Electrochemical cells, Daniell cell, Galvanic cell

Electrochemistry (Electrochemical cells, Daniell cell, Galvanic cell) The branch of chemistry which deals with the production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical transformations is called electrochemistry.

Electrochemical Cell

• An electrochemical cell is a cell used for generating electrical energy from chemical reactions, or for inducing chemical reactions from electrical energy.

• Daniel cell
• It is a type of galvanic cell.
• The anode is Zn rod dipped in ZnSO₄
• The cathode is Cu rod dipped in CuSO₄
• Generates electrical energy from the chemical reaction
• Electrode potential = 1.1 V, when = 1 mole dm⁻³

• When an external potential is applied and increased slowly, the reaction continues till E_ext = 1.1 V. At this external potential, the reaction stops; when E_ext is further increased, the reaction starts again, but in the opposite direction. Now, the device becomes an electrolytic device.
• When E_ext < 1.1 V
• (i) Electrons flow from the Zn rod to the Cu rod, so current flows from Cu to Zn
• (ii) Zn dissolves at the anode and copper deposits at the cathode

• When E_ext = 1.1 V
  (i) There is no flow of electrons, and hence, there is no current
  (ii) No chemical reaction takes place
  

• When E_ext > 1.1 V
  (i) Electrons flow from the Cu rod to the Zn rod, so current flows from Zn to Cu
  (ii) Copper dissolves at the Cu electrode and Zn deposits at the Zn electrode.

Galvanic Cells

• The devices which converts chemical energy of a spontaneous redox reaction into electrical energy are known as Galvanic cells.
• Example: Daniel cell
• The overall cell reaction is

• Reduction half-reaction:

• Oxidation half-reaction:

• Reduction half-cell:

Oxidation half-cell:

• Electrode potential – The potential difference developed between the electrode and the electrolyte is known as Electrode potential.

·         Standard electrode potential –It is the  electrode potential when the concentration of the species of the half-cell is unity at STP.

• Anode − Electrode where oxidation takes place
• Cathode − Electrode where reduction takes place
• Cell potential − Potential difference between the cathode and the anode is known as Cell potential.
• Cell electromotive force (emf) – The potential difference between cathode and anode when no current is drawn is known as Cell electromotive force (emf)

• Emf of a galvanic cell is positive, and it is given by

• For example:
Cell reaction

• Half-cell reactions are:

• Reduction half-reaction (cathode):

• Oxidation half-reaction (anode):

• The given cell can be represented as

      Therefore, 

·         Cell representation: The cell is representing by placing Anode first followed by salt bridge and which is followed by cathode.

Such as : If in a cell anode is of Cu and cathode is Ag, then first we have to write the anode Cu_(s)/Cu_(aq)²⁺ first which is followed by // and then the cathode Ag_(aq)⁺/Ag_(s).

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By: Satyam Kumar Nigam