Electrochemistry Class 12 Introduction about batteries and type of batteries

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:

1. A practical cell/battery should be light in weight, and compact in size.
2. 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.,

1. Primary cell/battery
2. Secondary cell/battery.

Primary Cells

• 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

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)

(anode)                        (cathode)

Daniell cell gives an emf of 1.1 V.

Dry Cell

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.