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What is a primary cell and what are some examples?

What is a primary cell and what are some examples?

A galvanic cell can be made by combining two different electrodes. But they cannot be used as practical cells or batteries. Battery is normally used for a few cells of a similar type. A battery is made by arranging electrochemical cells which is used as an energy source. Electrochemical cell is a result of oxidation-reduction reaction. A practical battery is said to have the following traits:

  1. It is light in weight and small in size.
  2. It should emit a voltage which should not change.
  3. The life span should be life-long both in cases of when it is used or unused.

The practical cells or batteries for commercial purpose are of two kinds.

  1. Primary cell
  2. Secondary cell

What is a primary cell?

A primary cell can be defined where the redox reaction happens only for one time and then the cell becomes dead and has to be discarded. For example, dry cell, LeClanche cell, mercury cell and Daniell cell.

Daniell cell

Daniell cell has a copper vessel which has a concentrated solution of copper sulphate. A porous pot which has dilute sulphuric acid is kept in the copper vessel. A zinc rod which acts as anode is dipped in dilute sulphuric acid. The copper vessel acts as the cathode. The following reactions takes place in the cell.

The Daniell cell is represented as follows.

It gives an emf of 1.1 V.

Dry cell

A dry cell is a smaller version of LeClanche cell. It has an outer container which is made of inc, and acts as anode. The zinc container has a porous insulating paper inside.

The cathode is made of carbon rod and has a brass cap. The space which is between the cathode and the anode is filled up of a mixture consisting of MnO2 and a paste of ammonium chloride, (NH4CL0), zinc chloride (ZnCl20), and charcoal. The porous paper acts a barrier thus preventing contact between zinc container and the paste. It serves as a salt bridge.

The cell is covered from the top with pitch or wax.

The reactions which take place during the discharge are:

The Zn2+ ions go towards carbon electrode (cathode).

The reaction which takes place during the discharge are:

It then acts as a depolarizer. Where the cathodic reaction takes place, the manganese goes from +4 state to +3 state. The ammonia molecules which is formed at the cathode reacts with Zn2+ ions coming from the anode and then it forms a complex ion Zn(NH3)42+. The complication of Zn2+ by NH3 molecules lessens the concentration of free Zn2+, and thus it increases the voltage of the cell. The potential of a dry cell is about 1.5 V.

The dry cells are actually not dry as they contain wet paste of NH4Cl and Zn Cl2. Dry cells can carry its functions only when the paste in it is wet. They cannot be recharged and have to be discarded. Since the NH4Cl paste is acidic in nature, it starts to eat away the zinc container even when it is not in use.

Mercury cell

Mercury cell has been recently introduced in the market. Its voltage is stable in nature. It has an emf of 1,35 V. The mercury cells are expensive. Thus, it is used in sophisticated instruments like camera, hearing aids, and watches etc. A mercury cell has an amalgamated zinc plate anode and it is covered by a steel top plate. A paste of Hg, HgO and carbon powder which functions as the cathode is kept closely to the outer steel case. The electrolyte which is apaste of KOH saturated with Zn (OHG0 2 is carried in an inert porous material. The Two electrodes are separated by an insulation seal made of neoprene rubber. The reactions which takeplace during the discharge are:

What are secondary cells?

Secondary cells are the ones where the reactions can be changed by external factors such as external electric energy source. Hence, secondary cells can be recharged by passing electric current through it and can be used multiple number of times. Secondary cells are also known as storage cells or accumulators. Some examples are lead storage battery and nickel-cadmium storage cell.

Lead storage cell

Lead storage cell has a number of voltaic cells connected in series. Three to six cells are joined to get 6to12 volt battery.

  • The anode is a spongy lead and the cathode is lead with PbO2.
  • The electrolyte has an aqueous solution of sulphuric acid.
  • At the anode, lead gets oxidized to Pb2+ions and then insoluble PbSO4 is formed. At the cathode, PbO2 turns to Pb2+ ions and then PbSO4 is formed.

To study further on similar questions, refer: Electrochemistry from Class 12 Chemistry – Find All the Questions & Answers

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