The device in which chemical energy is converted into electrical energy is called galvanic cell or electrochemical cell or voltaic cell. In a galvanic cell, a redox reaction is carried out in an indirect manner and the decrease in free energy during the chemical process appears as electrical energy. An indirect redox reaction is such that reduction and oxidation processes are carried out in separate vessels. In order to understand this phenomenon, let us consider the redox reaction between Zn and CuSO4 solution.
Zn(s) + Cu2+(aq) à Zn2+(aq) + Cu(s)
The overall reaction is split up into two half reactions,
Zn (s) à Zn2+ (aq) and Cu2+ (aq) + 2e - à Cu(s)
Half Reactions and Redox Couples
The half reactions are used to form redox couples as described below.
In its simple form, a zinc strip is dipped in the ZnSO4 solution and a copper strip is dipped in the CuSO4 solution taken in separate beakers. An arrangement involving contact between oxidised and reduced form of the substance such as Zn / Zn2+, or Cu/Cu2+ at the interface is called redox couple. The two metallic strips which act as electrodes are connected by the conducting wires through a voltmeter. The two solutions are joined by an inverted U-tube known as salt bridge. The U-tube is filled with the solution of some electrolyte such as KCl, KNO3 or NH4Cl to which gelatin or agar-agar has been added to convert it into semi-solid paste.
A schematic diagram of this cell has been shown in Fig. 33.1.
The deflection in voltmeter indicates that there is a potential difference between the two electrodes. It has been found that the conventional current flows through the outer circuit from copper to zinc strip. It implies that the electrons flow occurs from zinc to copper strip.
Let us now understand the working of the cell.
(i) Zinc undergoes oxidation to form zinc ions
Zn(s) à Zn2+(aq) + 2e – (oxidation)
(ii) The electrons liberated during oxidation are pushed through the connecting wires to copper strip.
(iii) Copper ions move towards copper strip, pick up the electrons, and get reduced to copper atoms which are deposited at the copper strip.
Cu2+(aq) + 2e - à Cu(s) (Reduction)
The overall cell reaction is combination of oxidation half and reduction half reactions.
Zn(s) + Cu2+ (aq) à Cu(s) + Zn2+ (aq)
The electrode at which oxidation occurs is anode and that at which reduction occurs is cathode. In the above cell, zinc strip is anode and the copper strip is cathode. Due to the oxidation process occurring at the anode it becomes a source of electrons and acquires a negative charge in the cell. Similarly, due to reduction process occurring at the cathode it acquires positive charge and becomes a receiver of the electrons. Thus, in the electrochemical cell, anode electrode acts as negative terminal and cathode electrode acts as positive terminal.
Salt Bridge and functions
Salt Bridge is a U-shaped tube containing a semi-solid paste of some inert electrolyte like KCl, KNO3, NH4Cl, etc., in agar-agar or gelatine. An inert electrolyte is one which:
(a) does not react chemically with the solution in either of the compartment.
(b) does not interfere with the net cell reaction. Function of Salt Bridge. In the electrochemical cell a salt bridge serves two very important functions :
(i) It allows the flow of current by completing the circuit.
(ii) It maintains electrical neutrality.
The transference of electrons from anode to cathode leads to a net positive charge around the anode due to increase in the concentration of cations and a net negative charge around the cathode due to excess of anions in solution. The positive charge around the anode will prevent electrons to flow out from it and the ·negative charge around the cathode will prevent the inflow of electrons at it. The reaction would thus, stop and no current will flow. The salt bridge comes to aid and restores the electroneutrality of the solutions in the two compartments. It contains a concentrated solution of an inert electrolyte the ions of which are not involved in electrochemical reactions. The anions of the electrolyte in the salt bridge migrate to the anode compartment and cations to the cathode compartment. Thus, the salt bridge prevents the accumulation of charges and maintains the flow of current. In the electrochemical cell, the salt bridge can be replaced by the porous partition which allows the migration of ions but does not allow mixing of the two solutions.
REPRESENTATION OF REDOX COUPLE AND GALVANIC CELL
Galvanic cell is a combination of two redox couples, namely; oxidation couple or oxidation half cell and reduction couple or reduction half cell. If M represents the symbol of the element and Mn+ represents its cation (i.e., its oxidised state) in solution, then
Oxidation half cell is represented as M / MD n+(c)
Reduction half cell is represented as MD n+(c)/M.
In both the notations c refers to the molar concentration of the ions in solution. Conventionally, a cell is represented by writing the cathode on the right hand side and anode on the left hand side. The two vertical lines are put between the two half cells which indicate salt bridge. Sometime the formula of the electrolyte used in the salt bridge is also written below the vertical lines.
For example, zinc-copper sulphate cell is represented as follows:
VARIOUS TYPES OF REDOX COUPLES OR ELECTRODES
Some important types of electrodes which are frequently used in electrochemical cells are described as follows:
1. Metal-metal Ion Electrode. Such type of electrodes include a metal rod dipped in the solution of its own ions. Some examples are Zn/Zn2+ Ag/Ag+, .Cu/Cu2+ etc.
2. Amalgam Electrodes. These are similar to metal-metal ion electrodes, but here the metal is used in the form of its amalgam with Hg. Amalgam is formed to modify the activity of metal. (Zn-Hg)/Zn2+ is one of the common example of this type.
3. Gas Electrodes. Such electrodes involve inert metal such as platinum dipped in the solution containing ions of the gaseous element. The arrangement is made in such a manner that gas and its ions are brought in contact at the surface of the inert metal. Some examples along with their electrode reactions are as follows: