Class 11 Chemistry Redox Reactions Electrochemical cell

Electrochemical cell

  • Electrochemical cell is the cell in which chemical energy gets converted to electric energy.
  • In it indirect redox reactions takes place.
  • These reactions are spontaneous that is free energy change for this reaction is negative.
  • This cell consists of two half cells.
  • In one half cell , there is a aqueous 1molar Zinc sulphate solution with Zinc rod dipped in it.
  • In other half cell, there is a 1 molar aqueous solution of Copper sulphate solution with Copper rod dipped in it.

Class_11_Chemistry_Redox_ElectroChemicalCell

  • These electrodes by means of wire are attached to galvanometer.
  • A U-shaped tube is taken, which is sealed from both the ends with cotton plug.
  • In this, the electrolyte that is inert electrolyte is taken like Potassium nitrate, Ammonium nitrate etc. The electrolyte present is in semi-liquid state.

Observations

  • With time we see that Zinc rod loses weight, as it has more tendency to loose electrons that is:

Zn -2 electrons -->  Zn2+ (Oxidation)

Zinc                       Zinc Ion

  • These electrons released by zinc, travel to another beaker by means of wire. In doing so, they cause deflection in galvanometer and produce current. This current travel in the direction opposite to the flow of electrons.
  • These electrons move to another half cell, where copper ions gain these electrons that is reduction occur. As a result, copper metal start depositing on electrode. The reaction that occurs is shown below:

Cu2+  + 2electrons --> Cu(reduction)

Copper ions                   Copper Metal

Functions of salt bridge

  • It connects the circuit internally by connecting the solutions.
  • It helps in maintain neutrality.

With passage of time, the left container will have excess positive charge around electrode. Due to which further oxidation stops .Whereas in other beaker negative charge will exceeds, which will start repelling electrons. Therefore, at that time salt bridge comes into action. The oppositely charged electrolyte ions start diffusing into half cells in order to neutralize the excess charge. Hence, the cell keeps on working.

The electrolyte that is selected must fulfill two conditions:

  • Size of its cation and anion should be equal.
  • Electrolyte in salt bridge should not interact with the main electrolyte of half cells.

The overall reaction that takes place is:

Zn + Cu2+ -->   Zn2+       +    Cu  

Zinc   Copper   Zinc Ion    Copper Metal

Representation of the cell

Class_11_Chemistry_Redox_Representation_Of_Cell

Formula to calculate standard electrode potential of cell (when concentration of electrolyte is 1 molar)

E=Ec-Ea (where ‘c’ is cathode and ‘a’ is anode).

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