A galvanic cell is obtained when two different electrodes are coupled together. But, all galvanic cells cannot be used as practical cells or batteries. Generally, the term battery is used for a combination of a few cells of the same type. A practical battery should have the following characteristics:
- A practical cell/battery should be light in weight, and compact in size.
- A practical cell/battery should give a constant voltage. The voltage should not change much during use.
The practical cells/batteries of commercial value are of two types, e.g.,
- Primary cell/battery
- Secondary cell/battery.
- Primary cells produce electricity by the virtue of chemical reaction. In primary cells,the reaction occurs only in one direction, and cannot be reversed. As a result these cells become deadover a period of time.
- Primary cells cannot be recharged/reused.
- Typical primary cells are, Daniell cell, Dry cell, and Mercury cell.
Daniell cell consists of a copper vessel containing a concentrated solution of copper sulphate. A porous pot containing dilute sulphuric acid is placed in the copper vessel containing copper sulphate solution. A zinc rod is dipped into dilute sulphuric acid. Zinc electrode acts as anode, while the copper container acts as cathode. The reactions taking place in the cell are:
At anode: Zn(s) →Zn2+(aq) + 2e–
At cathode:Cu2+(aq) + 2e– →Cu(s)
Net cell reaction: Zn(s) + Cu2+(aq) →Cu(s) + Zn2+(aq)
Daniell cell is shown in Fig. 5.20. The cell may be represented as,
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Daniell cell gives an emf of 1.1 V.
A dry cell is a compact form of LeClanche cell. It consists of an outer container made of inc, which acts as anode. The zinc container is lined from inside with a porous insulating paper.
The cathode is a carbon rod having a brass cap. The zinc container lined from inside with a porous insulating paper. The space between the cathode and anode is filled with a mixture of MnO2 and a thick paste of ammonium chloride, (NH4Cl), zinc chloride (ZnCl2), and charcoal. The porous paper lining prevents a direct contact between zinc container and the paste, and acts as a salt bridge.
The cell is sealed from the top with pitch or wax.
The reactions during the discharge are:
At anode: Zn(s) →Zn2++ 2e–
The Zn2+ ions migrate towards carbon electrode (cathode).
The reaction at cathode is,
At cathode: MnO2+ NH4++ e– → MnO (OH) + NH3MnO2
Acts as a depolarizer. In the cathodic reaction, manganese is reduced from + 4 state to + 3 state. The ammonia molecules formed at the cathode react with Zn2+ ions coming from the anode, to form a complex ion Zn(NH3)42+. The complication of Zn2+ by NH3 molecules lowers the concentration of free Zn2+, and results in an increase in the voltage of the cell. A dry cell has a potential of about 1.5 V.
The so called dry cells are not really dry. These cells contain wet paste of NH4Cl and ZnCl2. In fact, a dry cell operates only as long as the paste in it is moist. Dry cells cannot be recharged.
Dry cells do not have an indefinite life. This is because; NH4Cl paste is acidic in nature, and it continues to corrode the zinc container even when not in use.
This cell is a recent introduction to the market. It provides a more stable voltage. Its emf is 1.35 V. Mercury cells are costlier, and hence used only in sophisticated instruments like camera, hearing aids, and watches etc. A mercury cell consists of an amalgamated zinc plate anode which is covered by a steel top plate. A paste of Hg, HgO and carbon powder acts as the cathode. It is placed in contact with the outer steel case. The electrolyte is a paste of KOH saturated with Zn (OH) 2. This paste is carried in an inert porous material. The two electrodes are separated by an insulation seal of neoprene rubber. The reactions during discharge are,
At anode: Zn (Hg) + 2OH– →Zn (OH)2+2e–
At cathode: HgO + H2O + 2e– →Hg + 2OH–
Overall reaction: Zn (Hg) + HgO(s) →Zn (OH)2+ Hg(l)
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