Joseph William Mellor.

A comprehensive treatise on inorganic and theoretical chemistry (Volume 1) online

. (page 163 of 177)
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Some fused salts conduct electrolytically, e.g. with fused silver chloride and silver
electrodes, silver is dissolved at the anode and deposited on the cathode, so that the
total amount of silver chloride is maintained constant ; with carbon electrodes,
silver is deposited at the cathode, and chlorine evolved at the anode.

It is usually stated that an acid or alkali is added to water in order that the lull <-r
may be decomposed into its constituent elements by the electric current. The function
of the acid (or, mutatis mutandis, of the alkali) has been a subject of some speculation.
(i) It has been said that the mere presence of the acid simply makes the water a con-
ductor and that the water alone is decomposed by the current : 2H 2 0=2A 2 + -f0 2 ~ ;
it has also been said that the acid alone is decomposed by the current, and that
the water is attacked by the products of the electrolysis and chemically decom-
posed. Symbolically, 2H 2 S0 4 =2H 2 -f 2S0 4 (electrolysis) followed by 2S0 4 +2H 2


=2H 2 S0 4 -|-0 2 (chemical). As a matter of fact, it is very doubtful if pure anhydrous
acid or pure water is a conductor. Hence, it is not more logical to say that the
acid makes the water a conductor than that the water makes the acid a conductor.
Each constituent loses its individuality when mixed together. At first sight, it
seems as if during the electrolysis of acidulated water, the mixture must fee
regarded as a unit which (1) conducts the current from one electrode to the
other ; and which (2) suffers decomposition by electrical influences at the
surfaces of the electrodes. Several working hypotheses can now be devised e.g.
with dilute sulphuric acid, it is plausible to assume that the electrolyte contains
the complex H 2 S0 4 .wH 2 0. During electrolysis, neither the water nor the acid is
decomposed, but rather the complex: 2H 2 S0 4 .rcH 2 0=2rcH 2 +-|-w02~-|-2H 2 S04.
The action of the current is to deprive the complex of both hydrogen and oxygen in
the proportions 2H 2 : 2 .

In a general way it may be said that (i) electrolytic conduction is accompanied
by visible decomposition, or (ii) polarization phenomena (q.v.) may appear, (iii) An
assembly of metals at a constant temperature can give no current, but if an electro-
lyte be introduced into the series a current can be obtained, (iv) According to the
electromagnetic theory of light, if a conductor be transparent it will probably
conduct electrolytically, e.y. fused salts, hot glass, etc.

5. The Ionic Hypothesis

Let us learn to dream, then perhaps we shall find the truth.- A. KEKULK.
In framing hypotheses we must see that they agree with facts ; in other respects, they
may be as inconceivable (not self -contradictory) as any fairy tale.- M. M. P. MUIR.

The main facts so far established by the preceding discussion of the phenomena
attending electrolysis may now be summarized :

(1) Electrolytes in solution conduct electricity, and the process of electrical
conduction is attended by a splitting of the molecules of the solute into
anions and cations ; the anions appear at the anode, and the cations at the
cathode. The separation of a certain number of anions at the anode is
simultaneously attended by the separation of a chemically or electrically
equivalent number of cations at the cathode. During electrolysis, the anions
and cations appear to be discharged electrically, because electrically neutral
molecules appear as secondary products of the electrolysis.

(2) The anion which separates at the anode is not necessarily derived from
the same molecule as the cation which appears at the cathode.

(3) Solvent and solute together make a conducting medium, since as a rule
neither solvent nor solute alone shows any marked capacity for conducting

(4) No measurable time is needed to put an aqueous solution in a condition
to conduct the current. Immediately the necessary difference of potential
appears at the electrodes the process of electrolysis begins.

(5) Osmotic pressure and related phenomena show that electrolytes in
dilute solution have what seems to be a molecular weight, which suggests
that the ordinary chemical molecule of the electrolyte dissolved in certain
solvents is dissociated into two parts.

It is generally agreed that during electrolytic conduction there is a convection
of electricity by the atoms of matter, but there have been differences of opinion as
to the mode of transit of the atoms through the liquid :

(1) The molecular chain hypothesis of C. J. T. von Grotthus was generally
accepted in the first half of the nineteenth century. In his Theorie de la
decomposition des liquides par I'electricile (jahanique, he (1805) l assumed that
the molecules of salt in solution are distributed throughout the solvent in an
irregular manner without any signs of orientation, as represented diagrammatically
at A, Fig. 4. The molecules, in the presence of a pair of oppositely charged electrodes


range themselves in " chains," like little magnets. The positively charged ions
cations are directed towards the negatively charged cathode, and the negatively
charged ions anions to the positively charged anode, B, Fig. 4. If the charges
on the electrodes are great enough, the molecules in immediate contact with the
electrodes will decompose, C, Fig. 4, and the charge on one of the ions will be
neutralized by the charge on the electrodes. And the other ion will unite with the
neighbouring molecule and liberate an ion with a similar charge. The free ion
attacks the next molecule, and so the process is continued throughout the " chain."

To fix the idea, consider the end of the molecular chain at the cathode. There,
a negatively charged ion is se.t free when a positively charged ion is neutralized at
the cathode. This " negative " ion associates with the adjacent molecule of the
chain ; this molecule decomposes, forming a new molecule, liberating, at the same
time, a negatively charged ion which associates with the next molecule of the
chain, D, Fig. 4. This successive decomposition and recombination goes on
throughout the chain of molecules from electrode to electrode. The new molecules
so formed turn about, and are again ranged in a " chain " resembling I?, as shown
at E, Fig. 4. A cycle^of changes of this nature is supposed to be going on all the
time the current is passing through the electrolyte.

C. J. T. von Grotthus' mechanical interpretation of the phenomenon is very
_ , ingenious, and it satisfactorily

It explained the facts known in his

c* 5 *> 8 f> $ A day, but later knowledge has shown
_ _J. that the hypothesis is not tenable

I in its original form. If the elec-

909090 909090 9090 B tricity be conducted by Grotthus'
_ chain, no current can flow until the

^ electromotive force driving the

09090909090909 o C energy is equivalent to the energy
_ represented by the heat of forma-

Ition of the molecules undergoing
o.o.o*o*o.o*co. D decomposition in the solution.
_. When a sufficient electromotive

I force is applied at the electrodes,

ooooo*ooo E t h e decompositions and recom-

FIG. 4.-Diagrammatic Representation of C. J T. Potions ^ of the molecules might
von Grotthus' Chain Hypothesis. proceed m the way described by

Grotthus. No such critical electro-
motive force, however, has been found to be necessary for the passage of a
current through electrolytic solutions. In H. von Helmholtz's air-free cell, 2
polarization is produced by an infinitesimal element, but no permanent leakage
of electricity goes on through the cell until the applied current attains a
certain voltage. The smallest electromotive force hitherto tried causes a current
to flow when it is applied to copper electrodes immersed in a solution of copper
sulphate. The total energy consumption is then nothing but that due to the
" resistance " of the cell. Again, solutions of electrolytes, like metallic wires,
conduct electricity in such a way that the rate at which "electricity passes through
the system is proportional to the electromotive force. This is true whatever be the
magnitude of the force, and consequently, if a certain amount of electrical energy
be expended in breaking up the molecules, this proportionality cannot obtain.
Hence, very little electrical energy can be expended in breaking up the dissolved
molecules into their respective ions, and it has therefore been urged that " the ions
cannot be held together by a force of finite value." Consequently, in the homo-
geneous electrolyte without polarization no hypothesis which involves the tearing
of the molecules asunder against the chemical binding forces can be admitted ;
there is no chemical cling of the atoms, but only a frictional rub. Otherwise,
those electrolytes whose atoms or radicles are held together by the weakest


attractions would most readily decompose electrolytically. This is by no means the
case. For instance, mercuric chloride is much less stable than sodium chloride,
and yet the latter is much more readily decomposed by an electric current.

G. F. Fitzgerald 3 has pointed out that the difficulty with Grotthus' hypothesis
can be overcome if it be assumed that when the molecules are polarized, they draw
one another apart at a rate proportional to the polarization. This at once makes the
relation between electric force and decomposition a linear one, and so satisfies Ohm's
law in the case of small currents. It also so far agrees with Clausius's hypothesis
that it explains electrolysis and double decomposition as properties of the same
kind. The molecules in a liquid will occasionally be arranged by accident in a proper
polarized condition in a closed circuit for drawing one another apart ; and if the
circuit includes molecules of different kinds, there will result double decomposition.
He added :

The supposition that it is a particular arrangement that is required before exchanges
take place, and that with this arrangement exchanges take place of their own accord, seems
to explain electrolysis and double decomposition without supposing free atoms to exist
within the liquid.

(2) The electrostatic strain hypothesis of H. von Helmholtz. Here it is assumed 4
that each kind of matter has a specific attraction for electricity some kinds for
positive, other kinds for negative ; that accordingly, work must be done to separate
one atom from its electrical charge, or to remove electricity from an atom of high
specific attraction and give it to another lower in the scale. Further, the chemical
affinity is mainly due to the electrical attraction of oppositely charged atoms, and
that when such atoms combine into a compound molecule, they do not discharge
into each other, but retain their charge. During electrolysis, work is done, not in
tearing the atoms asunder, but in tearing their electrical charges from them.

(3) The ionization hypothesis of R. Clausius. As a trial hypothesis it may be
assumed that the mere presence of the solvent leads to the fission of the molecules
of the electrolyte into sub -molecules, each of which is charged with a definite amount
of positive or negative electricity equivalent to 96,540 coulombs per chemical
equivalent. The solution does not itself appear to be electrically charged, and
hence it is assumed that equal quantities of positive and negative electricity are
developed by the rupture of the molecules of the electrolyte during the process of
solution. Solutions of electrolytes are supposed to normally contain a definite
proportion of the sub-molecules charged with electricity. By a modification of
M. Faraday's definitions the " sub-molecules " are called ions, and consequently :
ions are atoms or groups of atoms which carry a positive or negative charge of
electricity, and they are formed by the dissociation of the electrolyte in the solu-
tion. Each molecule, on dissociation, furnishes two kinds of ions with equal
and opposite charges of electricity. Consonant with M. Faraday's work, it is
further assumed that each monad ion carries a definite charge of electricity (96,540
coulombs) ; each dyad ion, two such charges ; a triad ion, three such charges ;
etc. ; but never a fraction of such a charge. To avoid confusing the phenomenon
of dissociation, in which the products are not charged electrically, with the dissocia-
tion of a molecule into electrically charged ions, the term ionization is reserved
for the latter phenomenon. The ionization of hydrochloric acid is represented in
symbols : HCl^H'-j-Cl' ; and of sodium chloride : NaCl^Na'-f Cl'.

A. W. Williamson's theory of the continuous interchange of the atoms of the
molecules of a compound was suggested in 1850, and it was followed in 1857 by
R. Clausius' suggestion that the molecules of the solute are ionized when dissolved
in the solvent, but R. Clausius appears to have assumed that only an infinitesiuntlh/
small fraction of the total number of dissolved molecules are so ionized. As the ions
are discharged at the electrodes during electrolysis, more molecules are ionized.
The un-ionized molecules keep the electrolyte constantly supplied with a definite
number of ions. The ions conduct the current ; the " undissociated " molecules


are inactive. Further, at any given temperature, there is a constant relation
between the number of un-ioriized molecules, and the number of ions. S. Arrlienius
(1884), more bold or less cautious than R. Clausius, asserted that a considerable,
fraction of the dissolved molecules are ionized, and that the number of ions increases
more and more as the solution becomes more and more dilute. W. Ostwald, J. H.
van't Hoff, W. Nernst, and a large number of other workers have followed the logical
consequences of Arrhenius' hypothesis in a great many directions ; the results, on
the whole, have been satisfactory, and the theory has thus stimulated the study
of the properties of solutions in a remarkable manner. Some hold that the great
cloud of subsidiary hypotheses which is needed to make the ionic theory presentable,
serves also to obscure progress towards a more satisfactory view of the nature of
solution. It is also maintained that the " principle of exhausting hypotheses " has
not been followed, and that the favoured child the ionic hypothesis has grown
into a tyrannical master ; for instance, G. F. Fitzgerald (1896) has said that " the
supposed advantage of the free ion theory is not only illusory but misleading." If
this be a correct diagnosis of the ionic hypothesis we have some consolation in
H. Davy's words : " The destruction of an error hardly ever takes place without
the discovery of truth."

At first sight the ionic hypothesis appears so incredible and so opposed to the
instinct, common sense, or prejudices of the chemist that it has been assailed by
much wholesome criticism particularly by H. E. Armstrong. For instance, it is
asked :

1. In view of the great chemical activity of metallic sodium in contact with water,
is it profitable to postulate the existence of the element sodium in contact with water
without cliemical action ? This objection is said to " rest on a misunderstanding,"
because electrically charged ions of sodium in an aqueous solution of sodium chloride
are very different from neutral atoms of metallic sodium. The ions of sodium
carry large charges of electricity. It is urged that " chemists know practically
nothing about the properties of atoms carrying large charges of electrical energy,"
and also that " the chemical activity of an atom of sodium charged with its 96,540
coulombs of electricity is much less than a neutral atom of sodium." In other
words, the presence of the electrical charge on the sodium ion keeps the ordinary
chemical activities of the atom in abeyance. This means that whenever a chemical
difficulty arises in the application of the ionic hypothesis the assumption is made
that " neutral atoms or atomic groups and ions are different substances," because
the properties of a substance are determined as much by the energy it contains as
by the kind of matter. In this way, the ions have been invested with such imaginary
properties as may be needed to keep the ionic hypothesis consistent with facts.

2. Compounds like mercuric chloride, very prone to thermal dissociation, are not
readily ionized ; while compounds like calcium chloride ivhich resist thermal dis-
sociation are readily ionized. Would not the ionic hypothesis predict the converse
ptienomena ? Mercuric chloride is very volatile and readily dissociates into its
elements by heat ; calcium chloride, on the contrary, does not readily volatilize or
dissociate except at very high temperatures, yet it is said that the latter is readily
ionized in solution while the former remains all but unchanged. Here again it is
said that totally different phenomena are confused, and that the forces which
produce ionization are quite different from those which produce thermal dissociation.

3. Bodies carrying electrical charges of opposite sign are attracted and cling to one
another ; if therefore a mobile solution contains "free and independent " ions carrying
enormous electrical charges of opposite sign, how can the charged ions remain more
than momentarily free ? It is assumed that a certain proportion of the molecules of
the solute are continually breaking down into free (charged) ions, and a certain
proportion of the ions are continually recombining to form ordinary molecules, the
result is, that the ratio between the number of free ions and paired ions (molecules)
remains unchanged. This statement, of course, does not answer the perplexing
question. Attempts have been made to refer the difficulty to the specific insulating


properties the so-called dielectric constant of the solvent. The action of the
solvent has been compared with the function of the glass in a charged Leyden jar.
This agrees with the non-conducting qualities of pure water, but experiments have
shown that the relation between the insulating properties of a solvent and its
ionizing properties is not an adequate and sufficient explanation of the observed
facts. The two phenomena do not always vary concomitantly. A satisfactory
answer to the question, therefore, has not yet been found.

4. If an ionized salt, say, sodium chloride, is present in solution as a mixture of
Na' and Cl' ions, it might be thought possible to separate the two components by diffusion
or by some other mechanical process. When the molecules of certain gases hydrogen,
chlorine, etc. exist free in a liquid, they will escape ; but when, say, sodium chloride
is ionized : NaCl^Na'^-Cr, it is said that the chlorine ions do not escape because
of their electrical charge. S. Arrhenius also says that the great electrostatic attrac-
tion of the oppositely charged ions prevents any marked diffusion. W. Nernst,
however, has shown that the concentration currents produced when, say, a solution
of sodium chloride is carefully covered with a layer of water, leads to the conclusion
that the greater mobility of the chlorine ions charges the upper layer negatively,
and the lower layer positively, so that a current of electricity can be obtained by
placing the two layers in electrical contact. R. C. Tolman (1911) whirled aqueous
solutions of iodides sodium iodide, potassium iodide, hydrogen iodide, etc.
in tubes in a powerful centrifugal machine, and found that the two ends of the
tubes acquired charges of opposite sign. The extreme ends of the tubes acquired
a negative charge presumably because of the accumulation there of the heavier
positively charged iodine ions ; and the opposite ends of the tubes acquired a
positive charge presumably owing to the slight excess of positively charged sodium
ions at that end. There is the possibility that the electrification of the tube was
due to the friction against air.

5. Salts which form solid compounds with two or more different amounts of water
of crystallization have different solubilities in their different forms . Hence it is asked :
Is it not more reasonable to assume that the molecules of the solute exist in solution as
definite hydrates ? The ionic hypothesis answers : Only a definite fractional part
of the salt is ionized, and this part is proportionally less, the more concentrated the
solution. As a rule, in a saturated solution, only a small proportion of the solute is
ionized. A similar observation applies to the existence of liquid crystals. This
does not preclude the possibility that the un-ionized molecules and the ions are
themselves hydrated.

6. When a compound is formed from its elements ivith the loss of energy, the com-
pound cannot be resolved into its elements unless energy be supplied. It is therefore
pertinent to inquire : What is the source of the energy ivhich leads to the fission of the
molecule into ions carrying equal but opposite charges of electricity ? Here, again, it
is necessary to reiterate that the ionic hypothesis refers not to the separation of a
compound into its original constituents, but into charged ions ; and it is interesting
to observe that molecules of sodium chloride, etc., which appear to be very stable
when dry, react with great facility when in solution. A little heat is supposed to
be evolved during the ionization of many (not all) electrolytes, and the process of
ionization is then presumably accompanied by an exothermal reaction which more
than compensates for the energy needed for the fission of the molecule into oppositely
charged ions.

There are also hypotheses which suppose ionization occurs by collision.
G. T. Beilby (1905) considers that the ionization is essentially a mechanical operation,
the result of the kinetic activity of the solute molecules, for in a dilute aqueous solu-
tion of, say, hydrogen chloride, each molecule of the solute is surrounded by and
" at the mercy of " some millions of water molecules, all in a state of intense activity,
and the rude mechanical jostling to which the molecule of hydrogen chloride is
subjected will naturally tend to break it up into simpler portions mechanically more
stable. J. Kendall 5 made a similar suggestion, but B. de Szyskowsky and J. Perrin


regard this hypothesis as untenable the latter says la probabilite de rupture d'une
molecule ne depend pas des chocs qu'elle subit.

W. Nernst and J. J. Thomson have found that the ionizing power of a solvent is
related with its specific inductive capacity or dielectric constant as indicated in the
chapter on " Water." W. Nernst has pointed out that water has a higher specific
inductive capacity than other liquids, and that liquids like methyl alcohol, formic
acid, and others, which, as solvents, give solutions having electrolytic conductivity
also have high specific inductive capacities. From this it is argued that the dis-
sociating power of a solvent, or its power of producing ions, is greater the greater
its specific inductive capacity. C. B. Thwing's numbers show that the dielectric
constants, K, of the hydrocarbons and non-asssociated liquids approximate to
K~2'6D, where D is the specific gravity. Other liquids, particularly those which
contain hydroxyl groups, have higher values than correspond with this rule,
and H. Crompton has shown that if i be the association factor, C. B. Thwing's
data can be represented by K=2'6Dfi, or i=^(^L/2'6D), where the values of i so
obtained run quite parallel with the values obtained by I. Traube, but are a little
higher. R. Abegg also showed that the temperature coefficient of the dielectric
constant is very small for non-associated liquids there is only a slight change between
15 and 80 for toluene and ether, but with other liquids there is a larger change
as the temperature is lowered, probably due to increasing density and increasing
association with ethyl and amyl alcohol, and acetone, the change is quite marked.
H. Crompton therefore argues that it is almost impossible to doubt that association
plays an all-important part in determining the specific inductive capacity of a liquid,
and that if there is any connection between specific inductive capacity and the power
of ionization, it may be looked for rather in the fact that electrolytes are solutions
of approximately non-associated salts in an associated solvent than in there being
any peculiar ionizing power attaching to the solvent. According to P. Dutoit and
E. A. Aston, and P. Walden, the association of the solvent may in turn be referred

Online LibraryJoseph William MellorA comprehensive treatise on inorganic and theoretical chemistry (Volume 1) → online text (page 163 of 177)