ELECTROCHEMISTRY

DEFINITION OF ELECTROCHEMISTRY :-

It is branch of Physical chemistry which deals with the relationship between electric energy & chemical changes  taking place in redox reaction. 
It deals with production of electric current from energy released during spontaneous chemical reactions ( as in electrochemical cells) or the use of electric energy to bring about non-spontaneous chemical changes ( as in electrolytic cells )
In nut shell, we can conclude that electrochemistry is transformation of chemical energy to electric energy ( Electrochemical cells) or electric energy to chemical energy ( Electrolytic cells).
IMPORTANCE OF ELECTROCHEMISTRY :-
1) Extraction of metals like Na, Mg, Ca & Al
2) Electroplating ( Electroplating is the process of plating one metal onto another by hydrolysis, most commonly for decorative purposes or to prevent corrosion of a metal) 
3) Purification of metals 
4) Batteries ( Pd-Acid battery, Ni-Cd Battery ) & Cells ( Dry cell, Mercury cell)  & Fuel cells  used in various instruments.  
5) To find conductivity of solution or electrolytes .
ASPECTS OF ELECTROCHEMISTRY :-
There are three main aspects of electrochemistry  viz:- 
1) Electrolysis-Electrolytic cells 
2) Electrolytic conduction 
3) Electrochemical cells 
ELECTROLYTIC CELLS & ELECTROLYSIS 

• Electrolysis is the process of ionization of an electrolyte when electric current is either passed in its aqueous solution or molten state.
• It occurs in electrolytic cell. In electrolytic cell both oxidation & reduction takes place in the same beaker. 
• Anode is +vely charged & Cathode is -vely charged electrodes in electrolytic cells .
• During electrolysis, cations are liberated at Cathode & anions are deposited or liberated at anode.
• Cell reactions are redox & non-spontaneous.   

• No salt bridge is required. 

ELECTROLYTIC CELL 

NOTE:- For metals to be deposited on cathode during electrolysis , the voltage required is almost same as the standard electrode potential. However for liberation of gases, some extra voltage is required than theoretical value of the standard electrode potential. The extra voltage thus required is called as OVER VOLTAGE or Bubble Voltage. 

Working of Electrolytic cell:-

 It works on the the principle of ionization of electrolyte by passing electric current through it & then the ionized ions under influence of electric current move towards oppositely charged electrodes. Thus, cations move towards to Cathode ( & get reduced) , while anions move towards to anode ( & get oxidized.)  

PREDICTION OF PRODUCTS OF ELECTROLYSIS :-

The product of electrolysis are actually ions obtained from electrolysis of electrolytes , but situation is so simple, product of electrolysis also depends on :-

1) Presence of Aq. solution of electrolytes 

2) Discharge potential of ions -( Lower the D.P, higher will be preference for discharge of ions )

3) Sometimes on the material of the electrodes used -Inert or Reactive 

The first two are interconnected & can be explained , with suitable examples:-

A) ELECTROLYSIS OF (i) molten NaCl &(ii)  Aq. NaCl 

Electrolysis of Molten NaCl 

(i) In the electrolysis of molten NaCl, sodium is deposited at the cathode while chlorine is liberated at anode. 

From  the cell reaction it is clear that  when electric current passed through the molten NaCl, ionization takes place , the Sodium cation move towards cathode & Chloride anions move towards anode. Na is deposited as cathode & chlorine gas is liberated at anode. 

(ii) Electrolysis of Aq. NaCl solution :-

Herein , water & NaCl ionise as:- 

Thus, chlorine is liberated at anode. Because it has higher Reduction potential . 
in terms of Discharge potential - Chloride ion has lower D.P as compared to OH- ions , therefore chloride anions discharged in preference to Hydroxyl anions. NaOH will remain in  the solution

DISCHARGE POTENTIAL SERIES

NOTE:- At anode, the reaction with lower value of standard reduction potential is preferred  . But due to overvoltage , oxidation of chloride ions occurs & chlorine gas is obtained .

Discharge potential of ions 

• Li, Na , Mg, Al are hard to discharge ...
• while Cu, Hg & Ag are easily discharged.
• Cu(II) ions will be discharged in preference H+
• In metals series ions at lower position of electrochemical series will be discharged preferentially.
• In Anions - hydroxyl ions discharged preferentially 
• Cl, Br & I (anions ) can be discharged Ist when their conc. are higher  as compared to Hydroxyl anions.
• Hydroxyl ions discharged preferentially as compared to Sulphate & Nitrate ions
• PRODUCT OF ELECTROLYSIS ↓↓↓↓↓↓↓↓↓↓↓↓↓- For detailed summary click link https://drive.google.com/file/d/17sVRE6Fl6D_NNMPJFK5vcQAe0vTF-Kqb/view?usp=sharing

FARADAY'S LAWS OF ELECTROLYSIS :-

Based upon the studies on electrolysis, Michael Faraday in 1833 put forward the following two laws:-

(I) Faraday's first law of electrolysis :- the amount of the substance deposited or liberated at an electrode is directly proportional to the quantity of electricity passed through an electrolyte. 

   Mathematically:- 

W = Z Q    

where as w= weight of substance , Z=  electrochemical equivalent & Q= charge 

Q= Current(i) X Time (t) = i x t 

Thus , W= Z i t 

when i=1A & t=1sec, then W=Z

Therefore, Z is defined as the mass of substance deposited or liberated when current of 1A is passed for 1 second i.e a quantity of the electricity equal to 1 coulomb is passed.

As 1F deposits 1 gram equivalent of substance , hence Z can be calculated from the equivalent weight i.e., 

Z= E/ F     or   Z   = E/96500

 

(II) Faraday's Second law of electrolysis :- when the same quantity of electricity is passed through different electrolytes , the amounts of the products obtained at the electrodes are directly proportional to their chemical equivalents or equivalent weights. Thus 

Mass of A         Eq. weight of A 

------------   =    ------------------

Mass of B         Eq. weight of B

m1         E1      Z1Q       Z1

---- =      --- =    ----  =   ---

m2         E2      Z2Q       Z2 

Hence, Z is proportional to Eq. wt.