Chapter: Electrolysis

Electrolysis, Summary

Faraday's Laws of Electrolysis: (1) The weight of an ion deposited in a Riven time is proportional to the strength of the current; (2) 96,500 coulombs liberate i gm. equiv. of any ion. This quantity of electricity is called a faraday, denoted by F.

Theory of Electrolytic Dissociation: Salts in solution are dissociated into electrically-charged atoms or radicals, called ions. These ions carry the current in electrolysis. The charge on an ion is either positive (cation), or negative (anion), and is always equal to the fundamental charge; ε, multiplied by the valency of the ion. The unit charge, ε is identical with the charge of the atom of free negative electricity, or the electron. Its value is 1.59 x 10^-10 cmb.

The conductivity, k, of an electrolyte is the current in amperes which passes through the solution contained in a cubical cell, with sides 1 cm. long, when the opposite sides, forming electrodes, are at a potential difference of 1 volt.

The equivalent conductivity of a solution is the conductivity divided by the concentration, c, in gm. equiv. per cm³: Λ = k/c. It increases with dilution. On the assumption that the ionic mobilities are independent of dilution, this result is explained as due to partial ionisation; on dilution the ionisation increases. The degree of ionisation is measured by the ratio of the equiv. conduct, at a given concentration to the equiv. conduct, at infinite dilution, corresponding with complete ionisation, α = Λ_v/Λ_∞ In the case of strong electrolytes (largely ionised) it is now assumed that ionisation is complete at all except fairly high concentrations, and the change of equivalent conductivity on dilution is explained as due to the retarding influence exerted by an atmosphere of ions around the central ion, containing an excess of ions of opposite charge, this influence on a moving ion increasing with the concentration c, according to the equation Λ = Λ_∞-a√c (Debye and Hückel).

Kohlrausch's law states that the value of Λ_∞ is additively composed of the mobilities of the cation and anion: Λ_∞ = l_c + l_a. The ratio of the current carried by an ion to the total current is called the transport number: e.g., n = l_a/(l_a + l_c). It may be measured by the changes of concentration of electrolyte occurring around an electrode, due to the unequal speeds of the two ions. The absolute speeds of the ions, in cm. per sec. per volt per cm. are given by u = l_cF, v = l_aF; they may be measured directly with coloured ions, and are small, of the order of 0.0005 cm./sec. (except for the H˙ ion, 0.00326; and OH' ion, 0.0018).

Neutralisation of a strong acid by a strong base in aqueous solution is the union of the hydrogen ion of the acid with the hydroxide ion of the base to form undissociated water. The ionisation of water is very small. In general, a base is a substance which unites with protons: e.g. NH₃ + H˙ = NH₄˙; OH' + H˙ = H₂O.

Electrolysis, Articles

Protein crystallography
Chemistry guide
<<<	>>>