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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Electrochemistry Page 1

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1.

Electrochemistry

 

The study of the inter-relation between chemical reactions and electrical energy is called electrochemistry. There are two types of cells that can be used for a chemical reaction to take place:

a)

Electrochemical Cell: The device which is used to convert chemical energy into electrical energy at the expense of spontaneous oxidation reduction reaction is called an electrochemical cell. Example: Daniel cell.

 

b)

Electrolytic Cell:The device which is used to bring about a non-spontaneous chemical reaction using electrical energy is called an electrolytic cell. Example: Electrolysis of fused sodium chloride.

 

 

 

2.

Electrolysis

 

 

The process of decomposition of an electrolyte by the passage of an electric current through its aqueous solution or fused mass is called electrolysis. Example: Electrolysis of sodium chloride.
Electrolysis of Fused Sodium Chloride:
The electrolytic cell consists of 2 electrodes of platinum or graphite. Fused sodium chloride dissociates to form sodium cations and chloride anions. The sodium ions are discharged at the cathode as sodium atoms (metallic sodium). The chloride ions are discharged at the anode as molecular chlorine.
Reaction at cathode:
2Na+ + 2e-
2Na (Reduction)
Reaction at anode:
2Cl-
2Cl + 2e- (Oxidation)
2Cl
Cl2 (g)

 

3.

Faraday's Laws of Electrolysis

 

 

a)

Faraday's First Law of electrolysis: The amount (weight) of any substance deposited or

 

liberated or dissolved at an electrode is directly proportional to the quantity of electricity passed through the electrolyte.
W
Q. Hence, W it
W = Zit

where, i  

= current (ampere)

t   

= time in seconds

Z  

= electrochemical equivalent

b)

Faraday's Second Law of Electrolysis: The amounts (weights) of different substances

 

deposited or dissolved by passing the same quantity of electricity through different electrolytes, connected in series are directly proportional to their equivalent weights.

WA

=

EA

WB

EB

where,
WB = wt. of substance A
WA = wt. of substance B
EA = equivalent of A
EB = equivalent of B

 

4.

Faraday

 

 

Quantity of electricity passed in order to deposit or dissolve or liberate one gram equivalent (one equivalent) of a substance during electrolysis is called one Faraday (F).
1 Faraday = 96,500 coulombs

E = 96,500 Z or
E = F Z where,
E = chemical equivalent
Z = electrochemical equivalent
F = 96,500 coulombs

 

5.

Electrochemical Equivalent (Z)

 

 

The weight (amount) of the element deposited or liberated at the electrode when one coulomb of electricity is passed through the electrolyte is called the electrochemical equivalent.
W= ZQ
\ Z = W/Q kg/coulomb

 

6.

Electrical Units

 

 

a)

Ampere (I): It is the unit of current strength. It is defined as that current, which, when passed through a circuit for one second can liberate 0.001118 gram (1.118 10-6 Kg) of silver from silver nitrate solution (15% AgNO3 solution).

 

b)

Coulomb (Q): It is the unit of charge. It is the quantity of electricity, which passes through a conductor when a current of one ampere strength flows for one second.
1 coulomb = 1 ampere 1 second

 

c)

Volt (V): It is the unit of potential difference. It is defined as the difference of potentials between two points on a conductor, carrying a current of one ampere, when the power between these points is of one watt.

 

d)

Ohm (R): Ohm is the unit of electrical resistance. It is defined as the resistance between two points of a conductor when the potential difference of one volt is applied between the two points to produce a current strength of one ampere.

 

e)

Joule (J): It is the unit of electrical energy (work). It is defined as the work done per second by a current strength of one ampere flowing through a resistance of one Ohm.

 

f)

Watt (W): Watt is the unit of electrical power. It is defined as the electrical power, which does work at the rate of one joule per second.
\ 1 Watt = 1 joule/second.

 

 

7.

Electrochemical Cells

 

 

Electrochemical cells are cells in which chemical energy is converted into electrical energy. Electrical energy is made available at the expense of spontaneous oxidation reduction reaction taking place within the cell.

Examples:

a)

Daniel cell with a porous pot or porous partition.

 

b)

Daniel cell with a salt bridge



a)

Daniel Cell with a Porous Pot or Porous Partition: This type of cell consists of a copper

 

vessel containing copper sulphate solution. The vessel contains a porous pot of unglazed porcelain. This pot contains zinc sulphate solution. Copper vessel acts as positive electrode. A zinc rod placed in the zinc sulphate solution serves as negative electrode. These two electrodes are connected with an external wire.

b)

Daniel Cell with a Salt bridge: The cell consists of two beakers - one containing copper

 

sulphate solution and a copper rod that acts as a positive electrode and the other beaker contains zinc sulphate solution with a zinc rod that acts as a negative electrode. A metallic wire is used to connect the two electrodes. The two solutions are connected with a salt bridge.

 

Cell representation:

Cell reaction:
At anode (oxidation electrode)
Zn(s)
Zn(aq)2+ + 2e- (oxidation half cell reaction)

At cathode (reduction electrode):

(reduction half cell reaction)


Total reaction:

(Redox reaction)


The e.m.f. of the Daniel cell is about 1.1 volt.

 

8.

Salt Bridge

 

 

It is an inverted U-shaped glass tube filled with a saturated solution of KCl or KNO3 or NH4NO3< in agar-agar gel. The ends of the glass tube are plugged with glass wool. The two ends of the salt bridge are immersed in the solution of the two half cells.

 

9

Functions of the Salt Bridge

 

 

  • It connects the two half cells.
  • It prevents mixing of two electrolytes.
  • It minimizes the liquid junction potential between the two electrolytes.
  • It maintains electrical contact between the two electrolytes.
  • It maintains electrical neutrality in the cells.

 

 

10

Conventions Used for Representing the Voltaic Cells

 

 

  • Negative electrode (zinc electrode in the voltaic cell) is written on the left hand side.
  • Positive electrode (copper electrode in the voltaic cell) is written on the right hand side.
  • The vertical single line is drawn between the electrode and the electrolyte.
  • A vertical double line is between two electrolytes that indicate indirect contact of the two electrolytic solutions.
  • Concentration of the activities of the two solutes are written in brackets like (C1), (C2), or (a1), (a2).
  • In case of a gas electrode, the gas is shown along with an inert electrode, used on the left hand side.

 



 

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