Introduction to electrochemistry

Electrochemistry is the branch of chemistry that involve relations between electrical energy and chemical energy.

In electrochemistry a chemical reaction is driven by an electrical potential difference (Ex. electrolysis), or if there is a potential difference arises due to a chemical reaction as in a battery or fuel cell, it is called an electrochemical reaction. Unlike in other chemical reactions, in electrochemical reactions , these transformation takes place through te flow of electrons.

Application of electrochemistry :
   (1) Metal extraction from an ore or purification (Al extration by HH process)
   (2) Generation of electricity using chemical rxn (Battery)
   (3) Electroplating (Coating is done to protect metal from corrosive environment.)
   (4) Electropolishing
   (5) Produce chemical by supplying external current (H2 production by electrolysis of H2O

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Components of a electrochemical cell :
   (1) Electrolyte
   (2) Electrodes

(1) Electrolyte :
An electrolyte is a chemical compounds (fused or aqueous solution) which conduct electricity by undergoing chemical decomposition due to flow of current through it.
Examples:
   acidic solution : HNO₃, HCl, KCl
   Alkali solution : KOH, NaOH
   Ionic solution : CuSO₄,ZnSO₄, PbBr₂

(2) Electrodes :
An electrode is a an electrical conductor through which electron may leave or enter in the electrolyte solution..

Types of electrode:
   Anode : On which metal dissolution i.e., oxidation occures ( Ex. M ---> Mⁿ⁺ + ne^- )
   Cathode : On which metal deposition i.e., reduction occures ( Ex. Mⁿ⁺ + ne^- ---> M )

Sign on electrode :
We cannot say which will be +ve or -ve. It dependes on the type of cell.
For spontaneous reaction (Ex Galvanic Cell)
     Anode : -ve
    Cathode : +ve

For non - spontaneous reaction (Ex Electrolytic Cell)
     Anode : +ve
    Cathode : -ve

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Type of electrochemical cell :
There are basically 3 type of cell as Galvanic cell, Concentration cell and Electrolytic cell

(1) Galvanic Cell
These type of cell form between two dissimilar metal having different electrode potential or same metal having two dissimilar section. And the driving force for the reaction is the difference between electrode potentials. Galvanic cell convert chemical energy to electrical energy.

Galvanic cell can be form due to :-
(1) Two dissimilar metal in contact,
  Example : Fe bolt is anodic to Cu sheet
(2) Different heat treatments,
  Example : Tempered steel is anodic to annealed steel
(3) Differential strain,
  Example : Strained area anodic to unstrained area
(4) Grain boundaries,
  Example : Grain boundary is anodic to grain
(5) Differential grain size,
  Example : Smaller grain is anodic is large grain
(6) Scratches or abrasions,
  Example : Scratched area is anodic to remaining area
(7) Surface condition,
  Example : New section of pipi when welded in old pipeline then new section become anodic and corrode

(2) Concentration Cell
These type of cell form due to concentration difference between electrode solutions on which electrodes are dipped. Both electrodes are formed from same metal but the electrolyte they dipped in, have different concentraion. Electrode dipped in a lower conc. will acts as a anode to electrode which dipped in a higher conc.

Example :
   Zn ----> Zn²⁺(C₁) + 2e^-
   Zn²⁺(C₂) + 2e^- ----> Zn

Here C₁ < C₂. So initially the emf will be high but then it will fall gradually.

(3) Electrolytic Cell
These type of cell utilizes an external electrical energy source to drive a non - spontaneous chemical reaction that would not occur otherwise. Means these type of cell convert electrical energy to chemical energy.

Example :
   Zn ----> Zn²⁺(C₁) + 2e^-
   Zn²⁺(C₂) + 2e^- ----> Zn

Here C₁ < C₂. So initially the emf will be high but then it will fall gradually.

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Comparison between galvanic cell and electrolytic cell :

Electrochemical cell (Galvanic Cell)	Electrolytic cell
A Galvanic cell converts chemical energy into electrical energy.	An electrolytic cell converts electrical energy into chemical energy.
Here, the redox reaction is spontaneous and is responsible for the production of electrical energy.	The redox reaction is not spontaneous and electrical energy has to be supplied to initiate the reaction.
Here the anode is negative and cathode is the positive electrode. The reaction at the anode is oxidation and that at the cathode is reduction.	Here, the anode is positive and cathode is the negative electrode. The reaction at the anode is oxidation and that at the cathode is reduction.
The two half-cells are set up in different containers, being connected through the salt bridge or porous partition.	Both the electrodes are placed in a same container in the solution of molten electrolyte.
The electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit.	The external battery supplies the electrons. They enter through the cathode and come out through the anode.

Representation of electrochemical reaction :

   Anode electrode / Anode Electrolyte // Cathode electrode / cathode electrolyte
   Ex. Zn / Zn²⁺ // Cu²⁺ / Cu