What is a battery?
You can get a galvanic cell by combining two different electrodes together. However, you cannot use all the galvanic cells as practical cells or batteries.
Fig: BatteryUsually, we use the term battery for a combination of a few cells that are similar in nature. A practical battery must have the following characteristics:
• It must be light in weight and compact in size.
• The cell or a battery must be able to give a constant voltage. Moreover, the voltage of the battery or the cell must not vary during the use.
Types of Batteries:
The batteries or the practical cells of the commercial values are mainly of two types. These are:
• Primary cell/battery
• Secondary cell/battery.
The primary cells produce the electricity by the virtue of a chemical reaction. Here the reaction occurs only in one direction. We can not reverse this phenomenon. As a result, these cells become dead over a period of time. You cannot reuse or recharge a primary cell. Some of the examples of primary cells are Daniell cell, Dry cell, and Mercury cell.
The Daniell cell has a copper vessel which contains 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 an anode, while the copper container acts as a 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)
The cell may be represented as,
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Daniell cell gives an emf of 1.1 V.
Fig: Dry cellA compact form of the LeClanche cell is the dry cell. It comprises of an outer container made of inc, which acts as an anode. The zinc content of the cell is lined from inside with a porous insulating paper. The cathode is a carbon rod having a brass cap.
There is a space between the cathode and the anode which is filled with a mixture of MnO2 along with a thick paste of ammonium chloride, (NH4Cl), zinc chloride (ZnCl2), and charcoal. The lining of the porous paper prevents a direct contact between zinc container and the paste. It acts as a salt bridge. The cell is sealed from the top with pitch or wax.
Reactions during discharge
At anode: Zn(s) →Zn2++ 2e–
The Zn2+ ions migrate towards carbon electrode (cathode). The reaction at the cathode is,
At cathode: MnO2+ NH4++ e– → MnO (OH) + NH3MnO2
It acts as a depolarizer. State of manganese is reduced from + 4 to + 3 in cathodic reaction. 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.
Are dry cells really dry?
In reality, the dry cells aren’t really dry. They have a wet paste of NH4Cl and ZnCl2. In reality, a dry cell will function only as long as the paste in the cell is moist. Moreover, you cannot recharge a dry cell. So, naturally, the dry cells do not have an indefinite life. This is because the NH4Cl paste is acidic in nature and it goes on corroding the zinc container even when it isn’t in use.
Fig: Mercury cellMercury cell is recently introduced in the market. It offers a rather more stable voltage. The emf of the Mercury Cell is 1.35 V. Usually, the mercury cell is costlier. This is the reason, why they are used only in sophisticated instruments such as camera, hearing aids, and watches etc. Amalgamated zinc plate coated with a steel top plate acts as anode in Mercury cell.
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. An inert porous material carries this paste. 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)
On the other hand, the secondary cells are the repeated action cells. These cells can be recharged after every use. Passing electricity through the cells recharge them. As a result, you can use these cells over and over again. Some of the examples of secondary cells are the Lead-acid cell, (or lead storage cell), nickel-cadmium cell etc.
Ques: What is the other name given to secondary cells?
Ans: In the secondary cells, the electrical energy is stored in the form of chemical energy. This is the reason, why they are also known as the storage cells or, accumulators.