4.5 Galvanic Cells (Daniel Cells)

4.5 Galvanic Cells (Daniel Cells)

The cell which involves a spontaneous redox reaction to generate electricity is called a galvanic or voltaic cell. The name Voltaic is given to this cell because Alessandro Volta discovered the first such cell. The English chemist Fredrick Daniel constructed the first voltaic cell using zinc (Zn) and copper (Cu) electrodes. Therefore this cell is named Daniel Cell. A galvanic or Daniel cell is shown in Figure 4.7.

A galvanic cell consists of the following parts:

(1) A zinc bar dipped into a 1M ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{O}}_4$$\mathrm{ZnSO}_{4}$ solution.

(2) A copper bar dipped into a 1M ${\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}}_4$$\mathrm{CuSO}_{4}$ solution.

(3) A salt bridge is an inverted U tube containing an

inert electrolyte such as KCl. Its ions do not react with the electrodes or the ions in the solution. It makes the electrical contact between the solutions through which ions can move.

(4) A voltmeter to measure current.

Each compartment of the cell is called a half- cell. Thus a Daniel cell consists of two half- cells joined in series. When the circuit is complete, electrons begin to flow from the Zn rod through the external wires to the Cu rod. Thus Zn half- cell acts as an anode and the Cu half- cell as a cathode. Note that a half cell consists of a metal rod dipped in the solution of its salt.

Concept Assessment Exercise 4.2

Sketch a voltaic cell labeling the cathode, the anode and the direction of flow of the electrons. Use the following chemicals:

Silver, Zinc, Silver Nitrate and zinc sulphate (ZnSO4).

(Hint: Zn is more active than Ag)

4.5.1 Reactions in a Daniel Cell

In Daniel's cell, electrons flow from the Zn rod, through the external wire to the Cu rod. This is because Zn has more tendency to undergo oxidation than Cu. Zn atoms from the rod go into the solution as ${\mathrm{Z}\mathrm{n}}^{2+}$$\mathrm{Zn}^{2 + }$ ions leaving electrons on the rod. These electrons flow in the external circuit. Thus oxidation half reaction occurs at the anode compartment. ${\mathrm{C}\mathrm{u}}^{2+}$$\mathrm{Cu}^{2 + }$ ions in copper sulphate solution capture electrons from the Cu electrode and are reduced. Reduction half reduction occurs at the cathode compartment. Such oxidation and reduction reactions are called half- cell reactions.

At Anode (Oxidation half reaction):

At Cathode (Reduction half- reaction):

The tendency of a metal to become oxidized or reduced, when placed in its salt solution is called electrode potential $({\mathrm{E}}^{\circ })$$(\mathrm{E}^{\circ})$ . Electrode potential is the measure of the potential of a reaction that can occur at the electrode. It is measured in volts. The electrode potential of an electrode is determined experimentally by using a standard hydrogen electrode. The International Union of Pure and Applied Chemistry (IUPAC) has recommended the representation of electrode potential as reduction potentials. A table showing the arrangement of metals based on increasing reduction potentials is called an electrochemical series.

A metal higher in the series has a lower value of reduction potential than the metal below it in the series. So, it has a higher tendency to lose electrons and is more active than the metals below it. In a voltaic cell electrode with the lower value of reduction potential acts as anode and the electrode having a higher reduction potential act as a cathode.

Table 1: Electrochemical Series