EMF AND CONDTNCE

E.M.F. OF CELLSE.M.F. OF CELLS (Electrochemical)(Electrochemical)

&& THE CONDUCTANCETHE CONDUCTANCE (Electrolytic)(Electrolytic)

Department Of Chemical Sciences Department Of Chemical Sciences (N.V.P.A.S.)(N.V.P.A.S.)

Saturday, April 15, 2023

1Dr. S. Srivastava

Topics Included:Topics Included:

Redox Reactions .Redox Reactions . Electrodes of an electrochemical cell.Electrodes of an electrochemical cell. Cell Notation.Cell Notation. Measurement of Cell potential.Measurement of Cell potential. Reversible cells.Reversible cells. Types of Reversible Electrodes.Types of Reversible Electrodes.

Measurement of EMFMeasurement of EMF Thermodynamic properties of a Cell reaction.Thermodynamic properties of a Cell reaction.

Application of Potential Measurements.Application of Potential Measurements.

CONCENTERATION Cells.CONCENTERATION Cells.

The Salt Bridge. The Salt Bridge.


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Redox reactions:Redox reactions:

These are the reactions wherein the oxidation as well as the reduction processes take place simultaneously.

Ex:-

Fe3+ + V3+ Fe 2+ + V5+

MnO4 - + Fe2+ + 8 H+ Mn2+ + Fe3+ + 4 H2O

AgCl + ½ H2 Ag + H+ + Cl-

Mg + Zn2+ Zn + Mg2+


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Anode of Electrochemical Cell:- Anode of Electrochemical Cell:-

Here the electrolyte is ZnSO4 so the dissociation of Zn ions is via following process:

Zn Zn 2+ + 2e-

As anode has excess of electrons so it donates its electrons in the solution and attains a ‘–ve’ charge on it (due to excess electrons.)


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Cathode of an Electrochemical cell:-Cathode of an Electrochemical cell:-

Here the electrolyte is CuSO4 sothe Cu ions are generated in the following process:

Cu2++ 2e- Cu

Since the cathode is electron deficient so it needs electrons and thereforegets reduced by the gain of electrons and attains positive charge (due to deficiency in electrons).


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A standard Electrochemical CellA standard Electrochemical Cell (Daniel Cell)(Daniel Cell)


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Working of an Electrochemical Cell


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Cell Notation:Cell Notation:

The cell is represented by following notations:The cell is represented by following notations:

Pt Pt / / HH2 2 / / HClHCl(1M)(1M) //// AgCl AgCl //Ag. Ag.

(anode) (cathode) (anode) (cathode)

(Na-(Hg)(Na-(Hg)(c1)(c1) / Na / Na22SOSO44 / /(c2)(c2) (Hg)-Na. (Hg)-Na.

(Anode) (Cathode)(Anode) (Cathode)

Cu / CuCu / Cu++ // Zn // Zn2+ 2+ / Zn./ Zn. (anode) (cathode)(anode) (cathode)

Pt / FePt / Fe2+ 2+ ; Fe; Fe3+ 3+ // Cu// Cu2+ 2+ ; Cu; Cu+ + / Pt/ Pt (Anode) (cathode) (Anode) (cathode)

In a cell notation the anode is Always mentioned on the

left side ;whereas the cathode is mentioned on

the right side.


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Measurement of the Cell Potential:Measurement of the Cell Potential:

The potential of the cell is measured using the Voltmeter or the PotentiometerUsing the “Null point method”.

The following formula is used:

Es / Ex = AE / AD


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Reversible Cells:Reversible Cells:

Electrochemical Cells are reversible when following conditions holds true.

A] No reaction must occur at the electrodes when the potential of the electrodes are equal.

B] The reaction that could occur in the absence of the opposing potential, must occur if the opposing potential is infinitesimally small than the Cell Potential.

C] The reactions must occur if the opposing potential is infinitesimally greater than the Cell Potential.

Reversible Electrodes:Reversible Electrodes:

Electrodes which constitute the a Reversible cell are called Reversible Electrodes.


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Types of Reversible Electrodes:Types of Reversible Electrodes:

A] GAS Electrodes.

B] Standard Hydrogen Electrode.

C] OXIDATION and REDUCTION Electrodes.

D] Metal - Metal-Ion Electrodes.

E] AMALGAM Electrodes.

F] Metal-Metal Insoluble Salt Electrodes.

G] CALOMEL Electrodes.

H] WESTON Cell.


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A] GAS ELECTRODES:

A gas can be used to construct a gas electrode. One such electrode is used very importantly i.e. hydrogen electrode. While indicating the electrode ; the pressure of the gas, concentration, etc. must be indicated.

E.g.: Pt/H2(p atm) /H+(C M)

This type of electrode, consisting of the bubbling gas around the metal wire or foil immersed in a solution containing ions to which the gas is reversible.

The Nernst Equation for the above process is given as follows:

E oxi = E0 H2;H+ - {0.05915 / n} . Log[(a H+)2] / [(aH2)]

for the reaction ½ H2 H+ + e- .


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B] Standard Hydrogen Electrode:

This is the electrode constructed by dipping platinised plate in aqueous solution of H+ and bubbling Hydrogen gas over it.

By convention, the potential of this electrode is taken as 0.0 V.

This electrode is regarded as the Primary Reference Electrode . However the construction and uses of this electrode are very tedious.

Following are the difficulties faced when the electrode is used.

A)Absolutely pure Hydrogen is needed.B) Special devices are needed to control the pressure of the system.C) Bubbling of H + may change the activity of the ion.D) Impurity of Gas may poison the Platinum, thereby reducing the activity of Platinum.


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C] OXIDATION REDUCTION ELECTRODE:

As the basis of the reaction is the oxidation reduction reaction at all the electrodes ;hence the electrodes can be considered as the Oxidation Reduction Electrodes.

It is prepared by dipping the inert metal electrode in its ionic Solution.

Ex: Oxidation Electrodes. Reduction Electrodes A)Pt/Cu+/Cu2+ . A) Pt/Q; QH2 ; H+ . B)Pt/Fe2+/Fe3+ . B) Pt/Cu2+/Cu+ .

C) Pt /Mn2+/Mn7+ . C) Pt/Ag+/Ag .


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D] Metal-Metal-Ion Electrode:

It is prepared by dipping a active metal electrode in its aqueous solution of the salt.

Examples:

Zn/Zn2+ ; Cu/Cu2+ . The electrode reaction is given as Zn + Cu2+ Zn2+ + Cu .

Such type of Electrodes cannot use very active metals like Na, K etc. as they readily react with water.


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E] Metal-Metal-Insoluble Salt Electrode:

It is an electrode that consists of metal in contact with its insoluble salts , that is turn again in contact with a solution containing the anion of the salt.

Examples:Ag/AgCl(s)/Cl- ; Cu/CuCl(s)/Cl- ; Hg/Hg2Cl2/Cl-

½ Hg2Cl2 Hg+ + Cl- (Oxidation)

Hg+ + e- Hg (reduction)

½ Hg2Cl2 + e- Hg + Cl- (net cell reaction)


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½ Hg2Cl2 + e- Hg + Cl-

F] Calomel Electrodes:

It is one of the important secondary reference electrodes.

The Cell representation is as follows:

.

The potential of this cell depends upon the concentration of the KCl. These electrodes are commercially available with KCl solution having 0.1M concentration or 1M concentration.

Electrode constructed using this electrode is referred to as SCE.

The reduction potential of this electrode is 0.334V relative to the SHE.The y are constructed using various forms.


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G] WESTON CELL: A Weston cell is an example of the cell that can be made to definite specifications definite e.m.f long lived and produces an e.m.f that changes little with temperature.

The important precautions that are to be taken while using this cell:

The cell is an important one in the analysis of Potentiometer but current should not be drawn for a long time ; otherwise its potential would be affected.


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Measurement of EMF:

A single electrode cannot constitute to the generation of Electric Current ; but a combination of two can generate an appreciable amt of current. The difference in the Electrode Potentials of the two electrodes is termed as Electrode Potential.

E cell = E1 – E2 (E1= electrode potential of Electrode 1)

(E2= electrode potential of Electrode 2) The equations used to determine the E.M.F of the cell is as follows:

1.] E cell = E anode – E cathode (Oxidation potentials) 2.] E cell = E cathode – E anode ( Reduction potentials) 3.] E cell = E cathode + E anode (E1= reduction potential; E2=oxidation potential )

Examples: The e.m.f of the cells with SHE as one electrode is :

0.76 V (Zn/Zn2+) ; -0.34 V ( Cu/Cu2+) ; -0.799 V (Ag/Ag+)


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Thermodynamic Properties of Cell Reaction and Cell Potential:

The e.m.f can be used to calculate the free energy change of the cell ;

As G = - nFE; so [δ( G)/dT]p = - S

Now; [δ (-nFE)/dT]p = - S --------------- (1)

implies nF[δE/dT]p = S or nF [δEo /dT] = So --------- (2)

Now , Free Energy Change ( G) is given as

G = H – T S or Go = Ho –T So

Substituting the values of S and G in the above equation ;we get ; Ho = nFT [δEo/dT] –nFEo implies Ho = nF[T [δEo/dT] –Eo ] .


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Application of Potential Measurements:

The following are the applications of the EMF of the Cell:

A] Nernst Equation.B] Equilibrium constants (K eq and K sp)C] Mean Activity Coefficient of Electrolyte.

The following are the Equations for the above applications:

A] E = Eo - (0.05915/n) . log {activities of Products/activities of reactants}.

B] K eq = antilog[n Eo / 0.05915] .

C] K sp = antilog[ Eo / 0.05915] . D] γ = antilog [Eo-E/0.1183] – log(C).


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CONCENTRATION CELLS

Electrode Concentration Cells Electrolyte Concentration Cells

Electrolyte Cell without Liquid Junction

Electrolyte Cell with Liquid Junction


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Salt Bridge:

The salt bridge is a device that connects the two half cells without allowing them to mix with each other.

It eliminates the use of Liquid Junction containing the saturated solution of KClas 1.] there is high conc. of KCl.2.] similar values of t+ and t- for KCl.3.] similar diffusibilities of K+ and Cl- ions.

The cells with the salt brides are represented as:

anode/salt bridge/cathode

Examples: Pt/H2(1atm)/KCl/Hg2Cl2/Hg === Pt/H2(1atm) // Hg2Cl2/Hg .


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REFERENCES:

Principles of Physical Chemistry by

Puri; Sharma; Pathania…

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EMF AND CONDTNCE