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dissociates into phosphorus trichloride and chlorine (see page 466), according to the equation

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But if the active mass of either the chlorine or the trichloride be increased by adding more molecules of either one of these substances from some other source, the extent to which dissociation takes place will be proportionally diminished. Hence, by heating the pentachloride in an atmosphere of chlorine and thereby greatly increasing the molecular concentration of this gas, dissociation may be so far prevented that the density of the vapour is found to have practically the normal value for the compound PCI.

CHAPTER XI

ELECTROLYSIS AND ELECTROLYTIC DISSOCIATION

IF a strip of pure zinc and a strip of platinum be together dipped into a vessel containing dilute sulphuric acid, neither metal is affected by the acid, so long as the metals do not touch each other. If the ends of the strips outside the liquid be joined by means of a metal wire, the zinc gradually dissolves in the acid, and bubbles of hydrogen are disengaged from the liquid in contact with the surface of the platinum plate (which itself is otherwise unaffected by the acid), and at the same time an electric current passes through the wire. So long as the chemical action of the sulphuric acid upon the zinc proceeds, so long will the electric current continue to pass; in other words, chemical energy will be transformed into electrical energy. If the wire be severed, the electric current can no longer pass, and the chemical action at once stops.

Such an arrangement constitutes a galvanic or voltaic element or cell, and a series of such cells forms a galvanic battery. The zinc plate, or the end of a wire that may be connected to it, is termed the negative pole of the battery, while the end of a wire attached to the platinum plate is the positive pole. Other arrangements can be employed for generating a galvanic current, but in all cases the electrical energy is derived ultimately from chemical action.

If the two poles of a battery are connected together by placing them both in contact with various different substances, it is seen that in some cases the electric current passes, and in others not. For instance, if the poles are joined by placing them both in contact with a bar of sulphur, no current passes, whereas when connected by a rod of graphite the current freely passes. Substances which behave in this respect like the sulphur are said to be non-conductors of electricity, while those that allow the current to pass are distinguished as conductors. Substances capable of conducting electricity are of two kinds, namely, those which are merely heated,

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and those which undergo a chemical change in consequence. the metals, and a few of the non-metals, belong to the first of these classes; while the second includes a large number of compound substances, which are either in the liquid state or in solution in some solvent. Thus, if the poles of a battery are immersed in pure water, practically no current passes, because this liquid is a non-conductor; but if a quantity of hydrochloric acid (HCl) be dissolved in the water, the solution at once becomes a conductor, and it is seen that gas is disengaged from the liquid upon the surface of each wire. If the solution of hydrochloric acid is moderately strong, it will be found, upon examination, that the gas evolved at the negative pole is hydrogen, while that from the positive pole is chlorine the hydrochloric acid, therefore, is separated into its elements by the passage of an electric current through its aqueous solution. Such a process of decomposition is termed electrolysis; and the conducting liquid is known as an electrolyte.

The poles or terminals that are introduced into the electrolyte are called electrodes, the negative electrode being termed the cathode, and the positive electrode the anode.

Liquids which do not conduct electricity, or conduct only with extreme difficulty, such as water, benzene, aqueous solutions of alcohol or of sugar, are called non-electrolytes; while those which are good conductors, such as aqueous solutions of hydrochloric acid or of sodium chloride, are called electrolytes. Other liquids range themselves between these two extremes with respect to their conductivity, but those which may be said to fall about midway are sometimes spoken of as half-electrolytes. These terms, strictly speaking, apply to the actual liquids or solutions; thus in the above examples it is the aqueous solution of sugar which is the non-electrolyte, and the aqueous solution of sodium chloride which is the electrolyte. For brevity, however, it is usual to apply the terms to the substance in solution, and to understand that an aqueous solution is meant unless another solvent is specially mentioned. Thus, when we say that sugar is a non-electrolyte, and sodium chloride an electrolyte, it is the aqueous solutions of these substances that are referred to.

In the class of electrolytes are included the strong acids, such as nitric, hydrochloric, and sulphuric acids; the strong bases, such as the hydroxides of the alkali metals, and almost all the class of substances known as salts, irrespective of whether the acids and bases they are composed of are electrolytes or half-electrolytes.

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The half-electrolytes are the weak acids, such as acetic, tartaric, and oxalic acids, and the weal bases, as ammonium hydroxide and the hydroxides of divalent metals other than the alkaline earth metals. Non-electrolytes are substances of a neutral character such as sugar, this class including the large majority of organic compounds which do not happen to fall under the category of acids, bases, and salts.

In a great number of instances the electrolytic decomposition is accompanied by certain sendary reactions, caused by the action of the primary products the decomposition upon either the electrolyte or the solvent; for example, when a solution of sodium chloride (NaCl) is electrolysed, the primary products are sodium and chlorine, the latter appearing at the anode and the sodium making its appearance at the cathode. The metal sodium, however, in contact with the water in the neighbourhood of the cathode at once exerts chemical action upon the liquid, with the liberation of its equivalent of hydrogen, according to the equation—

2Na+ 2H2O=2NaHO+H2.

Similarly, in the case of hydrochloric acid, if the solution is sufficiently dilute the final products obtained by subjecting it to electrolysis are not hydrogen and chlorine, but hydrogen and oxygen. As before, the primary products are the same, but under the altered condition the chlorine which is discharged at the anode acts upon the water, combining with the hydrogen, and liberating an equivalent quantity of oxygen: the two actions being expressed by the equations

4HCl=4C1+2H2 4C1+2H,O=4HC1+Og

Again, when a dilute solution of sulphuric acid in water is electrolysed, the acid separates into the two primary products H, and SO4. The hydrogen as before appears at the cathode, while the group or radical SO, passes to the anode, where it undergoes decomposition in contact with the water, reforming sulphuric acid, while oxygen escapes. Thus

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2H2SO, 2H,+2SO4 2SO4+2H2O=02+2H2SO

It will be observed that the final products are oxygen and hydrogen in the proportion of two volumes of hydrogen to one

volume of oxygen; that is, the proportion in which they exist in water. This process is, in fact, the same as that frequently spoken of as the "electrolysis of water" (see page 207).

If instead of a solution of sulphuric acid, a solution of sodium sulphate, Na2SO4, is treated in the same way, this compound separates into the two primary products 2Na and SO; the sodium passing to the cathode and the SO, to the anode. The sodium in contact with the water interacts as explained above, liberating an equivalent quantity of hydrogen; while the SO group, as before, gives rise to the reformation of sulphuric acid and the liberation of oxygen. The final products, therefore, are again hydrogen and oxygen in the same proportions as before; while the sulphuric acid and caustic soda reunite to form sodium sulphate.

In the same way, when an aqueous solution of copper sulphate (CuSO) is submitted to electrolysis, the primary products are copper, Cu, and the group SO4. The copper is liberated at the cathode, and since it exerts no action upon the water, it is deposited as a metallic film upon the electrode.* The group SO4 again passes to the anode, where it undergoes decomposition in the presence of the water, as in the former cases.

The primary products of electrolysis are termed the ions. Those ions that appear at the anode (positive electrode) are those which are negatively electrified, or which convey negative electricity; such as the elements fluorine, chlorine, bromine, iodine, and a number of acidic groups or radicals, such as the SO, group already mentioned. Inasmuch as the negative ions appear at the anode, they are called anions.

Those ions, such as hydrogen and the metals, which travel to the cathode (negative electrode) are those that are positively electified, or in other words, which convey positive electricity: positive ions, therefore, are distinguished as cations.†

This is the essence of the process of electro-plating. The metal to be deposited, whether it be gold, silver, or nickeť, &c., in the form of a suitable salt (usually a double cyanide) in aqueous solution, forms the electrolyte. The object to be plated is made the cathode, that is, it is suspended in the liquid and is connected to the negative electrode of a suitable battery. The anode consists of a strip of the metal to be deposited. Thus in silver plating, a strip of silver is employed, and in this way the acidic radical that is liberated at the anode dissolves the metal, and thereby prevents the weakening of the solution, which would otherwise result from the gradual deposition of silver upon the cathode. + The student will sometimes meet with the words cation and cathode spelt kation and kathode, and in view of their Greek origin this no doubt is rigidly

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