Electrochemical Cell

Electrochemical cell

A device which is used to convert chemical energy into electrical energy is called electrochemical cell or galvanic cell.

Electrolytic cell

A device which is used to convert electrical energy into chemical energy is called electrolytic cell.

Difference between:

Electronic 

Concept of oxidation & reduction.

          1. Oxidation

Oxidation is a half reaction in which the loss of electron or increase in positive charge takes  place. The atom or ion which lose the electron is said to be oxidized. The oxidized atoms or ions  act as anode in electrochemical cell.

2.    Reduction

Reduction is a half reaction in which gain of electron or decrease in positive charge (+ve  charge) takes place. The atom or ion which gain electron is said to reduce. The reduced  atoms or ions act as cathode in electrochemical cell.

3.    Redox-reaction:

A reaction in which oxidation and reduction occur simultaneously is called redox-reaction.  In a redox reaction one of the substance lose electron while the other gains the same number  of electrons. Example: Zn + CusO₄ ⇾ ZnSO₄ + Cu

Oxidation: Zn –2e^-               ⇾ Zn⁺⁺

Reduction: Cu⁺⁺ + 2e^- ⇾ Cu

Symbolic representation of an electrochemical cell.

The ways of representation of electrochemical cell are:

       i.           Single vertical line (|) represents phase boundary which separate the different form of same species.

1. Double vertical line (||) represents the salt bridge which separates oxidation half and reduction half reaction.
   
   
2. Anode half-cell reaction is written on the left & cathode half-cell reaction is
   
   

written on the right from the self-bridge.

1. The symbol inert electrode like platinum (Pt) is enclosed in a bracket with  gas in respective side , if gas is involved in a reaction.
   
   
2. The concentration of ionic species are enclosed in a bracket with ions in
   
   

respective side.

1. The value of EME of the cell is written on the right-hand side of the cell notation.
   
   

Electrode potential

The tendency of an electrode to lose or gain electron when it is in contact with its  own ionic solution is called its electrode potential and denoted by a symbol" E".

The electrode potential is termed as oxidation potential if oxidation takes place as the electrode with respect to the standard hydrogen electrode and is called  reduction potential if reduction takes place at the electrode with respect to  standard hydrogen electrode. This oxidation potential or reduction potential is known as single electrode potential.

If the electrode potential measured under standard condition       is known as standard electrode potential and denoted by a symbol "E⁰". The standard conditions are:

1. Concentration of electrolytic solution is 1M.
   
   
2. Temperature of the system is 25ºC.
   
   

     iii.          Pressure of the gas is 1atm.

Electromotive force (emf) of cell

The force which causes the flow of electron from anode to cathode and result in the flow of current in a circuit is called electromotive force or  emf of cell. These forces arise due to the difference in potential between two electrodes.

Thus electromotive force may be defined as the difference of potential between two electrodes which cause the flow of current from an  electrode of lower reduction potential to an electrode of higher  reduction potential.

The emf of a cell determined under standard condition   is called  the standard emf and denoted by a symbol E⁰ _cell .The standard  conditions are:

       i.          Concentration of electrolyte is 1M.

      ii.          Temperature of rxn system is 25ºC. 

     iii.          Pressure of gas is 1 atm.

Reference electrode

The potential of unknown electrode can be calculated by combining (connecting) it with an electrode of known potential in an electrochemical cell. The electrode of known potential which is use to determine the potential of unknown electrode is called reference electrode. For example:

Standard hydrogen electrode, Calomel electrode

Standard hydrogen electrode.

In standard hydrogen electrode, hydrogen gas at 1atm pressure is  passed into 1M HCl solution in which platinum metal remains  immersed which is shown in a figure below.

The standard hydrogen electrode may act as a cathode or anode when it is combined with experimental electrode. So its notation may be represented as:

i.                 When it acts as anode then (Pt) H₂(g, 1atm)/H⁺(1M)// Cathodic rxn

or

ii.                When it acts as cathode then Anodic rxn // H⁺ (1M) / H₂(g, 1atm) (pt

It is difficult to maintain unit concentration of H⁺ ion.

ii.) It is difficult to maintain 1 atm pressure.

Sign convention for electrode potential

when Zn electrode comes in contact with Zn⁺⁺ ion, these exist an equilibrium between Zn and its ion

i.e.

Zn <=> Zn⁺⁺ + 2e^–

Here forward reaction is oxidation & backward reaction is reduction. The potential developed in  forward rxn is oxidation potential & backward rxn is reduction potential. The magnitude of oxidation  & reduction potential are same but different in sign.

i.e.

oxidation potential of an electrode = – reduction potential of same electrode