New Modified Presentation for Electrochemistry

8/3/2019 New Modified Presentation for Electrochemistry

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ELECTRO CHEMISTRY.


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References:

1. Engg.Chemistry by Jain and Jain

2. Engg.Chemistry by Dr. R.V.Gadag and Dr. A.Nithyananda Shetty

3. Principles of Physical Chemistry by Puri and Sharma

4. Engg. Chemistry ( Vol I & II) by J.C. Kuriacose and J Rajaram

5. ( Old question papers)


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Electrochemistry is a branch of chemistry which deals with

the properties and behavior of electrolytes in solution and

inter-conversion of chemical and electrical energies.


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An electrochemical cell can be defined as a single

arrangement of two electrodes in one or two

electrolytes which converts chemical energy into

electrical energy or electrical energy into chemical

energy.

It can be classified into two types:

Galvanic Cells.

Electrolytic Cells.


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Galvanic Cells:

A galvanic cell is an electrochemical cell that produces

electricity as a result of the spontaneous reaction

occurring inside it. Eg.: Daniel cell.


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Daniel Cell.


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ELECTROLYTIC CELL

An electrolytic cell is an electro chemical cell in which a

non- spontaneous reaction is driven by an external source

of current.


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Representation of galvanic cell.

Anode Representation:

ZnZn2+ or Zn ; Zn2+

Zn ZnSO4 (1M) or Zn ; ZnSO4 (1M)

Cathode Representation:

Cu2+ /Cu or Cu2+ ;Cu

Cu2+ (1M) ; Cu or CuSO4(1M)/Cu

Cell Representation:

Zn ZnSO4 (1M) CuSO4(1M)/Cu


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Emf of a cell.

The difference of potential, which causes a current toflow from the electrode of higher potential to one oflower potential.

Ecell = Ecathode- Eanode The E Cell depends on:

the nature of the electrodes.

temperature.

concentration of the electrolyte solutions.


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Standard emf of a cell(Eo cell) is defined as the emf of a cell when the

reactants & products of the cell reaction are at unit concentration or

unit activity, at 298 K and at 1 atmospheric pressure.

The emf of a galvanic cell can not be measured accurately using a

voltmeter


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Weston Cadmium Cell

Sealed wax

CorkSoturated solution of

CdSO4.8/3H2OCdSO4.8/3H2O

crystals

Cd-Hg

12-14%

Cd

Paste of Hg2SO4

Mercury, Hg


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Salt Bridge.

The liquid junction potential can be reduced (to about 1

to 2 mV) by joining the electrolyte compartments

through a salt bridge.


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Nernst Equation.

It is a quantitative relationship between electrodepotential and concentration of the electrolyte species.

Consider a general redox reaction:

Mn+(aq) + ne- M(s)

At 298K,

E= Eo-0.0592/n log 1/[Mn+]


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Energetics of Cell Reactions.

G = -nFE ( in k cal)

H = nF[T( E/ T)PE]

S = nF (E/ T)P


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Classification of Electrodes.

Gas electrode ( Hydrogen electrode)

.

Metal-metal insoluble salt (Calomel electrode).

Ion selective electrode.(Glass electrode).


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Construction and working of hydrogen electrode.


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Representation: Pt,H2(g)/ H+

Electrode reaction: H+ + e- 1/2 H2(g)

Applications:

To determine electrode potential of other unknown

electrodes.

To determine the pH of a solution.


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Limitations.

Construction and working is difficult

.

Pt is susceptible for poisoning.

Cannot be used in the presence of oxidisingagents.

Difficult to maintain the pressure as 1atmosphere, and concentration of electrolyte as 1M.


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Metal metal salt ion electrode.

These electrodes consist of a metal and a sparingly soluble

salt of the same metal dipping in a solution of a soluble

salt having the same anion.

Eg: Calomel electrode.

Ag/AgCl electrode.


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Construction.


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Representation: Hg; Hg2Cl2/ KCl

IAs anode: 2Hg + 2Cl-

Hg2Cl2 + 2e-

As Cathode: Hg2Cl2 + 2e- 2Hg + 2 Cl-

E= Eo -0.0591 log [Cl-] at 298 K

Its electrode potential depends on the concentration of KCl.

Conc. of Cl- Electrode potential

0.1M 0.3335 V

1.0 M 0.2810 V

Saturated 0.2422 V


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Applications:

Since the electrode potential is a constant it can be used

as a secondary reference electrode.

To determine electrode potential of other unknownelectrodes.

To determine the pH of a solution.


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Ion Selective Electrode.

It is sensitive to a specific ion present in anelectrolyte.

The potential of this depends upon the activity ofthis ion in the electrolyte.

Magnitude of potential of this electrode is an

indicator of the activity of the specific ion in theelectrolyte.

*This type of electrode is called indicator

electrode.


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Glass Electrode:`


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The overall potential of the glass electrode has threecomponents:

The boundary potential Eb

Internal reference electrode potential Eref.

Asymetric potential Easy.

Eg = Eb + Eref. + Easy.

Glass electrode is mainly used in the determination of pH of a

solution.


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CONCENTRATION CELLS.

Two electrodes of the same metal are in contact with

solutions of different concentrations.

Emf arises due to the difference in concentrations.

Cell Representation:

M/ Mn+[C1] Mn+/M[C2]


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Construction.


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Anode - electrode with lower electrolyte

concentration.

Cathode electrode with higher electrolyte

concentration.

Higher the ratio [C2/C1] higher is the emf.

Emf becomes zero when [C1] = [C2].


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At anode: Zn Zn2+(C1) + 2e-

At cathode: Zn2+(C2) + 2e- Zn

Ecell = EC-EA

= E0 + (2.303RT/ nF)logC2-

[E0+(2.303RT/nF)logC1]

Ecell = (0.0592/n) log C2/C1

Ecellis positive only if C2> C1

New Modified Presentation for Electrochemistry

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