Question

What is Voltaic Cell ? Explain it briefly and what is Salt bridge ?

Answer 1

VOLTAIC CELL

A voltaic cell, often known as a galvanic cell, provides electrical energy. The source of this energy is a spontaneous chemical reaction, more specifically a spontaneous redox reaction.

For example, all batteries are made of one or more voltaic cells; batteries go flat when most or all of their reactants have been converted to products, transforming their chemical potential energy into electrical energy

SALT BRIDGE

A salt bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. It maintains electrical neutrality within the internal circuit, preventing the cell from rapidly running its reaction to equilibrium.

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Answer 2

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→ When plates of two dissimilar metals are placed in a conducting liquid such as an aqueous solution of a salt, the resulting system becomes a source of electricity. When a piece of zinc is kept in copper sulphate solution for some time, the zinc piece is turned red. This is due to the deposition of copper on zinc. The reaction is given as,

                     Zn(s) + Cu²⁺(aq) ----> Zn²⁺(aq) + Cu(s)

→ In this redox reaction two electrons are transferred from zinc atom to cupric ion. The transfer of electrons does not appear as the reactants are mixed together. Electrons can be obtained in such a reaction by modifying the reaction condition. A device in which electrical energy is generated by performing a chemical reactions is called galvanic cell. Such a cell is also called voltaic cell.

→ A device in which an electrochemical reaction operates is called an electrochemical cell.

                                  IT IS OF TWO TYPES:-

1) Electrolytic cell

2) Galvanic cell

→ Electrical energy is converted into chemical energy in an electrolytic cell. Chemical energy is converted into electrical energy in a galvanic cell. A galvanic cell is commonly called electrochemical cell. A typical galvanic cell.

→ A galvanic cell consists of two half cells or electrodes. One electrode contains Zn metal plate dipped in ZnSo₄ solution and other is copper plate in CuSo₄ solution. The two half cells are joined by a salt bridge.

→ Now, When the Zn and Cu electrodes are joined externally by a wire the following observations are made:-

1) There is a flow of electric current through the external circuit.

2) 'Zn' rod loses its mass, while 'Cu' rod gains the mass.

3) The concentration of ZnSO₄ solution increases, while the concentration of CuSo₄ solution decreases.

4) The solutions in both the compartments remains electrically neutral.

⊕ CHEMICAL REACTIONS IN A GALVANIC CELL

→ Zinc goes into solution as Zn²⁺ ions in the half cell in the left hand side.

                      Zn(s) ----> Zn²⁺ (aq) + 2e⁻

→ Since the reaction involves the loss of electrons, it is known as oxidation half reaction. The electrode is known as ''Anode''. The electrons from the Zn electrode flow through the external wire to the Cu electrode. Cu⁺² ions of the CuSo₄ solution take up these electrons to form Cu atoms.

      →      Cu²⁺ (aq) + 2e⁻ ----> Cu(s)

→ Since this reaction involves gain of electrons it is known as reduction half reaction. This electrode is known as cathode.

     → At anode:- Zn ---> Zn²⁺ 2e⁻ (oxidation)

     → At cathode:- Cu²⁺ + 2e⁻ ----> Cu (reduction)

The overall cell reaction can be written as Zn + Cu²⁺ ----> Zn²⁺ + Cu

→ Voltmeter is used to measure the potential difference between the two electrodes.

⊕ SALT BRIDGE : Salt bridge is an inverted 'U' shaped tube filled with a jelly like substance agar-agar mixed with an electrolyte like KCl,KNO₃ or some times NH₄NO₃ . These electrolytes are not involved in any electrochemical change and do not react chemically with the two half cells. The salt bridge minimises the polarisation of the cell and maintains  the electrical neutrality of the cell.

  → Daniell cell is designed to make use of spontaneous redox reaction between zinc metal and aqueous cupric ions.

→ In the Daniell cell the salt bridge is replaced with a porous pot. Aqueous zinc sulphate is taken in the outer vessel. A zinc plate kept in this solution acts as anode.

→ Aqueous copper sulphate is taken in the inner porous vessel. A copper rod kept in this solution acts as cathode. The electrical neutrality of the Daniell cell is maintained by the porous vessel. The net cell reaction is:-

                     Zn(s) + Cu²⁺ (aq) ------> Cu(s) _ Zn²⁺ (aq)

→ The standard EMF of a Daniell cell is 1.1 V. If an external potential of 1.1 V is applied on this cell, there will be no chemical reactions and no flow of electrons. If the external potential is less than 1.1 V, the spontaneous redox reaction continuous. If the external potential is more than 1.1 V, the cell functions as an electrolytic cell and the electrical energy is used to carry non-spontaneous chemical reactions.


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