Electrolysis : Electrolytic Dissociation

The picture of the mechanism of electrolytic conduction employed above suggests that ions move independently through an electrolyte. They behave as if they were free, and each ion responds to the attraction of the electrodes as if the other ions were not present. If the current is switched off, no visible change occurs in the solution, so that we may assume that the ions "still remain in the solution free and independent of each other.

Clausius (1857) assumed that in the solution of an electrolyte a few molecules of the salt are broken up into ions, the processes of decomposition and recombination going on continually, and the free ions present at any instant are transported as the current. Williamson (1851) had previously assumed an exchange of atoms between different molecules of the electrolyte, and thought that during the exchange the atoms or radicals existed transitorily in the free state.

It was Arrhenius, in 1887, who first made the bold assumption that nearly all the molecules of the electrolyte may be dissociated into free ions. According to his theory of electrolytic dissociation, or of ionisation, an electrolyte (salt, acid, or base), when dissolved in water or certain other solvents which yield conducting solutions (such as ethyl and methyl alcohols, pyridine, anhydrous hydrocyanic acid, or formamide), undergoes change in such a way that from the electrically neutral molecule two or more charged ions are produced. The sum of the positive and negative charges on the ions must always be zero, since the solution as a whole is uncharged.

The current in the solution is due solely to the free ions; the un-dissociated salt molecules do not move to the electrodes. When the ions reach the electrodes their charges are neutralised, and the uncharged atoms or molecules are deposited. Sodium chloride, when dissolved in water, is largely ionised into the sodium ion and the chloride ion: NaCl = Na˙+Cl'. This takes place whether the solution is electrolysed or not. In electrolysis, the negative chloride ions are attracted to the positive anode, and on reaching it give up their electronic charges, becoming chlorine atoms: Cl' = Cl + e. These cannot exist as such, but combine in pairs to form chlorine molecules, which escape as chlorine gas. The positive sodium ions, on reaching the cathode, take from it the negative electrons which have passed round the metallic wire circuit from the chloride ions discharged at the anode, and so become neutral sodium atoms: Na˙ + e = Na. These may dissolve in mercury, if the cathode is metallic mercury, or react with water, forming caustic soda and hydrogen, if the electrode is of platinum.

The atoms of the substances, at the moment of liberation at the electrodes, may be very reactive. Hydrogen liberated by the electrolysis of an acid can bring about the reduction of a ferric salt added to the solution, in the same way as nascent hydrogen.

The electrolytic dissociation, or ionisation, of a dissolved electrolyte is different from the thermal dissociation of a gas. Ammonium chloride on heating dissociates into ammonia and hydrochloric acid: NH₄Cl = NH₃ + HCl, but in solution it is electrolytically dissociated into the ammonium and chloride ions: NH₄Cl = NH₄˙ + Cl'.

The reader will have no difficulty in representing the reactions at the electrodes during the electrolysis of salts by means of the ionic theory. The electrolysis of copper sulphate may be taken as an example:

Protein crystallography
Chemistry guide
Electrolysis
-E-
<<<	>>>