Electro Chemical Cell

 

Electro Chemical Cell : Whenever a redox reaction is allowed to take place directly in a single beaker, it is found that the solution becomes hot. For example, when a zinc is placed in a copper solution (Fig. 1), the solution is found to warmer as the reaction proceeds according to the equation.

Zn (s) + CuSO4 (aq) ---> ZnSO4 (aq) + Cu(s)

Similar results are observed when a rod of copper is placed in silver solution. The reaction taking place as follows:

Cu(s) + 2 AgNO3 ---> CuNO3 + 2 Ag

Thus, we conclude that whenever a redox takes place directly in a single beaker, chemical energy in the form of heat is produced . By suitable means it is possible to bring out the redox reaction indirectly so as to convert the chemical energy into the electrical energy.

A device used to convert the chemical energy into electrical energy is called Electrochemical cell O₂ Voltaic cell after the name of Luigi Galvani (1780) and Alessandro volta (1800) who were the first to perform experiment on conversion of chemical energy into electrical energy.

 

Experimental Arrangement

 

A zinc rod is placed in the zinc sulphate solution taken in a beaker. A copper rod is placed in the copper sulphate solution taken in another beaker. The two rods are connected by a wire and two solutions are connected by a salt bridge.

 

Representation of

Electrochemical Cell

 

An electrochemical cell is represented in a manner as illustrated below for the denial cell.

Zn / Zn2+ (C1) || Cu2+ (C2) / Cu

i.e. by convention, the electrode on which oxidation takes place is written on the left hand side and the other electrode on which reduction takes place is written on the right hand side. The electrode of the left hand side is written by writing the symbol of the metal (of the gas) first followed by the symbol of the ion with its concentration in brackets. The electrode on the right hand side is written by first writing the ion along with its concentration in brackets followed by the symbol of the metal (or the gas) single vertical line represent the phase boundaries of the electrode and the double line represents the concentration of Zn2+ and Cu2+ ion respectively.

 

Salt Brigade and Its Function

A salt- brigade is a U-shaped tube containing concentrated solution of an inert electrolyte like KCI, KNO3, K2SO4 etc. or solidified solution of such an electrolyte in agar-agar and gelatine. An inert electrolyte is one whose ions do not take part in the redox reaction and also do not react with electrolyte used. The function of the salt bridge is to allow the movement of the ions from one solution to the other without mixing of the two solutions. Thus, whereas the electrons flow in the outer circuit in the wire, the inner circuit is completed by the flow of ions from one solution to the other through the salt bridge moreover, it helps to maintain the electrical neutrality of the solution is the two half cells.

Thus main function of the salt brigade are :-

(i) To complete the electrical circuit by allowing the ions to flow form one solution to the other without mixing of the two solutions.

(ii) To maintain the electrical neutrality of the solutions in the two half cells.

Let us see what would happen if the salt bridge were not used in the cells shown in Fig. 2 electrons given out by the zinc electrode where they will neutralize some of the Cu2+ ions of the solution. Thus So42- ions will not eave and the solution will acquire a negative charge. At the same time, Zn2+ ions produced from zinc plate will enter into ZnSo4 solution thus giving will stop and hence the current stops working.

 

To Calculate the Standard of emf of

Any Electrochemical Cell

 

An electrochemical cell is based on reaction which can be split into the two half reaction, viz.

(i) Oxidation half reaction

(ii) Reduction half reaction

Standard EMF of the cell = (Standard reduction potential of the reduction half reaction)

-(Standard reduction potential of the oxidation half reaction)

 

Some Important Features

Above cell may be summed up as follows :-

i) The zinc electrode at which oxidation takes place is called the anode. The copper electrode at which the reduction takes place is called the cathode.

ii) Since electrons are produced at the zinc electrodes, this electrode is rich in electrons pushes the electrons into the external circuit and hence it is designated as the negative pole. The other electrode i.e., the copper electrode is in the need of electrons for the reduction of Cu2+ ions into the Cu i.e., this electrode is deficient in electrons and pulse the electrons from the external circuit, therefore it acts as the positive pole.

iii) The electrons flow from the negative pole to the positive pole in the external circuit. However, conventionally, this current is set to flow in the opposite direction.

iv) The oxidation of zinc into ions produces excess of zn42+ ions in the left beaker. Similarly, the reduction of copper ions to copper atoms leaves the excess. So, the ions in the solution in the right beaker. To maintain electrical neutrality of the solution in the two beakers, the cations and anions move through the salt bridge. The cells to complete the inner circuit as already mentioned.

v) As copper form copper sulphate solution is deposited on the copper electrode and sulphate ions migrate to the other side, the concentration of the copper sulphate solution decreases the cell operates consequently the current falls the passage of time.

vi) Evidently, the weight of the copper rod will increase while that of zinc rod will decrease as the cell works.