portion of the substance. The molecular conductivities in comparison with those necessary to produce anything are, therefore, values which express the degree of con- like the same degree of change in the case of dichloracetic ductivity due to the introduction of equal numbers of acid quoted above. Moreover, the initial value differs molecules of dissolved substances between the electrodes. but slightly from the final value—which is an indication If mv be the molecular conductivity of a substance in a that the acids are almost completely dissociated even in solution of dilution v, and m ^—deduced by extrapola- concentrated solutions; in fact, if 90 be taken as the tion from the mv values—that in an infinitely dilute limiting value, in the case of the halhydrides, HCl,HBr, solution, in the case of compounds of univalent ions, the and HI as well as in that of nitric acid, about 80/90 = -9 degree of dissociation or ionization iv in the solution of the of a molecular proportion of the electrolyte is resolved concentration v and the molecular conductivity of the dis- into its ions when dissolved in only 2 litres of water. solved substance are so related that The different behaviour of hydrogen fluoride is of interest, iv = f^-v/^^co • as this hydride, it is well known, tends to form complex In the case of compounds containing multivalent ions a aggregates; apparently these only gradually undergo more complex relation obtains. In a system consisting of simplification on dilution, and consequently the electro-
