269. Poles or Electrodes. If the wire were cut, the electricity coming from the + plate would be stopped at the end of the wire marked +, [Fig. 84], after passing through the acid and up Cu. This end of the wire is called the + pole or electrode (positive). The end of the wire joined to Zn is called the - pole or electrode; that is, the + electrode is the end of the wire attached to the - plate. The tops of Cu and Zn may be considered electrodes. The top of Cu is the + pole of the cell, while Cu is the - plate.
270. Chemical Action in the Simple Galvanic Cell. The chemical formula of sulphuric acid is H2SO4 (read H, 2, S, O, 4). This means that it is a compound of hydrogen (H), sulphur (S), and oxygen (O). The H2SO4 stands for molecule of acid, and the small figures show that the molecule is made up of 2 atoms of hydrogen (H2), one of sulphur (S), and 4 of oxygen (O4). The atoms are held together by chemical attraction or affinity.
There is a stronger chemical affinity between zinc (Zn) and SO4 than between H2 and SO4; so, as soon as the Zn gets a chance, as it does in the cell, it drives out the H2, and it takes its place in the molecule. This chemical reaction may be shown by the following chemical equation:
Zn + H2SO4 = ZnSO4 + H2.
Zinc and sulphuric acid produce zinc sulphate and hydrogen.
The zinc sulphate produced weakens the effect of the acid; in fact, the acid has to be renewed occasionally, as it is changed to the sulphate which remains in solution.
271. Action in Cell Using Impure Zinc. The above action takes place in the cell when impure zinc is used, even when no current passes, heat being produced by the reaction instead of useful electricity. (See Local Currents.)
Action in Cell Using Pure Zinc. When pure zinc (or impure zinc that has been properly amalgamated) is used in the cell, it dissolves, or is eaten away, only when the current passes. It should be noted that the bubbles of hydrogen do not even then appear at the zinc; they are not seen throughout the body of the liquid. There seems to be an unseen transfer of hydrogen from the zinc, through the liquid, to the copper (or other - plate used), and it appears there in the shape of bubbles. The larger the quantity of pure zinc dissolved, the stronger the current, and the larger the amount of hydrogen produced.