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is evident, therefore, that the latter is the correct formula.

Hydrogen peroxide, in solution in water, is a feeble acid. As an
acid it enters into double decomposition readily, and the peroxides
are really salts in which the negative radical is 2 ". Thus, when
hydrogen peroxide is added to solutions of barium and strontium
hydroxides, the hydrated peroxides appear as crystalline precipitates :

Sr(OH) 2 + H 2 O 2 < 2H 2 + Sr0 2 .

The precipitation involves another equilibrium: Sr0 2 +8H 2 + Sr0 2 ,
8H 2 (solid).

The formation of a beautiful blue substance by the action of
hydrogen peroxide upon dichromic acid is used as a test. The test
is carried out by adding a drop of potassium dichromate to an
acidulated solution of the peroxide. The acid interacts with the
dichromate, giving free dichromic acid:

H 2 S0 4 + K 2 Cr 2 O 7 ^H 2 Cr 2 7 + K 2 S0 4 .

The composition of the blue substance, which is very unstable and
quickly decomposes, is not certainly known, so that no equation for
its formation can be given. The blue substance has the property,
unusual in inorganic compounds, of dissolving much more readily
in ether than in water. It is also much less unstable when removed
from the foreign materials in the aqueous solution. Hence the test
is rendered more delicate by extracting the solution with a small
amount of ether. In the ethereal layer the color of the compound is
more permanent, as well as more distinctly visible on account of the
greater concentration.

Hydrogen peroxide is a much more active oxidizing agent than is
free oxygen. This would be expected from the fact, that it contains
so much more internal energy than the water and oxygen into which
it decomposes (p. 194), that 23,100 cal. are liberated in the decom-
position of one mole. Thus, it liberates iodine from hydrogen iodide,
an action which, in presence of starch emulsion (cf. p. 165), is used
as a test for its presence:

2HI + H 2 2 -> 2H 2 + I 2 .



HYDROGEN PEROXIDE 213

It converts sulphides into sulphates. The white lead (q.v.) used in
paintings is changed by the hydrogen sulphide in the air of cities to
black lead sulphide, Pb 3 (OH) 2 (CO 3 ) 2 + 3H 2 S-3PbS + 4H 2 O +
2C0 2 . This may be oxidized to white lead sulphate by means of
hydrogen peroxide:

PbS + 4H 2 2 - PbS0 4 + 4H 2 0,

and in this way the original tints of the picture may be practically
restored. Organic coloring matters' are changed into colorless sub-
stances by an action similar to that of hypochlorous acid (cf. p. 192).
Hence hydrogen peroxide is used for bleaching silk, feathers, hair,
and ivory, which would be destroyed by this more violent agent.
The products of its decomposition, being water and oxygen only,
are harmless, and, on this account, it is used as a bactericide in
surgery.

Hydrogen peroxide exercises the functions of a reducing agent in
special cases, also. Thus, silver oxide is reduced by it to silver:

Ag 2 + H 2 2 -* 2Ag + H 2 + 2 .

A solution of potassium permanganate, in which the permanganic acid
has been set free by an acid: KMnO 4 +H 2 SO 4 ?= HMn0 4 + KHSO 4 ,
is rapidly reduced. The permanganic acid, with excess of sulphuric
acid, tends to undergo the first of the following changes, provided a
substance, such as hydrogen peroxide, is present which can take
possession of the oxygen that would remain as a balance :

2HMnO 4 4- 2H 2 SO 4 -> 2MnS0 4 + 3H 2 O (+ 50) (1)

f (50) + 5H 2 O 2 -> 5H 2 O + 50 2 (2)

2HMnO 4 + 2H 2 SO 4 + 5H 2 O 2 -* 2MnSO 4 + 8H 2 O + 5O 2

Exercises. 1. What volume of ozone will be taken up by 100
c.c. of water at 12 from a stream of oxygen containing 7.5 per cent
of ozone (p. 103)?

2. At what temperature will a ten per cent solution of hydrogen
peroxide freeze (p. 205) ?

3. Write the thermochemical equations for oxidation of indigo
by ozone (pp. 194, 212), and by hydrogen peroxide.



CHAPTER XIX
IONIZATION

Introductory. As we have seen, acids, bases, and salts, when
dissolved in water, interact with one another by interchanging radicals
(p. 201). We have also learned that the same solutions have abnor-
mal values for their freezing-points and for two other properties.
These facts indicate dissociation into the radicals (p. 207). Now
precisely these solutions have a property which is not shared by any
other solutions, namely, that of being conductors of electricity and
suffering chemical decomposition by the passage of the current. Such
solutions are called, in consequence, electrolytes, and the process is
named electrolysis (r/AeKi-pov, and Xvetv, to loosen, i.e. to decompose,
by means of electricity). Now the natural inference from the fore-
going facts is that the electricity is carried by the liberated radicals.
Our first aim in the present chapter is to show by a study of the
chemical changes taking place in electrolysis that this inference is correct.
We then proceed to discuss the hypothesis of ions, by means of which
these facts are harmonized with the molecular hypothesis. Next,
we apply the hypothesis to the explanation of electrolysis, to the
equilibrium between the ions and the remaining, undissociated molecules,
and to conductivity phenomena as a means of measuring the fraction
ionized. Finally we deduce the relation between extent of ionization
and chemical activity.

Incidentally, the facts to be given provide the means of under-
standing the electrolytic processes, many of them of great importance
in chemical industries, to which frequent reference is made in later
chapters.

Non- Electrolytes. To clear the ground, we should first note the
fact that only solutions (as a rule) possess both of the properties
in question, namely that of conducting and that of being decom-
posed by the current. Some substances, notably the metals and
materials like carbon, are conductors. But they are not changed

214



ELECTROLYSIS 215

chemically by the current. Again, single substances, even when
they are such as, if mixed, yield electrolytes, are not conductors at
ordinary temperatures. Thus hydrogen chloride, whether gaseous
or liquefied, is a nonconductor, and water is a very feeble conductor,
although the solution of the two conducts exceedingly well. Dry
acids, bases, and salts,, except when at a high temperature and fused,
are likewise nonconductors. Furthermore, even amongst solutions,
not all are conductors. Solutions of sugar and other substances of
the same class (p. 205), which have normal freezing-points, are non-
conductors. Only solutions of acids, bases, and salts in certain
specified solvents, of which the commonest is water, are electrolytes
at ordinary temperatures.

Chemical Changes Taking Place in Electrolysis: at the
Electrodes. When the wires from a battery are attached to plati-
num plates immersed in any electrolyte (e.g. Fig. 21, p. 64), we
observe that the products appearing at the two electrodes are always
different. They may be of several kinds physically, and will be
secured for examination variously according to their nature. Thus,
when they are gases which are not too soluble, they may be collected
in inverted tubes filled with Tihe solution. Solids, if insoluble in the
liquid, will either remain attached to the electrode or fall to the
bottom of the vessel as precipitates. Soluble substances on the
other hand will usually not be visible. They may be handled by
interposing a porous partition of some description which will restrain
the diffusion of the dissolved body away from the neighborhood of
the electrode, while not interfering appreciably with the passage
of the current. Surrounding one electrode with a porous battery
jar is a convenient method for effecting this.

Of the various illustrations which we have encountered, the elec-
trolysis of hydrochloric acid (p. 64) happens to have been the only
one which delivered both components of the solute with a minimum
of modification at the electrodes:

Neg. wire, H 2 < H.C1 > C1 2 , Pos. wire.

Hydrogen does not interact with water, and chlorine interacts very
slightly, so that the molecular substances H 2 and C1 2 are promptly
formed from the elements H and Cl which are liberated. The chlo-
rides, bromides, and iodides of those metals which do not interact
with water give equally simple results:

Neg. wire, Cu < Cu.Br 2 > Br 2 , Pos. wire.



216 COLLEGE CHEMISTRY

Thus, in electrolysis, the solute seems to split into its radicals, and the
radical, if it does not interact with water, is set free. A substance
thus set free is called a primary product of the electrolysis. In the
foregoing instances both products are primary.

Usually the chemical change is more complex. Thus, when dilute
sulphuric acid is electrolyzed, hydrogen and oxygen are liberated at
the negative and positive electrodes, respectively. But these prod-
ucts do not account for the whole of the constituents (H 2 SO 4 ). We
therefore proceed to examine the materials in solution round the
electrodes. It is found that, as the action progresses, sul-
phuric acid accumulates round the positive wire, while the liquid in
the neighborhood of the other pole is gradually depleted of this sub-
stance. In view of this fact we easily explain the phenomenon.
Evidently the substance divides into its radicals, H and S0 4 , but
SO 4 , not being a known substance, must interact with the water to
produce sulphuric acid and oxygen: 2S0 4 + 2H 3 2H 2 S0 4 + 2 .
The whole change may therefore be tabulated as follows:

Neg. Wire, H 2 < H 2 .S0 4 > 2 and H 2 S0 4 , Pos. Wire.

Hence the hydrogen is a primary product, but the oxygen and sul-
phuric acid are secondary products. All acids give hydrogen alone at
the negative electrode, whatever may be the product at the positive.
If we electrolyze cupric nitrate solution, we obtain a red deposit of
metallic copper on the negative plate and at the positive end oxygen
and nitric acid are formed. We infer, therefore, that the division
of the original molecule was into Cu and N0 3 , but that the latter
interacted with the water: 4N0 3 + 2H 2 4HN0 3 -f O 2 :

Neg. Wire, Cu. < Cu.(N0 3 ) 2 > 2 and HN0 3 , Pos. Wire.

With a solution of potassium nitrate we find hydrogen and oxygen
appearing at the negative and positive electrodes respectively. Lit-
mus paper, however, shows the presence in the solution of a base
(potassium hydroxide, KOH) at the negative and an acid (nitric
acid) at the positive end. Secondary chemical changes have
occurred at both poles. We infer that the parts of the parent mole-
cules are K and N0 3 . The former, since it resembles sodium (p. 66),
instead of being liberated, gave rise to free hydrogen and potassium
hydroxide :

Neg. Wire, H 2 and KOH < K.NO 3 > O 2 and HN0 3 , Pos. Wire.



ELECTROLYSIS 217

We are confirmed in these conclusions when we employ a pool of
mercury in place of the negative wire. A portion of the potassium
is then found to have dissolved in the mercury and escaped inter-
action with the water.

Having now before us the results of electrolyzing some typical
substances, we bring these results into relation with the facts
described in the last chapter. Acids contain hydrogen which pos-
sesses certain specific properties (p. 201), and by electrolysis they all
divide so as to give up this constituent alone at one electrode. The
evidence that the other radical has different electrical properties
which carry it to the opposite plate is conclusive. Again, salts
undergo double decomposition in which they exchange radicals with
acids, bases, and other salts (p. 201), and we find that it is these very
radicals which are withdrawn from the solution by the influence of the
electricity. Furthermore, the radicals exist free in the solution, being
formed by dissociation of the molecules (p. 207). Hence the function
of the electricity seems simply to consist in sifting apart the two kinds of
free radicals which each solution contains. It only remains for us
to construct an hypothesis (see below) to account for the sifting
action of the current. Before turning to this explanation of the
phenomena, however, there is one question which may be answered
in passing. Since a solution may eventually be cleared of all the
hydrochloric acid, for example, which it contains, we should like to
know how the free radicals in the center of the cell reach the electrodes.

Ionic Migration. To know how the free radicals reach the elec-
trodes, all that is necessary is to take a material, one (or both) of
whose radicals is a colored substance, and watch the movement of
the colored material as it drifts towards the electrode. Most salts
which give colored solutions are suitable. In very dilute cupric
sulphate solution, for example, a freezing-point determination shows
that thedepressionhas practically double the normal value. In other
words, the dissociation into the radicals, CuSO 4 =(Cu) + (S0 4 ),
is almost complete. Now, the blue color of the solution cannot be
due to the few remaining molecules of CuSO 4 , for anhydrous cupric
sulphate is colorless. Nor is it due to the color of the (SO 4 ) radicals,
for dilute potassium sulphate and dilute sulphuric acid are both
colorless. On the other hand, all cupric salts, in dilute solution,
have the same tint. The color is therefore that of the free cupric



218



COLLEGE CHEMISTRY



radical (Cu). In order most clearly to see the motion of the cupric
radical, we place the cupric sulphate solution in the middle of the
space beween the electrodes, and place between it and the latter a
colorless conducting solution. The motion of the blue material
across the boundary may then be easily observed.

The most convenient arrangement is to dissolve the cupric sul-
phate in warm water containing about 5 per cent of agar-agar, and
to fill with this mixture the lower part of a U-tube (Fig. 51). The

setting of the jelly prevents subse-
quent mixing of the cupric sulphate
system of materials with the rest
of the filling of the tube, and the
consequent disappearance of the
boundary. A few grains of char-
coal may be scattered on the sur-
face of the jelly to mark the present
limits of the colored substance,
and a solution of some colorless
electrolyte, such as potassium ni-
trate, is added on each side. To
prevent agitation of the liquid by
the effervescence at the electrodes,
it is well to use agar-agar with the
lower part of the colorless liquid
also. The whole is finally placed in ice and water, to prevent
melting of the jelly by the heat caused by resistance, and the current
is then turned on.

After a time, we observe that the blue cupric radicals ascend above
the mark on the negative and descend away from it on the positive
side. In each case there is no shading off in the tint. The motion
of the whole aggregate of colored radicals occurs in such a way that,
if the contents of the tube were not held in place by the jelly, we
should believe that a gradual motion of the entire blue solution was
being observed. With a current of 110 volts, and a 16-candle power
lamp in series with the cell, the effect becomes apparent in a few
minutes.

Although the (S0 4 ) radicals are invisible, we may safely infer that
they are drifting towards the positive electrode. Indeed, this can
be demonstrated by interposing a shallow layer of jelly containing




FIG. 51.



IONIZATION. 219

some barium salt a little distance above the charcoal layer on the
positive side. When the (SO 4 ) reaches this, barium sulphate begins
to be precipitated and the layer becomes cloudy. In similar ways
the progress of other colorless ions may be rendered visible.

It appears, therefore, that electrolysis is not a local phenomenon,
going on round the electrodes only, but that the whole of the products
of the dissociation of the solute are set in motion. It is on account of
this remarkable property of traveling or migrating towards one or
other of the electrodes that the individual atoms (like Cu), or groups
of atoms (like SO 4 ), have been named ions (Gk. iW, going). The
term was first applied by Faraday to the materials liberated round
the electrodes.

Different ionic substances move with different speeds when pro-
pelled by the same current. The hydrogen radical of acids (H) is
the most speedy, the hydroxyl radical of bases (OH) comes next.
The actual speeds of several ions, in dilute solutions at 18, when
driven by a potential difference of 1 volt between plates 1 cm. apart,
expressed in cm. per hour is: H 10.8, OH 5.6, Cu 1.6, S0 4 1.6,
K 2.05, Cl 2.12.

The Hypothesis of Ions. That the molecules of certain classes
of substances, although seemingly without chemical interaction
with the water in which they are dissolved, should nevertheless be
decomposed by the influence of the water, is strange, but not incon-
ceivable. Heating produces a somewhat similar effect on many sub-
stances. The novel fact, for which an explanation is demanded, is
that the molecules of the products of the dissociation appear to be
attracted by electrically charged plates, which have been lowered
into the solution, while molecules of dissolved sugar, for example,
are not so attracted. Now the only bodies which we find to be
conspicuously attracted by electrically charged objects are bodies
which are already provided with electric charges of their own. Thus
we are led to add to the molecular hypothesis the assumption that
substances which undergo dissociation in solution divide themselves
into a special kind of electrically charged molecules.

Since the solution, as a whole, has itself no charge, equal quantities
of positive and negative electricity must be produced:

HC1 <= H + Cl NaCl ^ Na + Cl NaOH <=> Na + OH.



220 COLLEGE CHEMISTRY

This means that bivalent radicals, on dissociation, will become ions
carrying a double charge and trivalent ions must carry a triple
charge:

CuCl 2 < Cu + 2C1 CuS0 4 <=> Cu + SO 4

K 2 SO 4 <= 2K + S = O 4 FeCl 3 <=Ve + + 3C1

In these equations, the coefficients multiply the charges, as well as
the radicals bearing the charges, and it will be seen that the numbers
of + and charges produced by each dissociation are equal. Hence,
univalent ions all possess equal quantities of electricity, and other ions
bear quantities greater than this in proportion to their valence. This is
an inevitable inference from the electrical neutrality of all solutions.
It is confirmed by actual measurement.

To show that this hypothesis is adequate, we next apply it to the
explanation of the phenomena of electrolysis. After that some
seeming objections will be discussed.

Application to the Explanation of Electrolysis. A

battery is a machine which maintains two points, its poles, or two
wires connected with them, at a constant difference of potential.
One cell of a storage battery, for example, maintains a potential
difference of two volts. When the wires are joined, directly or
indirectly, the poles are immediately discharged, but the cell con-
tinuously reproduces the difference in potential by generating fresh
electricity. Now the effect of immersing two plates, one of which is
kept by the battery at a definite positive potential and the other at
a definite negative potential, into a liquid filled with floating multi-
tudes of minute bodies, already highly charged, may easily be foreseen.
The figure (Fig. 52) will convey some idea of the behavior of the
parts of a system such as we have imagined. The electrodes are
marked - and +. The negatively charged plate attracts all the
positively charged particles in the vessel, and, although these parti-
cles are in continuous and irregular motion, they nevertheless begin,
on the whole, to drift toward the plate in question. On the other
hand, the negatively charged particles are repelled by this plate and
attracted by the positive plate, so that they drift in the opposite
direction. Those which are nearest each plate, on coming in contact
with it, will have their charges of electricity neutralized by the oppo-
site charge on the plate, turning thereby into the ordinary free forms



O
O
O
O







e



O
C?



Ill



IONIZATION 221

of the matter of which they are composed. The continuous removal
of the electrical charges of the plates through contact with ions of
the opposite charge furnishes occasion for recharging of the plate
from the battery, and thus gives rise to a continuous current in each
wire. Again, the continuous drifting of positively and negatively
charged particles in opposite

directions through the liquid, cathode + Anode

constitutes what, in the view 4 cation=A 4.

< 11 i f / "\ anion^NOa

of all external means of fo
observation, appears to be
an electrical current in the
liquid also. A magnetized
needle, for example, which is
deflected when brought near
to one of the wires of the
battery, is influenced in the FIG. 52.

same way by being brought

over the liquid between the electrodes. The illusion, so to speak, of
an electric current is complete, although in reality it is a connection of
electricity that is taking place. Furthermore, the quantity of elec-
tricity being transported across any section of the whole system is
the same as that across any other, whether this section be taken
through one of the wires, through the electrolyte, or even through
the battery at any point. As fast as the ions are thus annihilated as
such, the undissociated molecules (mingled with the ions, but not
shown in the figure) dissociate and produce fresh ones, as in all
chemical equilibria. Eventually, by continuing the process long
enough, if the substances set free are actually deposited and do not
go into solution again in any form, the liquid can be entirely deprived
of the whole of the solute which it contains.

The analogy to the transportation of a fluid like water is notice-
able, although not complete. Water may be transported in three
ways. It may flow through a pipe, it may pass by pouring freely
from one container to another, and it may be carried in vessels.
Thus a stream of water, essentially continuous, might be arranged,
in which part of the passage took place through the pipes, part by
pouring from the pipes into buckets, and part by the carrying of
those buckets between the ends of the pipes. The quantity of water
passing a given point per minute in this system would be the same



222 COLLEGE CHEMISTRY

at every part, although the actual method by which the water was
transported past the various points might be different. In such a
disjointed circuit we suppose the electricity to move when carried
from a battery through an electrolytic cell. It flows in the wire,
passes by discharge between the pole and the ion, and is transported
upon the ions in the liquid. The parallel is imperfect, however,
because we have used the conception of two electric fluids and because
the ions are already charged in the solution, and before any connection
with the battery is made. They do not, so to speak, transport the
electricity of the battery, but their own.

Difficulties Presented by this Hypothesis. The question
was raised (p. 207), as to how we can imagine separate atoms of
sodium to exist in water without acting upon it, as the metal sodium
usually does. But the ions of sodium in sodium chloride solution
are not metallic sodium. They bear large charges of electricity.
They possess an entirely different, and in fact, by measurement,
much smaller amount of chemical energy than free sodium. And,
as we have seen, the properties of a substance are determined as
much by the energy it contains as by the kind of matter. Metallic
sodium and ionic sodium are, simply, different substances.

We think of hydrogen chloride and common salt as exceedingly
stable substances, and are averse to believing that precisely these
compounds should be highly dissociated by mere solution in water.
But it must be remembered that in solution they undergo chemical
change very easily, and it is only in the dry form that they show
unusual stability.

Again, why do not the ions combine, in response to the attractions
of their charges? The answer is that they do combine, but the rate
at which combination takes place is no greater than that at which
the molecules decompose, so that on the whole the proportion of
ions to molecules remains unchanged.

Finally, it might appear that the assumption that bodies could
retain high charges in the midst of water is contrary to all experience.
It must be remembered, however, that the molecular, pure water,
which separates the ions from one another, is a perfect nonconductor.
The moisture which covers electrical apparatus and causes leakage



Online LibraryAlexander SmithGeneral chemistry for colleges → online text (page 20 of 47)