Electrochemistry: Kohlrausch՚s Law, Electrochemical and Electrolytic Cell (For CBSE, ICSE, IAS, NET, NRA 2022)

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Kohlrausch՚s Law

“At infinite dilution, each ion of the electrolyte makes a definite contribution towards conductivity of the electrolyte and it is independent of the presence of other ions of the electrolyte.” This is called Kohlrausch՚s Law of independent migration of ions.

For a salt like , molar conductivity at infinite dilution can be written

In general, for a salt of formula

indicates molar conductivity at infinite dilution.

This law is used to calculate the molar conductivity at infinite dilution for weak electrolytes.

Electrochemical Cell

  • An electrochemical cell is a device used for the inter conversion of electrical and chemical energy.
  • An electrochemical cell contains two electrodes (cathode and anode) and an electrolyte.

These are of two types, based on the nature of conversion of energies:

Electrolytic Cell (Faradaic Cell)

  • In electrochemical cells, a battery is used to carry out redox reactions on the electrodes of the cell.
  • These cells convert electrical energy into chemical reaction.

Galvanic Cell (Voltaic Cell)

  • In galvanic cells an emf is developed as a result of redox reaction occurring on the electrodes.
  • These cells convert chemical energy into electrical energy.

Electrolytic Cell

An electrolytic cell consists of two electrodes connected to a battery as shown in fig.

Electrolytic Cell
  • In an electrolytic cell electrical energy is converted into chemical energy.
  • The process of decomposition of an electrolyte into its ions when an electric current is passed through it, is called electrolysis.
  • When electricity is passed through an electrolyte, a chemical change i.e.. decomposition of the electrolyte into ions takes place at the electrode.
  • ions migrate to the electrode (anode) and undergo oxidation by losing electrons in the electric field.
  • ions will go to electrode (cathode) and undergo reduction in the electric field.

The process can be represented as:

Oxidation at anode

Reduction at cathode

Faraday՚s Law of Electrolysis

Faraday՚s First Law of Electrolysis

  • It states that the mass of a substance liberated during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte taken as a solution or as melt.
  • Mathematically, if is the mass of a substance liberated or discharged on passing coulomb of electricity.

  • is the constant of proportionality is called electrochemical equivalent of the substance.
  • The quantity of electricity passed, , is related to the strength of the current passed in amperes , and the time in seconds, .

Faraday՚s Second Law of Electrolysis

It states that when the same quantity of electricity is passed through different electrolytes (taken in different electrolytic cells which are connected in series) the masses of different substances liberated are proportional to their chemical equivalent masses.

Let and be the masses of two substances liberated in two electrolytic cells connected in series, then:

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