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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Electrochemistry Page 2

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11.

Electrodes Used in Electrolytes

 

 

  1. Indicator Electrodes: The electrode system which is used to determine the concentration of the solution used as an electrolyte in the half cell is known as indicator electrode.

Example:

  1.  
  2. Reference Electrode: The electrode whose potential is arbitrarily fixed to a certain value (zero volt) or is exactly known is called a reference electrode. Commonly used reference electrodes are:

a)

  Standard Hydrogen Electrode (S.H.E.) - Primary reference electrode.

b)

  Standard Calomel Electrode (S.C.E.) - Secondary reference electrode.

 

a)

Standard Hydrogen Electrode (S.H.E.): The electrode consists of a glass jacket

 

consisting of dry hydrogen gas bubbled at one atmosphere. There is a platinum wire sealed in the glass jacket. This entire unit is immersed in 1 M HCl solution.
Pt, H2(g) H+(aq)
P = (1 atm) (1M) or (aH+ = 1)
Working: H2(g)
H+ + e- Oxidation
H+ + e-
H2(g) Reduction
Advantages of Standard Hydrogen Electrode (S.H.E.):

  • Small potential is developed on the hydrogen electrode, hence it can be taken as zero.
  • In determining the single electrode potential, using S.H.E. as a reference, the potential of the unknown potential will be equal to the e.m.f. of the cell.

Disadvantages of Standard Hydrogen Electrode (S.H.E.):

  • It is not convenient to assemble the apparatus.
  • It is difficult to maintain the pressure of hydrogen gas and concentration of HCl.
  • It is difficult to get pure, dry hydrogen gas and prepare ideal platinised platinum plate.
  • The impurities present in H2 and HCl poison the Pt, and affect the equilibrium at the electrode.

b)

Standard Calomel Electrode (S.C.E.): The electrode consists of a broad glass tube with

 

a side tube. The broad glass tube consists of pure Hg at the bottom, covered with a saturated paste of Hg2Cl2and Hg. The tube is then covered with saturated KCl. Electrical contact with Hg is made by a platinum wire sealed in the glass tube. The side tube is immersed in the desired solution.
Electrode potential depends upon concentration of KCl solution.
Pt, Hg (l) | Hg2 Cl2 (s) | KCl (aq) (a = x)

At 298K, oxidation potentials are:

a)

sat. KCl: - 0.242V

b)

1N KCl or 1M KCl: - 0.280V

c)

0.1 N KCl or 1M KCl: - 0.334V


Oxidation: 2Hg (l) + 2Cl- (aq) Hg2Cl2 (s) + 2e-
Reduction: Hg2Cl2 (s) + 2e- 2Hg (l) + 2Cl- (aq)

Advantages of Standard Calomel Electrode:

  • It is very handy, compact and easy to transport.
  • Its potential can remain constant and it can easily be reproduced.
  • It is easy to construct and maintain.

 

 

12.

Measurement of Electrode Potential

 

 

The electrode potential of a single electrode can be measured by combining it with a reference electrode to form a cell. The e.m.f. of the cell is measured with a potentiometer.

 

13.

Concept of Electrode Potential

 

 

When an electrode is dipped in its solution, containing its ions, there are two opposing processes taking place - electronation and de-electronation. This is known as Nernst theory of electronation and de-electronation.

a)

  De-electronation: The metal atoms have a tendency to pass into the solution as cations and leave behind electrons on the metal rod. This produces an electrode potential and the electrode undergoing de-electronation is anode (oxidation electrode).

 

b)

  Electronation: Cations of the solution accept electrons from the metal to form neutral

 

metal atoms. This produces an electrode potential, and the electrode undergoing electronation is the cathode (reduction electrode).
The resultant potential of the metal depends upon
P(s) (solution pressure) and
Po (osmotic pressure).

  • Ps >Po, the electrode gets negatively charged.
  • Ps < Po, the electrode gets positively charged.
  • Ps = Po, the electrode becomes chargeless. (Null electrode).

 

14.

Nernst Equation for Single Electrode Potential

 

 

 

15.

Single Electrode Potential

 

 

The difference in potentials between the electrode and the surrounding solution at equilibrium in a half cell is called single electrode potential or half cell potential.

 

16.

Standard Oxidation Potential (E oxd)

 

 

The electric potential developed between an electrode and the surrounding electrolyte due to the oxidation process when a metal is dipped into an electrolyte containing the same metal ions at 1 M concentration at 298 K is called standard oxidation potential.

 

17.

Standard Reduction Potential (E red)

 

 

The electric potential developed between an electrode and the surrounding electrolyte due to the reduction process when a metal is dipped into an electrolyte containing the same metal ions at 1M concentration at 298 K is called the standard reduction potential.

 

18.

E.M.F. of the Cell

 

 

The potential difference, which is responsible for the flow of current from an electrode of higher potential to the electrode of lower potential is called the electromotive force (e.m.f.) or the effective voltage of the cell and expressed in volts.

Standard of cell:

E cell = E oxd (anode) - E oxd (cathode)

or

E cell = E oxd (anode) + E red (cathode)

 

19.

E.M.F. Series

 

 

It may be defined as the series of elements in which elements are arranged in the decreasing order of their standard oxidation potential as compared to the standard hydrogen electrode. Also, called as electromotive series or electro-chemical series.

 

20

Heat of Neutralisation (DHn):

 

 

Applications of the E.M.F. Series

a)

  Gives a quantitative idea about the tendency of an element to lose or gain electrons.

b)

  Oxidising and reducing abilities of the elements.

c)

  Displacement reactions.

d)

  Corrosion behaviour.

e)

  E.M.F. of a cell.

f)

  Spontaneity of Redox reactions.

 

21

Types of Electrochemical Cells

 

 

There are two types of cells: (1) Primary and (2) Secondary
1. Primary Cell
Dry Cell: It is used to convert chemical energy into electrical energy. In this cell, the reaction occurs once and the cell, then, becomes dead. The cell cannot be recharged.
Zinc vessel is the anode and graphite (carbon) rod is the cathode.
The cell voltage is around 1.5 volts.
Representation of dry cell:
-
Zn(s) | Zn++ || NH4Cl, ZnCl2 MnO2, Mn2O3 | C+(s)
Applications of Dry Cell:

  1. It is convenient to carry as solution is not used.
  2. It is used in calculators, watches, torches, transistors.

2. Secondary Cell
Lead Accumulator:

It is known as a storage cell or secondary cell. Electrical energy is not generated but stored in it from an external source. It is a reversible cell, because the supplied electrical energy is stored in the form of chemical energy and then supplied in the form of electrical energy during its operation. It works on the principle of charging and discharging of cells. Lead plates act as an anode and lead plates impregnated with lead dioxide act as a cathode. A fully charged cell has a voltage of around 2 volts.
Representation of lead accumulator:
-Pb, PbSO4 (s) | H2SO 4 (aq) | PbSO4 (s), PbO2(s), Pb(s)+ 20%
Applications of Lead Accumulator Cell:

  • It is used in automobiles
  • It is used in telephones and telegraph offices
  • It is used in laboratories as a D.C. supplier.

 



 

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