Imágenes de páginas
PDF
EPUB
[merged small][merged small][merged small][ocr errors][merged small][merged small]

K

= Reaction (or ionisation) constant (see p. 76).
L = Latent heat (see p. 178).

Q = Heat of reaction (see pp. 154 and 179).
R = Gas constant (8.316 joules per mol).

T = Absolute temperature.

Ил Ис= Mobility of anion and cation (see p. 43).

W = Work (see p. 179).

A = Equivalent conductivity (see p. 58).

dE

dᎢ

[ocr errors]
[blocks in formation]

i = van't Hoff's factor (see p. 70).

Ionic conductivity of anion and cation (see p. 62).

[merged small][merged small][ocr errors][merged small]

ELECTRO-CHEMISTRY

CHAPTER I

MECHANISM OF CONDUCTION IN ELECTROLYTES

§ I. FARADAY'S LAWS: MEASUREMENT OF
QUANTITY OF ELECTRICITY

THERE is a group of substances which, when an electric current is passed through them, suffer chemical decomposition. These substances are called electrolytes; the process of decomposition electrolysis; and reactions occurring in electrolysis may be described as electro-chemical. It is the study of such electro-chemical reactions that forms the subject of the present

book.

As a typical process of electrolysis we may take the decomposition of dilute sulphuric acid between platinum plates. Imagine, then, this arrangement of

apparatus (Fig. 1)-Place some dilute + sulphuric acid in a beaker; insert into

it two plates of platinum, so as to be partially immersed, taking care that they do not touch each other; connect the upper parts of the plates by wires to a source of electric current (battery, dynamo, thermopile, etc.). Such an arrangement is known as an electrolytic cell. The platinum plates

FIG. I.

serve to convey the current into and out of the liquid, and are called electrodes; that by which the current is led in, the anode,

T. P. C.

B

that by which it is led out the cathode. The liquid is the electrolyte, and the reaction that occurs when a current is passed through is the decomposition of water into its elements, oxygen and hydrogen.

Now, the leading peculiarity of this, as of all electro-chemical as distinguished from ordinary chemical reactions, lies in the appearance of the products of reaction at the electrodes only; not, as usual, throughout the mass of the reacting material. In the case chosen as example, all the oxygen appears at the surface of the anode, all the hydrogen at that of the cathode. It is therefore convenient to consider the amount of material evolved, or in the case of a solid deposited, at either electrode, and in order to arrive at the true nature of electrolysis, it is first necessary to find on what this amount depends. This was accomplished by Faraday, and the laws in which he formulated his observations are commonly known by his name. They

are

(i.) The amount of any substance deposited is proportional to the quantity of electricity which flows through the electrolyte. (ii.) The amounts of different substances deposited by the same quantity of electricity are proportional to their chemical equivalent weights.

What is implied by the first of these is, essentially, that the rate at which the electricity flows is of no consequence, provided the total quantity be the same. If we follow the usual exposition of electrical science, and regard the current-defined by means of its magnetic action —as fundamental, we may say that the total quantity of electricity is measured by the product of the current into the time that it flows. Current is measured in amperes, and the meaning of Faraday's first law is illustrated by saying that five amperes will in two minutes effect exactly the same amount of electro-chemical reaction as ten amperes in one minute.

It is consequently more natural for our purpose to look upon quantity of electricity as fundamental; the unit in which this is measured is called the coulomb, the connection between the two being that (a) a coulomb is the quantity of electricity conveyed by a current of one ampere in a second, or (b) an

ampere is a flow of electricity at the rate of one coulomb per second.

The most exact measurements on the mass of substance deposited by the current have been made on silver, and it appears that one coulomb deposits o'0011175 grm. of that metal. This quantity is known as the electro-chemical equivalent of silver, and by means of Faraday's second law we may calculate from it the electro-chemical equivalent of any other substance, eg. that of oxygen. The atomic weight of silver is 107'93, and its equivalent weight the same; the atomic weight of oxygen is 16, but, being a divalent substance, its equivalent weight is 8. Hence the weights of silver and of oxygen liberated by the same quantity of electricity are in the ratio 107.93 8, and the electro-chemical equivalent of oxygen is 107.9 X 0.0011175 0.00008283 grm. per coulomb. = other numbers, see table, p. 255.

8

For

Faraday's two laws may be conveniently summed up in one statement. In accordance with the second law a gramequivalent (the chemical equivalent weight taken in grams) of any substance must require the same quantity of electricity to deposit it. This quantity may be calculated from the data for silver just given; to deposit one gram-equivalent of silver will require 96580 coulombs. (As the fourth significant figure has not been settled with certainty we shall adopt the approximate value 96600.) Therefore

107.93 0.0011175

=

96600 coulombs are required for the deposition of one gram equivalent of any substance.

This fundamental quantity of electricity, which occurs constantly in all writings on electro-chemistry, is called by the Germans a "faraday,"1 a term which we in England may very well adopt.

Faraday's laws have been found to hold exactly in all cases that have been satisfactorily measured-that is to say, the passage of one faraday through an electrolyte is always accompanied by the appearance at the anode of one gram-equivalent of new material, and at the cathode of one gram-equivalent; To be carefully distinguished from the unit of electrostatic capacity known as a "farad."

« AnteriorContinuar »