Ionic theory of electrolysis

Ionic theory of electrolysis

The use which Sir Humphrey Davy had made of electrolysis as a new method of chemical analysis attracted a great deal of attention in the scientific world, and a number of different ideas were put forward to explain it. The most important suggestion was made by the German physicist Rudolf Clausius, who considered that the current was conveyed through an electrolyte by charged particles which. are now called ions. This theory was later developed by the Swedish scientist Servant Arrhenius.

Ions have already been mentioned on page 375, where it was explained that an ion is an atom or a group of atoms possessing either a positive or a negative electric charge. We shall now explain what is believed to occur when an electrolyte dissolves in
water. Take copper sulfate as an example. This is a substance consisting of equal numbers of copper ions, Cue+, and sulfate ions, S04 z-, arranged in a definite pattern so that the whole is electrically neutral. A copper ion is a copper atom which has lost two electrons, and hence has a net positive charge. A sulfate ion is an S group which has acquired the two electrons lost by the copper atom and is therefore negatively charged. When copper sulfate is dissolved in water the ions dissociate or separate and are able to move freely in the electrolytic solution; in fact, the word ion means “wanderer”.

Other substances ionize in a similar manner. Sulfuric acid, HZ, dissociates into two hydrogen ions, H + (H minus one electron), and one S04 z- ion (S04 plus two electrons). Silver nitrate, Agni, consists of positive silver ions, Ag “, and negative nitrate ions, N -, and these separate when in solution. Water Adonises only very slightly into hydrogen ions, H +, and hydroxide ions, OH -. Generally speaking, all acids, bases and salts are ionized in solution, but not organic compounds such as sugar, alcohol, and naphthalene. When two electrodes are dipped into an electrolyte and connected to a battery positive ions (cations) are attracted towards the negative cathode and negative ions (anions) towards the positive anode. This migration of ions in both directions is the process by which an electric current flows through the electrolyte. In the following sections we shall describe the action which occurs at the electrodes when a current is passed through various electrolytes.

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