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
* α = Λ / Λ o ... K = Λ2C / Λ0(Λ 0 - Λ )
* Measurement of Conductivity
* Determination of Cell Constant
* Determination of Conductivity : k = b/R
* Equivalent Conductivity = Conductivity / No. of Valence Electrons

## Kohlrausch Law

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

## 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 .

## Electrochemical Cells

 Electric Energy is converted to Chemical Energy

## 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

## 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 .

## Test Yourself !

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Take a 30 minute test here : Test : Electrochemistry