THE SIMPLE VOLTAIC CELL

                                                     THE SIMPLE VOLTAIC CELL

Cell consists of a copper plate and zinc rod placed in dilute sulfuric acid. When terminals of the cell are joined by a wire, then hydrogen is given off at the cu plate and a current is found to flow from Cu to Zn in the external circuit and from Zn to Cu within the cell. Cu plate forms the positive electrode anode and zinc the negative electrode cathode. While the current is flowing, some of the zinc is found to dissolve in the acid.

The production of the e.m.f. can be explained with the help of contact difference of potential. Whenever a metal plate is brought in contact with an electrolyte, there always develops a tendency either for some of the metal to go into the solution in the form of positive ions, thus leaving the plate negatively charged or for some of the positive ions in the solution to be deposited on the plate which consequently becomes positively charged. In the case of a simple voltaic cell, when Zn is immersed in dilute H2SO4, some of the Zn enters the electrolyte in the form of positive ions which further combine with the negative sulphions in the electrolyte to form zinc sulfate. However, this action stops when the contact potential between Zn and electrolytic solution reaches the value of 0.62 V, Zinc being at the lower potential negative with respect to the solution film adjacent to it.

Similarly when Cu plate is placed in contact with the electrolyte, then the positive hydrogen ions in the solution have a tendency to get deposited on it until its potential rises nearly to 0.46 V above that of the solution. Hence, a total potential difference of 0.62-(-0.46)= 1.08 V is developed between the two electrodes.

As more and more of positive zinc ions pass into solution, an increasing number of electrons becomes available at the zinc electrode. These electrons run towards the copper anode via the external wire. There these electrons combine with the positive hydrogen ions, thus converting them into electrically neutral atoms. These atoms then combine in pairs to form molecules of hydrogen which bubble off from the copper plate. The chemical action taking place may be represented by the following equation

Zn+H2SO4+H2