Electrolysis : Dualistic Theory Of Berzelius

Lavoisier showed that non-metals (except hydrogen), when burnt in oxygen, yield acidic oxides which produce acids with water. He regarded oxygen as the principle of acidity (Greek oxus, sour). Davy found that sodium and potassium burnt in oxygen to form basic oxides, which gave alkalies with water; hence oxygen is also a constituent of bases. When baryta, or barium oxide, a basic oxide, is mixed with the acidic sulphur trioxide, both solids, the mass becomes red-hot and the neutral salt barium sulphate is formed: BaO + SO₃ = BaSO₄. Lavoisier considered salts as compounds of acidic and basic oxides, e.g., BaO,SO₃, and this idea of two parts contained in a salt was amplified by Berzelius (1811) into the dualistic system.

Berzelius found that solutions of the salts of the alkalies, when decomposed by an electric current, liberate alkali at the negative pole and acid at the positive pole, and he considered that the alkali and acid possessed positive and negative charges, respectively, and were drawn to the poles by the attraction of unlike charges.

Expt. 1. - Pour a solution of sodium sulphate, coloured purple with neutral litmus, into a U-tube with electrodes,

Fig: U-tube with, electrodes

and connect with a battery. The liquid around the positive pole becomes red, showing that an acid (sulphuric acid) is set free, whilst that surrounding the negative pole becomes blue, from liberation of alkali (caustic soda). Oxygen and hydrogen are liberated at these poles, respectively.

According to the theory of electrochemical dualism, salts are binary compounds of two oxides, the acid and the base, which are themselves binary compounds of elements with oxygen :

Sulphate of soda Na₂⁺O (Soda Na⁺ + O^-), S^-O₃ (Sulphuric acid S⁺ + O₃^-).

Elements giving basic oxides were called electropositive elements, those giving acidic oxides were called electronegative elements. Oxygen was assumed to be always electronegative; it was "the pole around which the whole chemical system revolved."

This dualistic system in its original form was soon shown to be untenable by three circumstances: (1) the recognition of the elementary nature of chlorine, which, since it forms salts, had previously to be regarded as an acidic oxide of an unknown element; (2) the discovery of the true character of electrolysis, which accounted for the simultaneous production of hydrogen and oxygen in the decomposition of salts; (3) the investigation of substitution reactions in organic chemistry - thus, an electronegative atom of chlorine can replace an electropositive atom of hydrogen without altering very much the chemical nature of the compound:

C₂H₄O₂(Acetic acid) + Cl₂ = C₂H₃ClCO₂(Chloroacetic acid) + HCl.

Many complicated equations involving oxidation and reduction are, however, most simply written down by making use of the obsolete dualistic notation, and the latter is still of service in this way.

For example, the oxidation of ferrous salts by acidified permanganate or dichromate solutions may be represented as an oxidation of 2FeO to Fe₂O₃, and a reduction of Mn₂O₇ to 2MnO or of 2CrO₃ to Cr₂O₃. The dualistic notation is also frequently used in representing the composition of minerals, e.g., felspar, K₂O,Al₂O₃,6SiO₂.

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