CHAPTER XII.

THE CHEMICAL PHENOMENA ACCOMPANYING THE PASSAGE OF

THE CURRENT.

§ 1. Introductory. In this chapter we shall consider the chemical phenomena that accompany the passage of an electric current. We shall assume that the student is acquainted with the elements of theoretical chemistry.

Of the other classes of phenomena, the heat effects will be only briefly mentioned here, a fuller treatment being found in Chapters XV. and XVI.; the magnetic effects will be discussed in Chapters XVII. and XVIII., &c.; and the induction phenomena form a large portion of our subject and will occupy our attention in Chapters XXI.-XXIV.

§ 2. Heating Effects; a Brief Account-When a current passes through a conductor it is found that the said conductor is heated. The stronger the current, and the greater the resistance of the wire, the greater is the quantity of heat evolved. The exact law relating to this matter will be given in Chapter XV. A battery of from three to six large bichromate cells will render white-hot, or even melt, fine platinum wire, and will illumine small 'glow' lamps of ten candle-power.

Now, whence does this heat-energy come? Our source of energy is the battery-cell. If in this a certain amount of zinc be dissolved, there will be a certain amount of heat evolved. If the circuit of action be contained within the cell, then all the heat due to the quantity of chemical action will appear in the cell. But if the cell drive a current and this current heat a wire external to the cell, so much the less heat will appear in the cell. The total amount of heat evolved during the solution of a given mass of zinc will be constant; but, by making the external circuit of

some substance that offers a very high resistance to the current, we may cause nearly all the heat, due to the amount of chemical action in the cell, to be evolved outside the cell. We lose chemical-potential-energy, and we gain equivalent heat-energy (see Chapter X. § 4).

$ 3. Chemical Effects; General View. Bodies may be roughly divided into three classes with regard to their behaviour as to allowing a current to pass through them.

(i.) Conductors.-All those solids and liquids that allow a current to pass, while themselves undergoing no change saving a rise in temperature, are called conductors. Such are metals whether solid or molten, carbon, the bodies of animals, &c.

(ii.) Insulators.-Bodies that do not allow any appreciable current to pass are termed insulators. A few examples of insulators are glass, ebonite, paraffin oil, dry vapours, &c.

These two classes merge the one into the other. The reader should refer to Chapter XIV. § 14, for tables of resistances.

(iii.) Electrolytes.- A great many bodies allow a current to pass, but themselves suffer a chemical decomposition that proceeds step by step with the current; not allowing any appreciable current to pass without this chemical decomposition occurring.

Such bodies are termed electrolytes. This class consists almost entirely of compound liquids or solutions of salts, or of molten salts; the word 'salt' being understood in its widest sense. A few examples are aqueous solutions of acids, of metallic salts, or of alkalis; also such liquids as melted potash or soda, &c. The action, as we shall see hereafter, requires that the molecules of the bodies shall move with freedom; hence all electrolytes are liquids or pastes.

Explanation of terms used.-Referring to fig. (ii.), § 4, we see that usually the electrolyte is introduced into a glass vessel, while the current enters by a metal plate A and leaves by another plate B. This cell is called an electrolytic cell; A is called the anode, B is called the kathode. These plates are usually of platinum, this being unacted upon by most liquids. In what follows it is understood that they are of this metal, unless the contrary is stated.

It is found that the electrolyte is invariably split up into two molecular (or atomic) groups, the metallic radicle and non-metallic

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radicle respectively. Thus H2SO, splits into H, and SO, not into H2O and SO,; NH4Cl into NH, and Ci, not into NH, and HCI; CuSO, into Cu and SO, ; and so on.

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These groups are set free at the two electrodes; the metallic groups (as H2, Cu, (NH4)2, &c.) at the kathode; the non-metallic groups (as O, SO4, Cl2, &c.) at the anode.

These groups are, from their 'travelling,' called ions; the metallic groups are called kations or electropositive, from being set free at the kathode or negative plate; the non-metallic groups are called anions or electronegative, from a similar reason.

The process of chemical decomposition through the agency of a current is called electrolysis.

Note.-A slight acquaintance with Greek will enable the reader to discover for himself the derivation of the terms anode, kathode, electrode, ions, &c. Experiments in electrolysis. -Experiments to illustrate electrolysis may be devised almost ad infinitum. We here describe a few typical cases.

(i.) Decomposition of water.--It is not certain that pure water can be electrolysed. Certainly as we approach purity the water becomes almost an

'insulator'; it being remarkable that mere traces of acids or salts in solution have a very great influence in destroying the insulating power and in rendering the water an electrolyte.

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When we wish to decompose water we usually mix it with about one-tenth its volume of strong H.SO. What part the H,SO, plays is not known for certain. Here we will assume that it only renders

the water capable of electrolysis.

The figure represents the usual arrangement, or one form of it. The vessel contains the acidulated water; A and B are the anode and kathode respectively; C and D are glass vessels at first filled with the liquid and inverted over the electrodes in order to collect the gases.

When the current passes, the H, and the O of the HO are set free in chemical equivalents at the kathode and anode respectively. Errors in the volumes collected occur from the greater solubility of oxygen, from part of the oxygen being set free in the form of ozone, and from hydrogen being 'occluded' by the platinum electrode to a greater extent than is oxygen. The first and third errors can be nearly eliminated by allowing the action to proceed for some time before collecting the gases; the second error by heating the tube, or by having electrodes of such area that the current is not too dense.

In several of the following experiments it is very convenient to have a V-shaped tube; the electrodes occupying the two arms of the tube respectively. Or we may have an ordinary cell in which the two electrodes are separated by a porous earthen diaphragm, or two vessels connected by wet cotton wick. Each of these methods enables us to examine at leisure the condition of the liquid about the two electrodes after the action has proceeded for some time; the mixing of these two portions of the liquid being to a greater or less extent prevented.

(ii.) Electrolysis of CuSO,.—If we employ a solution of CuSO,, we find Cu set free at the platinum kathode; while from the platinum anode is set free O, the liquid about this electrode at the same time losing colour and showing the presence of free H2SO.

If we employ copper electrodes we find fresh Cu coating the kathode, while the anode is dissolved with the formation of CuSO..

We shall in § 5 argue that in both cases there is primarily set free Cu at the kathode and SO, at the anode.

(iii.) Electrolysis of Na,SO,.-In this case (using platinum electrodes) we find H, and 2NaHO appearing at the kathode; from the anode is set free O, while the liquid about it shows signs of free H,SO,. We shall in § 5 show that this is equivalent to a setting free of Na, at the kathode, and of SO, at the anode.

If some extract of red cabbage be mixed with the solution, and a drop or so of dilute acid be added (if necessary) until the whole is of a dull purple colour, the alkali and acid will be indicated by green and red colourations about the kathode and anode respectively.

(iv.) Electrolysis of NH, Cl. In this case we get Cl at the anode, and NH, together with H at the kathode (NH,Cl=NH,+Cl). If, however, the kathode be of mercury, this latter swells up and forms what is by some considered to be an amalgam of mercury and the metal NH,.

The solution of NH. Cl must be weak and cold; otherwise we may get the very unstable and dangerous 'chloride of nitrogen' formed.

(v.) Electrolysis of KHO.—Some potassium hydrate is fused and is placed on a piece of platinum foil, which forms the anode. In a cavity on the upper surface of the salt is placed a drop of mercury which is made to form the kathode.

The salt will have absorbed from the air, when it cooled after fusion, enough water to render it an electrolyte ; it will, in fact, be a very stiff 'paste.' After passing a current from a battery (say of four bichromate cells) for a few moments, we drop the globule of mercury into water. It is seen to give off hydrogen, while KHO is found in solution.

This indicates that the KHO (2KHOKO+H,O) was decomposed, K, being set free at the kathode and there forming an amalgam with the mercury, while O was set free at the anode.

(vi.) Electrolysis of Pb. A-We here use a fairly strong solution of plumbic acetate, and a pair of lead electrodes.

The Pb is set free at the kathode, there forming a beautiful 'lead tree.' With a small cell this can be projected on the screen by means of a lantern. A battery of four bichromates causes a very rapid growth of the tree; any battery-cell will give the result in time.

The anode will at the same time be dissolved, giving Pb.A..

§ 4. Grothüss's Hypothesis. Nature of Electrolysis. In a later section we shall see that, as we should have expected, the ions set free at the two electrodes are always chemical equivalents of one another. Thus (we assume the reader to be acquainted with the exact meaning of chemical symbols) for H, and 2 NaHO at the kathode we get Ở and H2SO, at the anode ; we could not get O, since that is equivalent to 2H, or to 4NaHO.

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It may be noticed further that no signs of decomposition can be detected saving at the surfaces of the electrodes.

We give here a view of electrolysis that is consistent with the above facts; it is the view of Grothüss, or Grothüss's hypothesis, slightly modified to suit the modern theory of compound liquids or of salts in solution.

It is believed that, in a compound liquid, not only are the molecules in continual 'slipping' motion, but the atoms themselves are being continually dissociated and recombined. Thus, in water, neighbouring molecules of H2O are continually exchanging partners, the H, of one molecule taking the O of the other, and reciprocally. This cannot be observed, because the interchange is molecular only, and the average constitution of the liquid remains the same; for much the same reason, indeed, we cannot observe the molecular motion called 'Heat.'

It is considered that the presence of the electrodes, connected with the + and poles of the battery respectively, has the effect of directing this interchange; or, when one electrode is made + and the other then the interchange of partners is such that on the whole the H..s in their changes move towards the - electrode, and the O.s towards the + electrode.

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Probably the 'current' is thus, and thus only, conveyed; passing by convection in an electrolyte. In a metallic conductor it passes by conduction.