Electrochemistry

Electrochemistry Class 12 Notes:

Conductance-Conductance is the measure of the ease with which current flows through a conductor.

Conductivity- it is defined as conductance of a solution of 1 cm length and having 1 sq. cm as the area of cross-section.

Molar conductivity-Molar conductivity is described as the conducting power produced by the ions by dissolving one mole of electrolyte in a solvent.

m = / C

Conductivity and molar conductivity-

m= κ×1000/M

Equivalent conductivity- Equivalent conductivity is defined as the conductivity power of combining ions formed by the dissolution of electrolyte of one gram equivalent in a solution.

e= κ×1000/Ceq

Variation of conductivity and molar conductivity with concentration-

Electrolytic conductance decreases with an increase in concentration or increases with the increase in dilution.

Molar conductivity increases with dilution.

The equation of Debye Huckel Onsager shows the variation of molar conductivity along with concentration for strong electrolytes.

m=m⁰-AC1/2

Variation of molar conductivity with concentration for weak electrolytes- Molar conductivity of weak electrolytes cannot be found for weak electrolytes because dissociation of weak electrolytes is much lesser compared to strong electrolytes.
For weak electrolytes, molar conductivity at infinite dilution can be found using Kohlrausch law.

Kohlrausch’s Law-

At infinite dilution when ions are completely dissociated, every ion makes its unique contribution to the molar conductivity of the electrolyte irrespective of the nature of the other ion with which it is associated.

0=v++ + v--

Applications of Kohlrausch’s law-

Calculation of molar conductance at infinite dilution for weak electrolytes
The degree of dissociation of weak electrolytes is calculated as

α= c⁰

Calculation of dissociation constant of weak electrolytes.

K=C(0)2(1-0)

Calculation of solubility of sparingly soluble salts.

solubility= κ×1000

Galvanic cells- Converts chemical energy of spontaneous reaction into electrical energy.

Two half cells -

Cu+2 +2e-→Cu(s) (reduction half cell)

Zn(s)→Zn2++2e- (oxidation half cell)

Overall cell reaction-

Zn(s)+ Cu+2(aq)→ Zn2+(aq)+Cu(s)

Cell potential- Potential difference between two electrodes of a galvanic cell is called cell potential.

EMF (electromotive force)- the potential difference between anode and cathode when no current is drawn through the cell.

Feasibility of a reaction-

Ecell= Eright-Eleft

Reduction Half-

Oxidation Half-

For the above reaction, the reaction is feasible if

Ecell=EAg+/Ag -ECu+2/Cu is positive.

SHE (Standard hydrogen electrode) is assigned a zero potential to determine the potential of individual half cells.

-It is denoted by Pt(s)│H2(g)│H+(aq)

Nernst equation-

For reaction-

Mn+aq+ne-→Ms

E(Mn+│M) =EoM-RTnF ln[M(s)][Mn+(aq)]

E(Mn+│M) =EoM-2.303RTnF log[M(s)][Mn+(aq)]

E(Mn+│M) =EoM-2.303×8.314×298n×96500 log[M(s)][Mn+aq]

E(Mn+│M) =EoM-0.059nlog[M(s)][Mn+(aq)]

Here [M(s)] is taken as zero

E(Mn+│M) =EoM-0.059nlog1[Mn+(aq)]

E=E0-0.059nlog1[Mn+(aq)] at 25⁰C

Applications of Nernst equation-

Determining the cell potential using Nernst equation-

For a chemical reaction-

aA+bB→cC+dD

Ecell=E0cell-RTnFlnQ

At 298K, Ecell=E⁰cell-0.059nlog[C]c[D]d/[A]a[B]b

Determination of concentration of a solution of half-cell-

Using the Nernst equation, the concentration of the unknown species can be found out.

To find equilibrium constant using Nernst equation-

At equilibrium, Nernst equation takes the form of –

E⁰cell=2.303 RTnF logK

Electrochemical cell and Gibbs Energy-

∆rG=-nFEcell

This equation can help to predict the feasibility of the reaction.

Electrolysis- The process in which chemical changes take place due to the passage of current.

Faraday’s Law of electrolysis-

Faraday’s first law of electrolysis – It says that the quantity of substance settled at the electrode is in direct proportion with the amount the electricity passed through the solution.

w α ZQ

where w is the gram of substance deposited on passing Q coulombs of electricity if a current of 1 ampere is passed for t seconds.

Faraday’s second law of electrolysis- It says that when the equal amount of electricity is passed through different solutions lined up in series, the mass of the substance settled at the electrodes is in direct proportion with the equivalent weight.

Weight of Cu deposited Weight of Ag deposited=Eq. wt. of Cu Eq. wt. of Ag

Batteries-

Primary batteries

Dry cells- Found in torches, flashlights, calculators, tape recorders, and many other devices.

Reactions occurring at the electrode are-

Anode

Zn→Zn+2+2e-

Cathode

2NH4+(aq) + 2MnO2+2e-→Zn+2 + 2MnOOH+2NH3

Overall-

Zn+2NH4+ (aq) + 2MnO2 + Zn+2 + 2MnOOH+2NH3

Mercury cell-

-Found in electrical circuits.

-Reactions occurring at the electrodes are-

Anode-

ZnHg+2OH-→ZnOs+H2O+2e-

Cathode-

HgO(s)+ H2O+2e-→Hgl+2OH-

Overall-

ZnHg+ HgO(s)→ ZnO(s)+2OH-

Secondary batteries-

Lead storage batteries-

-Battery used in automobiles.

-Reactions taking place at electrodes-

Anode-

Pbs+SO42-(aq)→PbSO4(s)+ 2e-

Cathode-

PbO2(s)+SO42-(aq)+ 4H+ (aq)+ 2e-→PbSO4+2H2O

Overall-

Pb+ PbO2+2H2SO4(aq)→ 2PbSO4+ 2H2O

Nickel-cadmium storage cell-

-Has a longer life than the lead storage battery.

-Reactions occurring at electrodes-

Anode-

Cd+2OH-→CdO + H2O+2e-

Cathode−

2Ni(OH)3 + 2e-→2Ni(OH)2+2OH-(aq)

Overall-

Cd+2Ni(OH)3→CdO+2Ni(OH)2(s) + H2O(l)

Fuel Cells-

Features
Reactants are supplied continuously.
The energy of Combustion of fuels such as H2, CO, CH4, etc. is converted to electrical energy.
Reactions taking place at electrodes-

Anode-

2[H2+2OH-(aq)→2H2O+2e-]

Cathode-

O2+2H2O + 4e-→4OH-(aq)

Overall-

2H2(g)+ O2→2H2O