Write explanation of galvanic cell? 

A galvanic cell is an electrochemical cell that uses the transfer of electrons in  redox reaction  to supply an electric current.This cell is driven  by spontaneous chemical  reaction that produces an electric current through an  outside  electrical circuit .Galvanic cell reactions supply energy  , which is used to perform work. For this reason galvanic cells are used  as  batteries .

 Galvanic cells harness the electrical  energy available  from the electron transfer  in a redox reaction to perform useful electrical work. The key to gathering the electron flow is  to separate the oxidation and reduction  half reactions., connecting them by a wire , so that the electrons must flow through that wire.  The electron flow called a current   can be sent through a circuit which could be part of  any number of electrical devices  such as radios ,  televisions , watches  etc.

A schemetic line diagram of  a galvanic cell may be represented in a simple way as-

............. Oxidation half cell --------> Salt bridge <---------Reduction half cell

                             |____________connecting wire_____________|

The salt bridge :   The salt bridge or a porous disk is necessary to maintain the charge neutrality of each half cell by allowing the flow of ions with minimal mixing of the half cell solutions .  As electrons are transferred from the oxidation half cell to the reduction half cell , a negative charge  builds

in the reduction half cell  and a positive charge in the oxidation half cell. The charge building up would serve to oppose the current from anode to cathode - effectively stopping the electron flow  - if the cell lacked a path for ions to flow between the two solutions. From  the schemetic diagram as illustrated above  it can easily be gathered that  that the electrode in oxidation half cell is called anode and the electrode in reduction half cell is called the cathode.. A good mnemonic to help remember that is " the Red  Cat  ate an Ox " meaning that reduction takes place at the cathode  and oxidation takes place at the anode .

The anode , as the source of negatively charged electrons is usually marked with a minus ( - ) sign and cathode is marked with a plus  ( + ) sign.

Line notation for a Galvanic Cell 

Instead of drawing a cell diagram  a shorthand way of describing  a cell is called  line notation. This  notation scheme places the constituents of cathode on the right and the anode components on the left. The phases   of all reactive species  are listed and their concentrations or pressures  are given if those species are not at standard states  ( ie. 1 atm . for gases  and 1M for solutions ). All phase  interfaces are noted with a single line (  |   ) and multiple species  in a single phase  are separated  by  commas. For example  a half cell containing  1M solution of CuO and  HCl and  a  Pt electrode  for reduction of Cu²⁺ would be written as :

Pt (s) |  Cu²⁺ ( aq ) , H⁺  ( aq )

Note that the spectator ions , oxide and chloride have been left out of notation  and that banode is written  to the far left.

The salt bridge or porous disk is shown in the notation  as  a double line  ( ||  ) . Therefore a cell that undergoes oxidation  of magnesium by  Al³⁺  would have the following cell notation if the anode is  magnesium  and the cathode is aluminium

Mg (s)  |  Mg²⁺ ( aq ) | |  Al³⁺ ( aq ) |  Al   (s)

Standard Reduction Potentials

One can measure the cell potential E_cell   , in volts , of any  galvanic cell using  a potentiometer. However it is directly impossible  to measure the potential of each individual half cell.. Hence , a method has been devised to measure thhe ability of a chemical species to reduce  by compiling  tables of standard reduction potentials E^o . Arbitrarily assigning a value of exactly zero to potential of  of the standard hydrogen electrode allows us tommeasure the E^o of any half reaction. That measurement is made by constructing  a galvanic cell between  the STandard Hydrogen Electrode (SHE) and the unknown half cell at  standard state conditions .For example  , if the following cell is constructed , an E^o _cell   of 0.34 V is observed

Pt (s ) |  H₂ (g) |  H⁺ ( aq )  | |  Cu²⁺  (aq)  |  Cu (s)

Because SHE has a potential of exactly zero volts , the reaction  Cu²⁺  + 2e  ------------>   Cu  has a value of  0.34 V for its  E^o  .  Note  that ,

E^o _cell  =  E^o SHE    +  E^o 

Fortunately , every important reduction potential has been measured  and tabulated . Useful list of reduction potentials are available in most of the introductory chemistry   texts , including yours. Using this list the EMF of galvanic cells are calculated.The following expression is helpful to calculate emf of a galvanic cell under Standard conditions  ( ie. 1 atm pressire  and  at  278 K ) 

E^o _cell    =    E^o _R   -   E^o _L  where  , R  &  L  mean  the  right  and  left   half cells when the line notation for  cell composition is written , repectively.