Eletrochemistry Class 12th Chemistry notes

Electrochemistry

What is Electrochemistry ??

Electrical energy plays an important role in many chemical reactions. The branch of 
science which deals with the 
production of electricity from energy released during spontaneous chemical reactions 
and the use of electrical energy to bring about non-spontaneous chemical transformations is called electrochemistry. 

By Yugal 

The basis of these types of processes are 
redox reactions, which we have learnt in the previous class. Large number of chemical and biological reactions are redox reactions. These are used in burning of fuels for obtaining energy for domestic, transport or industrial purposes, digestion of food in animals; photosynthesis to capture energy from the sun, many industrial processes for extracting metals from their ores and manufacture of important chemicals, operation of dry and wet batteries, fuel cells etc. 

ELECTROCHEMICAL CHANGES : 
ELECTROCHEMICAL CELLS AN 
ELECTROLYTIC CELLS

The chemical changes which involve the flow of electric current are called 
electrochemical changes. These are broadly of two types :

1.  Electrochemical cells or Galvanic cells
These constitute the electrochemical reactions in which chemical energy 
is converted to electrical energy. In these cells, spontaneous redox reaction is 
used to generate an electric current. 
The devices in which chemical energy of a spontaneous redox reaction 
is converted into electrical energy are called electrochemical cells or 
galvanic cells.
These cells are very important because of their many practical applications. 
An early example of a galvanic cell is a Daniell cell which was invented by 
the British chemist John Daniell in 1836. Daniell cell was constructed on 
the basis of the following spontaneous redox reaction :
Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)


2.  Electrolytic cells or Electrolysis
These constitute the electrochemical reactions in which electrical energy 
is converted into chemical energy.
The phenomenon of chemical changes taking place by the passage of 
electrical energy from an external source is called electrolysis. The devices 
or cells used to carry out electrolysis are called electrolytic cells. For 
example, when electric current is passed through molten sodium chloride, 
sodium is produced at cathode and chlorine is liberated at anode.
2NaCl Electric current 2Na Cathode + Cl2 Anode 2Na Cathode.

The electrolysis is used to extract many metals such as Na, K, Ca, Sr, Mg, 
Al, etc. and manufacture of chemicals such as NaOH, Cl2, F2, etc. H. Davy 
isolated the element potassium by passing electric current through molten KOH.

ELECTROCHEMICAL CELL OR GALVANIC CELL

The devices in which electrical energy is produced from chemical reactions 
are called electrochemical cells or galvanic cells or voltaic cells. In these 
cells, oxidation and reduction reactions occur in separate containers called 
half cells and the redox reaction is spontaneous. 

Electrical energy is produced 
during such reactions. Let us explain the working of an electrochemical cell, 
known as Daniell cell with the help of a redox reaction:
Zn(s) + Cu2+(aq)  →  Zn2+(aq) + Cu(s)

The arrangement consists of two beakers, one of which contains 1.0 M solution 
of zinc sulphate and the other 1.0 M solution of copper sulphate. A zinc rod is dipped into ZnSO4 solution while a copper rod is dipped into CuSO4 solution. These metallic rods are known as electrodes. 

The metallic rods in the beaker are connected to the ammeter by means of an insulated wire through a key. Ammeter is used to know the passage of current which moves in opposite direction to the flow of electrons. 

The solutions in the two beakers are connected by an inverted U-tube containing saturated solution of some electrolyte such as KCl, KNO3 or NH4NO3 which does not undergo a chemical change during the process. 

The saturated solution is generally taken in agar-agar jelly or gelatin.

The two openings of the U-tube are plugged with some porous material such as glass wool or cotton. The U-tube which connects the two glass beakers is called a salt-bridge. 

When the circuit is completed by inserting the key in the circuit, it is 
observed that electric current flows through external circuit as indicated by 
the ammeter. The following observations are made :

(i) Zinc rod gradually loses its weight.
(ii) The concentration of Zn2+(aq) in the ZnSO4(aq) solution increases.
(iii) Copper gets deposited on the electrode.
(iv) The concentration of Cu2+(aq) in the CuSO4(aq) solution decreases.
(v) There is a flow of electrons in the external circuit from zinc rod to 
copper rod. Therefore, the current flows from copper to zinc. It may 
be noted that as a convention, the flow of electric current is taken 
opposite to the flow of electrons.
These observations can be explained as follows :
During the reaction, zinc is oxidised to Zn2+ ions which go into the solution. Therefore,
Zn(s) → Zn2+ (aq) + 2e– (Oxidation)

the zinc rod gradually loses its weight. The electrons released at the zinc 
electrode move towards the other electrode through outer circuit. Here, these are accepted by Cu2+ ions of CuSO4 solution which are reduced to copper. 

The metal gets deposited on the copper electrode.   Cu2+ (aq) + 2e–  →  Cu(s) (Reduction)

The zinc electrode where electrons are released or oxidation occurs is called 
anode while the copper electrode where electrons are accepted or reduction 
occurs is called cathode. As the electrons move from zinc rod to the copper 
rod, the zinc rod is regarded as negative terminal while copper rod is regarded 
as positive terminal. There is flow of electrons from negative terminal (anode) 
to positive terminal (cathode).
The two containers involving oxidation and reduction half reactions are 
called half cells. The zinc rod dipping into a ZnSO4 solution is oxidation half 
cell and the copper electrode dipping into a CuSO4 solution is reduction 
half cell.

Salt Bridge and its functions. A salt bridge has a vital role to play in an 
electrochemical cell. It is usually an inverted U-tube filled with concentrated 
solution of inert electrolyte.

 The essential requirements of electrolyte are :

(i) The mobility of the anion and cation of the electrolyte should be 
almost same.

(ii) The ions of the electrolyte are not involved in electrochemical change.

(iii) The ions do not react chemically with the species of the cell. Generally, salts like KCl, KNO3, NH4NO3, etc. are used. The saturated solutions of these electrolytes are prepared in agar agar jelly or gelatin. The jelly keeps the electrolyte in semi-solid phase and thus prevents mixing.

The necessary functions of the salt bridge square measure :

 (i) Salt bridge completes the circuit. The salt bridge connects the 2 solutions of the 0.5 cells and their electrodes square measure connected by suggests that of a wire. Therefore, the salt bridge completes the circuit.

 (ii) Salt bridge maintains electrical neutrality of 2 0.5 cell solutions. To know the performance of the salt bridge, allow us to think about the cell while not a salt bridge. The electrons free by the oxidisation of metal to Zn2+ ions are going to be accepted by the Cu2+ ions of CuSO4 within the spouse cell and therefore, the latter are going to be reduced to copper. The charged Zn2+ ions pass into the answer.

 When typically, this results into accumulation of additional electric charge within the resolution round the anode. Similarly, due to reduction of some metal2+ ions to Cu, the answer around cathode can acquire further electric charge thanks to way over SO4 2– ions. The accumulate  of electric charge around metal rod can stop the any flow of electrons from the metal rod. Similarly, the buildup of electric charge around copper conductor can stop the flow of electrons to the copper.

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