Electrochemistry

Conductance in Electrolytic Solutions

 * An electrolytic solution is a solution through which electric current can be passed.
 * Conductance (G) is the reciprocal of resistance.
 * Conductivity (k) is the reciprocal of resistivity.
 * Molar Conductivity(Λ) of a solution is defined as the electrolytic conductivity k divided by the molar concentration (C) of the dissolved electrolyte . Λ= k/C
 * Λ = kV = 1000 k / C.  (Concentration in terms of Molarity)
 * Molar Conductivity of strong electrolytes varies as Λ = Λo - a √C
 * Kohlrausch law states that at infinite dilution, each ion migrates independently of its co-ion and makes its own contribution of an electrolyte irrespective of the nature of other ion .                                        .   for AxBy ;                            Λo = xλ+o + yλ-o
 * α = Λ / Λ o... K = Λ2C / Λ0(Λ 0- Λ )
 * Measurement of Conductivity
 * Determination of Cell Constant
 * Determination of Conductivity
 * Equivalent Conductivity = Conductivity / No. of Valence Electrons

Electric Conduction
Metallic Conduction : The current flowing through metals due to potential difference.

Ionic or Electrolytic Conduction : The current flowing through a solution due to the migration of cations and anions.

Electrodes
Cathodes

Redution takes place at Cathode.

Anodes

Oxidation takes place at Anode.

Electrolytic Cells
Electrolysis is defined as passing of electric current through a solution to make a chemical reaction occur spontaneously. An electrolytic cell provides potential difference for carrying out electrolysis.

Electrolysis of NaCl
Fused NaCl

Reduction : Na++ e- ---> Na

Oxidation : 2Cl- > Cl2 + 2e-

Net Reaction : 2Na+ + 2Cl- ---> 2 Na + Cl2

Cl2 has been collected at the anode.

Na has been collected at the cathode.

Aqueous NaCl

Reduction : Na+ + e- --> Na

2H2O + 2e- --> H2 + 2OH-

Oxidation : Cl- --> Cl + e-

2H2O --> O2 + 4H+ + 4e-

Net Reaction : 2Cl- + H2O ---> Cl2 + H2 + 2OH-

H2 gas is liberated at cathode.

Cl2 gas is liberated at anode.

OH -ions are are formed in reduction half reaction. Na remains unatracted

NaCl is converted to NaOH.

This, we can conclude that Sodium cannot be obtained by electrolysis of Aqueous solution. Like Sodium, none of the s-block elements can be obtained by electrolysis of their aqueous solutions.

Faraday's Laws of Electrolysis
Faraday's First Law of Electrolysis :

The amount of substance that undergoes oxidation or reduction is directly proportional to the amount of current passed through it.

w α q

w α It

w = zIt, where w is the weight deposited at the electrode , z is the electro chemical equivalent

z = grams deposited by 1 Coloumb of electricity

Faraday's Second Law of Electrolysis :

When same amount of electricity is passed through different cells containing different electrolytes and arranged in series, the amount of substances oxidized or reduced at the respective electrodes are directly proportional to their equivalent masses.

W1 / W2 = E1 / E2

1 Farad = 6.022 x 1023 x 1.6022 x 10-19 = 96500 Coloumb mol -1

1 Farad of electricity, deposits 1 equivalent weight of the substance

Electrochemical Equivalent and Equivalent Weight

w / eq. wt = z / eq. wt

equivalent weight = electrochemical equivalent x 96500

Galvanic or Voltaic Cells
In a galvanic or voltaic cell, a spontaneous chemical reaction generates electric current.

Salt Bridge
A salt bridge is needed in Voltaic cells, to provide electrical contact between two solutions. It also prevents the diffusion of ions into each other. In most galvanic cells, a salt bridge is needed for reaction to occur. A salt bridge is a narrow tube, filled with agar agar gel and KCl (Cl ions).

Electrode Potential & Cell Potentials
Cell Potential

The cell potential, also called the Electro-Motive Force , measures the tendency of the cell reaction to occur. The standard cell potential is the cell potential when reactants & products are in the standard states.

The cell potential is related to Gibbs Energy changes to the of the cell reactions by the equations

Δ G = - nFE

Electrode Potential

The electrode potential is the measure of tendency of an element to undergo reduction.

Common Types of Cells
Dry Cell

Lead Accumulators

Nickel - Cadmium Cell

Fuel Cells
Hydrogen - Oxygen Fuel Cell

Electrochemical Series
Electrochical Series is an arrangement of electrodes in decreasing order of their standard reduction potentials.

Higher the reduction potential, greater is the tendency to get reduced , better is the oxidizing agent.

Lower the reduction potential, greater is the tendency to get oxidized , better is the reducing agent.

Some important comparisons :

Elec

Corrosion
Corrosion is an electrochemical process in which metal (Fe) is oxidized in an anode region of metal surface and oxygen is reduced in the cathode region.