ELECTROCHEMISTRY Note by CBCS NOTE

 

Electrochemistry deals with the study of electrical properties of solutions of electrolytes and with the interrelation of chemical phenomenon and electrical energies. Electrical energy is carried through matter in the form of electric current with the help of suitable source and charge carriers (ions or electrons).

1 . CONDUCTORS AND NON CONDUCTORS :

Substances are devided into two classes 

(a) Non conductor orinsulato

(b) Conductor

( a ) Substances which do not allow electric current to pass through them are called non-conductors or insulators examples - pure water, ice, glass, rubber etc.

( b ) Conductor : Substances which allow electric current to flow through them are called conductors. Examples - Metals, Aqueous solution of acids, bases and salts, fused salts and impure water etc.

Conductors are of two t ypes :

(i) Metallic conductors

(ii) Electrolytic conductors or electrolytes.

(i) Metallic conductors :

( i ) The conductors which conduct electric current by movement of electrons without undergoing any chemical change are known as metallic conductors.

Metals (Cu,Ag,Fe,Al etc), non metals (graphite) and various alloys and minerals are examples.

( i i ) Electroly t ic conductors :

Those substances whose water solution conducts the electric current and which are decomposed by the passage of current are called electrolytes. In this case, conduction takes place by movement of ions.

electrolytes also conduct electricity in fused state and undergo decomposition by passage the electric current.

      Substances whose aqueous solution does not conduct electric current are called non-electrolytes.

They do not conduct electricity in the fused state also. Solutions of cane suger, glycerine, glucose, urea etc. are the examples of non electrolytes.

Strong electroly te :

Electrolytes which are completely ionized in aqueous solution or in their molten state, are called strong electrolytes. Example – all salts, strong acid and strong base

Weak electrolyte :

Electrolytes which are not completely ionized in aqueous solution or in their molten state, are called weak electrolytes.

Examples :- All carbonic acids (except sulphonic acid), CH3COOH, HCN, NH3, amine, etc.

DIFFERENCE BETWEEN METALLIC AND ELECTROLYTIC CONDUCTION :

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2 . ELECTROLYSIS :
The process of decomposition of an electrolyte by the passage of electricity is called electrolysis or electrolytic dissociation. It is carried out in electrolytic cell where electrical energy is converted into chemical energy. For electrolysis to take place two suitable electrodes are immersed in the liquid. The solution of an electrolyte contains ions. When an electric potential is applied between the electrodes, the positively charged ions move towards the cathode and negatively ions move towards the anode, when a cation reaches the cathode, its takes up electron(s) and thus gets its charge neutralised. Thus the gain of electrons (decrease in oxidation number) means reduction takes place at the cathode.
    Similarly an anion when it reaches the anode, gives up electron(s) and thus gets discharged. Loss of
electrons(Increase in oxidation number) means oxidation takes place at anode.

•  The tendency of an electrode to loose electrons is known as the oxidation potential.
  
•  The tendency of an electrode to gain electrons is known as the reduction potential.
  ( a ) Electrolysis of fused sodium chloride :
  When fused sodium chloride is electrolysed, Na

( a ) Electrolysis of fused sodium chloride :
When fused sodium chloride is electrolysed, Na+ ions moves towards the cathode and Cl ions moves towards the anode. At cathode Na+ ions accept electrons to form sodium metal. At anode each Cl ion loses an electron to form Cl2 gas.

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At anode     Cl-  —(-e)  —> Cl   
Cl + Cl —> Cl

At cathode  Na+ —(+e)—> Na

(b) Electrolysis of Aqeous Solution of NaCl:

The solution of NaCl contain Na+, Cl and small amounts of H+, OH (due to small dissociation of water)

•  If more than one types of ions are present at a given electrode, then the one ion is the liberated which requires least energy. The energy required to liberate an ion is provided by the applied potential between electrodes. This potential is called discharge or deposition potential. requires least energy. The energy required to liberate an ion is provided by the applied potential between electrodes. This potential is called discharge or deposition potential.

∆ Order of Discharge Potential:

Higher be the discharge potential, lower will be the tendency of ion to get discharged at the respective electrode. The decreasing order of discharge potential or increasing order of deposition of some of ions are given below :

For cations: k+, Na+, Ca²+, Mg²+, Zn²+, H+, Cu+, Ag²+, Ag+

For anions: SO