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Faraday's Law.-When the same quantity of electricity is passed through different electrolytes, the ratio between the quantities of the liberated products of the electrolysis is the same as that between their chemical equivalents.

Thus, if the two electrolytes, hydrochloric acid and dilute sulphuric acid, be introduced into the same electric circuit, hydrogen and chlorine are evolved in the one case and hydrogen and oxygen in the other. If the gases be all collected in separate measuring vessels, it will be seen (1) that the hydrogen and chlorine evolved from the hydrochloric acid are equal in volume; (2) that the volume of hydrogen collected from the other electrolyte is the same, while that of the oxygen is equal to only one-half this amount. Knowing the relative weights of equal volumes of these three gases to be hydrogen, oxygen, chlorine, as 1, 16, 35.5, we see that they must have been liberated in the proportions by weight of—

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Similarly, if the same quantity of electricity be passed through aqueous solutions of hydrochloric acid (HCl), silver nitrate (AgNO3), copper sulphate (CuSO4), and gold chloride (AuCl3), by the time that I gramme of hydrogen has been liberated from the hydrochloric acid, there will be deposited upon the cathodes of the other electrolytic cells 108 grammes of silver, 31.7 grammes of copper, and 65.6 grammes of gold. These numbers, which are the electrochemical equivalents, are identical with the chemical equivalents of those elements, the chemical equivalent of an element being its atomic weight divided by its valency.

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Regarding the quantity of electricity required to liberate I gramme of hydrogen as the unit, we may say that 16 grammes of oxygen require 2 units of electricity for its liberation, 108 grammes of silver I unit, 63.5 grammes of copper 2 units, and 197 grammes correct. But as it is undesirable to introduce unnecessary variations in the spelling of the same words, and as spelt with a "C" these terms have now become established in our language by use, they will be uniformly so printed in this book. Moreover, to be consistent quite a number of other terms which are always spelt with a "C" would have to be changed, such as catalysis, catalytic, calorimeter, calorie, &c., &c.

of gold 3 units; or, in other words, the number of units of electricity required to liberate a gramme-atom is identical with the number representing the valency of that atom in the particular electrolyte employed.

Some metals, such as copper, mercury, tin, &c., are capable of functioning with different degrees of valency. Thus copper is divalent in copper sulphate and in cupric chloride, but monovalent in cuprous chloride. If, therefore, I unit of electricity be passed through aqueous solutions of each of these copper chlorides, 63.5 in the case of cupric chloride

2

31.7 grammes of copper will

be deposited, while in the cuprous chloride are formed.

63.5

=

63.5 grammes

I

The Ionic Theory.-The modern theory now generally held, to explain the phenomena of electrolysis, is known as the theory of electrolytic dissociation or the ionic theory. The passage of electricity through conductors of the two classes above mentioned, that is, through conductors such as metals, and those which are electrolytes, may be compared with the two ways by which heat is transmitted, namely, by conduction and convection. When a bar of metal is heated at one end, the heat travels along the bar, the metal remaining stationary; but when water is contained in a tube which is heated at its lower end, the heated particles of water travel along the tube, conveying the heat to the other extremity. In a similar manner, when electricity passes through a metallic conductor, the electricity travels through, or along, the metal, which itself does not move; but when it is passed through an electrolyte, it is conveyed or transported through the liquid by the moving ions. One set of ions charged with negative electricity travels towards the anode, while another set conveying positive electricity moves towards the cathode. In the earlier stages of the development of the present theory it was supposed that the electrolyte was only separated into its ions as the electric current was passed into it, that the electricity was the prime cause of the dissociation of the electrolyte, hence the expression electrolytic decomposition, still commonly used. It was believed (Grotthus) that the first effect of the current was to cause the molecules in the solution to take up positions towards each other and the electrodes which may be crudely represented by the top line in the following diagram, where the

cathode

molecules of hydrochloric acid, for example, are arranged with their electro-negative constituents all directed to the anode, and their electro-positive elements towards the cathode-precisely as a number of separate cells in a battery would be arranged. Then that a disruption of the molecules took place in which those nearest to the electrodes parted with their positive and negative ions to their respective electrodes (where they would be disengaged as free hydrogen and chlorine in the case of hydrochloric acid), while an exchange of partners between the other molecules all along the line took place, as represented in the second line, resulting in the formation of fresh molecules of the original compound. These would then immediately assume the position of those in the upper row. This theory, while affording an explanation of many

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of the phenomena connected with electrolysis (such as the fact that the ions are disengaged only at the surface of the electrodes, and not in the intervening space; that the appearance of the liberated ions takes place simultaneously at the two electrodes, however far removed from each other, &c.), was not capable of satisfying all the facts of the case. It was pointed out (Clausius) that if the electric current were the actual cause of the separation of the molecules into their constituent ions, this ought to be made manifest by the fact that the current would have to expend energy in doing the work of effecting such decomposition. But exact experiment shows that this is not the case. It is found that when an electric current passes through an electrolyte, no electric energy is absorbed in causing the dissociation of the molecules of the dissolved substance; but that the current is conducted by electrolytes with the same freedom as it is by metallic conductors. In other words, it has been shown that Ohm's law is equally applicable to electrolytes as it is to metals, namely, that the current is proportional to the electro-motive force for all values of that force. The theory of electrolytic dissociation, first proposed by Arrhenius,

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and now generally accepted by chemists and physicists, is that all solutions which are capable of conducting electricity contain molecules which are already in a state of dissociation. That is to say, the electrolyte (using the term to denote the dissolved substance, as explained on p. 97) consists of molecules which are already dissociated into their constituent ions to a greater or less extent. The simple act of solution in water results in the dissociation of a portion of the molecules into their positive and negative ions. For example, sodium chloride is an electrolyte; when, therefore, this substance is dissolved in water a certain proportion of the molecules immediately undergoes ionic dissociation, so that the solution contains some molecules of sodium chloride, some sodium ions, and some chlorine ions. In such solutions it is the ions alone which take any part in the conduction of the electric current, the undissociated molecules being entirely inoperative. Obviously, therefore, when a substance dissolves in water without undergoing ionic dissociation, the solution will be a non-electrolyte; while if dissociation only takes place to a limited extent the solution will come under the head of the half-electrolytes. Strong acids, bases, and salts, which are good electrolytes, are therefore the substances which undergo dissociation to the greatest extent. For any given solution the extent to which dissociation takes place increases as the solution is diluted until a point is reached at which all the molecules are dissociated into their ions.

At first it might appear contrary to established ideas that in such a case as sodium chloride, for instance, the sodium and chlorine in the free or separated state should be capable of existence side by side in the same liquid—a liquid, moreover, upon which one of these elements, namely, the sodium, is under ordinary circumstances capable of exerting a chemical action. Similarly, that with such a compound as sodium sulphate there should not only be the same element, sodium, existing in contact with water, but also a group of elements, or radical, SO4, which is not known in a state of separate existence. These ions, however, whether elementary like sodium or compound like the group SO4, are all united with and carry with them enormous electrical charges, positive or negative, as the case may be; and it is only so long as they retain their electrical charges that they can retain an independent existence and exhibit their own special properties. When the electrodes from an electric battery are introduced into a solution of sodium chloride, the sodium ions with their positive charges are attracted

to the cathode; they there discharge their loads of electricity, and thereupon become ordinary atoms of sodium, possessing the properties usually associated with that metal. Hence, since ordinary sodium cannot exist in contact with water, the metal immediately upon its liberation at the cathode reacts upon the water with which it is in contact in the manner usual to sodium. Similarly, the chlorine ions with the negative electric charges are endowed with their own characteristic properties, which are retained so long as the atom is united to the electricity. So soon as it loses its charge, which it does when it conveys it to the anode, the chloride ion then becomes a chlorine atom, two of which immediately unite, forming a molecule of the element possessing the ordinary properties of chlorine gas. If, therefore, we use the term radical to embrace single atoms as well as groups of atoms, we may describe an ion as a radical united to an electric charge—a positive ion being one which carries positive electricity, and a negative ion being a radical which is united to a negative charge.

Indeed, instead of regarding this subject as one presenting a new difficulty to the mind, we may even trace an analogy between it and another set of ideas with which we are already quite familiar. We know that when atoms enter into chemical union with each other they lose their own characteristic properties, and that the resulting compound is endowed with new and different properties. When an atom of sodium combines with an atom of chlorine the sodium no longer exhibits the properties of metallic sodium; and similarly, when sodium is combined with a negative electric charge it possesses properties differing from those of metallic sodium. The exact "how" and "why" are equally mysterious in both cases, and in neither case are we able to explain the precise nature of the union for which in both instances we employ the word " combine."

If we take as our unit the amount of electricity which is carried by one atom of hydrogen, then of all monovalent ions we may say that they convey one unit of electricity, for all such ions are united to equal amounts of electricity, whether they be simple or complex radicals. Divalent and trivalent ions respectively are united to two and three units of electricity. In chemical notation, when it is wished to indicate ions as such, it is customary to do so by the use of a dot () or a dash ('), as the case may be, to denote positive or negative charges. Thus Na' signifies a sodium ion, the one dot conveying the information both that it is a cation, and also

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