Introduction

  1. In today’s article, we are going to study about ‘Facts about Daniell cell you should know’ How beneficial it is and what is the Daniell cell. We will also read about some more topics like mechanism of Daniell cell, polarisation, Concentration polarisation, demonstration of Polarisation etc in Detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

Daniell Cell

This is simple galvenic cell which consists of copper and zinc rod as electrodes which are dipped in CuSO4 and ZnSo4 solutions respectively. Solutions are separated by a porous pot. Here we have showed a Fig of Daniell cell. When two electrodes are connected by a metallic wire, the cell starts producing current. If a voltmeter is connected between two electrodes, it shows deflection.

Daniell cell

Mechanism of Daniell cell

Every metal or hydrogen has a tendency to convert into cation by loosing electrons when it is placed either in water or in its solution. In Daniell cell Zn and Cu are in contact with their solutions (ZnSo4 and CuSo4) so they also have a tendency to liberate electrons and acquire following equilibrium.

Zn ⇔ Zn^+2 + 2e^-

Cu ⇔Cu^+2 + 2e^-

Zn due to its higher electrolytic pressure has more tendency to liberate electrons than Cu, therefore, Zn electrode has higher density of electrons. When two electrodes are connected with a metallic wire, electrons start to flow from Zn to Cu electrode. Following reactions take place due to movement of electrons :

Zn → Zn^+2 + 2e^-  (oxidation)

Cu^2+ + 2e^- → Cu  (Reduction)

Complete reaction

Zn + Cu^2+ → Zn^2+ + Cu (Redox)

The reaction takes place at electrode are termed as Electrode reactions or Half cell reactions. These reactions may be classified as oxidation and reduction reactions or simply, cell reactions are redox reactions. Hence, in galvanic cell the current is produced due to redox reactions.

Conventionally, the electrode at which oxidation takes place is known as anode. In Daniell cell, Zn rod acts as anode and the electrode at which reduction takes place is known as cathode. Cu rod acts as cathode in Daniell cell. When electrodes are connected with a voltmeter, the deflection in voltmeter show potential difference. When the potential difference is measured in open circuit (i.e. when no current is flowing) it is called electromotive force (emf) of the cell. Electromotive force of Daniell cell at 298 k is 1.0988 volt.

Polarisation

It is a general observation that emf of a galvanic cell decreases with time. This may be attributed to the fact that a layer of hydrogen is deposited on the copper Electrode. Due to this current, strength decreases and finally reaches to zero. This effect is known as polarisation of the cell. The phenomenon of polarisation may be explained as follows :

(1) When two electrodes of a galvenic cell are connected by a wire the positive H+ ions moves towards Cu electrode. These H+ should then deliver charges and bubble away in the form of H2 gas. But, infect, a film of H2 gas is found to collect on the Cu plate. This neutral H2 gas is a bad conductor of electricity and thus reduces the current strength. As the thickness of this layer increases, its resistance to current increases and strength of current further decreases which finally reaches to zero.

H2SO4 ‌⇔ 2H^+ + So4^2-

So4^2- + H2O ⇔ H2SO4 + O^-2

2H^+ + 2e^- → H2 (Cathode)

2O^2- → O2 + 4e^- (anode)

Hydrogen being electropositive with respect to zinc, local current starts flowing from hydrogen layer to zinc i.e. in the direction copper to zinc. This current is opposite to the main current in outer circuit i.e. from zinc to copper and this sets up a back emf in the cell. Which reduces the emf of the cell. A stage is reached when this back emf (also known as polarisation emf) does not further allow any further H+ ions from moving towards copper electrode. At this stage the current finally reaches to zero and the cell is supposed to be completely polarised.

Concentration polarisation

When the current passes through the cell there occur a change in the concentration at the electrodes and this change develops a back emf. This changes the net emf of the cell. Thus, the phenomenon of the departure of the electrode potential from the reversible value as a result of the change of concentration in the vicinity of the electrodes is known as concentration polarisation.

Demonstration of Polarisation

The polarisation effect of the cell can be demonstrated by connecting a bell in the circuit. When the cell works, initially the sound of bell is loud but it goes on fainting with time and finally it will stop when the cell is completely polarised.

Conclusion

So our article is finished and after completely reading this article, one can easily tell what a Daniell cell and polarisation is. And how we can explain it.

‌So one can say that they got a detailed information about mechanism of Daniell cell, polarisation, Concentration polarisation, demonstration of Polarisation

Must read : 

Faraday law of chemistry

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