Electrolysis : Theory Of Electrolysis

The facts of electrolysis are summarised in the two laws of Faraday. An explanation of the phenomena must include these laws. Since the ions move to the electrodes, it is simplest to assume that they are attracted by the electrodes and are themselves charged, the sign of the charge on an ion being opposite to that of the electrode towards which it moves. Anions are negatively charged atoms or radicals: cations are positively charged atoms or radicals. In the electrolyte two streams of charged ions move in opposite directions to the two electrodes.

Fig: Migration of ions in electrolytic cell

These streams of charged ions constitute the current; electricity is ferried across from one electrode to the other by the charged ions, and this convective current completes that passing through the metallic circuit outside the cell. When a negative anion touches the anode, its charge passes into the latter, which is able to conduct the electricity without simultaneous movement of ions. The positive cation touching the cathode neutralises its charge, and the two uncharged atoms or molecules are liberated at the electrodes They may then react with the water to form secondary products. The strength of the current is uniform throughout the whole circuit, whether the latter is all metallic or composed of metal and electrolytes. Since the current in the electrolyte is composed solely of charged ions, the weight of the latter moving to the electrodes in a given time is proportional to the current strength. This is Faraday's First Law.

Faraday's Second Law is simply explained by the assumption that the quantity of electricity associated with an ion is the same for all ions of the same valency, and is proportional to the valency. A univalent cation such as sodium carries one unit positive charge, a bivalent cation such as copper carries two unit positive charges. A univalent anion, such as chlorine, carries one unit negative charge, equal in magnitude but opposite in sign to the charge on a univalent cation, whilst a bivalent anion such as the sulphuric acid radical, SO₄, carries two unit negative charges, and so on.

The ionic charges carry the matter with which they are associated. When the ions reach the' electrodes, the charges leave them, and the matter is deposited. Since the current is uniform throughput the circuit, the quantities of the ions deposited must all be proportional to the amounts associated with the same quantity of electricity. According to the theory advanced above, these amounts are in the proportion of the chemical equivalents. Thus, the same current deposits amounts of the ions which are proportional to the chemical equivalents. This is Faraday's Second Law of Electrolysis.

Faraday (1833) had remarked that the second law would be explained on the assumption that "the atoms of bodies which are equivalents to each other in their ordinary chemical action have equal quantities of electricity naturally associated with them," but he hastens to state that he does not believe in the existence of atoms.

The ionic charges are large. To liberate 1 gm. of hydrogen, the current which lights an electric lamp (0.5 amp.) would have to pass for nearly fifty-four hours. If charges equal to that associated with 1 mgm. of hydrogen could be imparted to each of two small spheres placed 1 cm. apart, they would repel each other with a force of about 10¹⁴ tons weight. As Faraday remarks, the electric charges concerned in the most violent flash of lighting would barely serve to decompose a single drop of water.

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