Electrolysis

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The word ‘electrolysis’ is derived from two words ‘electro’ meaning electrical energy and ‘lysis‘ meaning dissociation (breakdown). The process of decomposition of a molten substance or its aqueous solution by passing an electric current is called electrolysis. In other words, electrolysis is a process in which electrical energy is used to bring about a non-spontaneous chemical reaction.

William Nicholson showed the electrolysis of water.

Components Required for Electrolysis

The three components required for the process of electrolysis are

Electrolyte

The compound which can conduct electricity in liquified state or in an aqueous state is termed as the electrolyte. e.g., acid, base, salt, etc.

On the basis of dissociation, electrolytes are of two types: strong electrolyte and weak electrolyte. Strong electrolytes dissociate completely while weak electrolytes dissociate only partially, NaCl, KCl, CaCl2, MgSO4 are the examples of strong electrolytes while CH3COOH is an example of weak electrolyte.

Non-electrolytes are bad conductor of heat because they do not dissociate into their ions when dissolved in water. e.g., urea, glucose, sugar, etc.

Electric Current

The flow of electrons in a conductor is termed as electric current. This result in the transfer of ions to the respective terminal.

Electrode

A solid electric conductor through which an electric current enters or leaves electrolytes is termed as an electrode. It is used to make electrical contact with some part of circuit

Arrhenius Theory of Electrolytic Dissociation or Decomposition

This theory was proposed by Sweden chemist Arrhenius in 1894 in order to explain the behavior of electrolytes in aqueous solutions.

Main postulates of this theory are as follows

  1. When an electrolyte is dissolved in water, it dissociates into its ions i.e., cations (positive ions) and anions (negative ions) and this phenomenon are called ionization.
  2. In ordinary conditions, weak electrolytes dissociate in solution to a small extent and the solution of these electrolytes contains ions which are in equilibrium with unionized molecules. Such an equilibrium is called ionic equilibrium.
  3. Electrolysis takes place only at electrodes
  4. The conductivity of the solution depends upon the number of ions present in the solution.

Faraday’s Laws of Electrolysis

Michael Faraday performed various experiments on the phenomenon of electrolysis and their results were published in 1833-34. On the basis of these experiments, Faraday gave the following two laws called the Faraday’s laws of electrolysis.

 First Law

The amount of chemical reaction which occurs at an electrode during electrolysis by current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).

m =ZQ = Zit

where, Z = electrochemical equivalent, m =substance deposited in gram

i= current in ampere, t = time in second, Q = charge in coulomb

When, i = 1A and t=1s then m=Z

i.e,. The electrochemical equivalent is defined as the mass of substance deposited when 1-ampere electricity is passed for 1 second (or 1-coulomb charge is passed) through a solution.

Second Law

A number of different substances liberated at the electrodes by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic mass of metal ÷ Number of electrons required to reduce the cation).

If W1, W2, W3 are the deposited amounts of the substances and E1, E2, E3 are their respective chemical equivalnet weights then

W1/W2 = E1/E2 and W1/W2=E2/E3 or Z∝ E

Thus, the electrochemical equivalent of a substance is directly proportional to the chemical equivalent of the substance

  • Faraday If an electric current due to 96487 coulomb charge is passed for 1 second through an electrolyte, it will deposit one equivalent weight of the substance. This amount of electric current is called 1 Faraday

1 Faraday = 96500 C mo-1

  • It is basically the charge on 1 mole of electron, its exact value is 96487.

Products of Electrolysis

Products of electrolysis depend upon the nature of material being electrolysed and the type os electrodes being used. e.g., if we use molten NaCl, the products of electrolysis are sodium metal and chlorine gas. During the electrolysis of an aqueous sodium chloride solution, the products are NaOH, Cl2 and H2. Electrolysis of an aqueous solution of copper sulphate using copper elecctrodes produces copper at cathode.

Products of electrolysis also depend on the different oxidizing and reducing species present in the electrolytic cell and their standard electrode potentials.

  • A potential difference develops between the electrode and the electrolyte, this is called electrode potential
  • When the concentration of all the species involved in a half cell is unity, then the electrode potential is known as standard electrode potential.

Application of Electrolysis

In Electrorefining of Metals

Pure form of copper, silver gold are obtained by electrorefining process in which anode is made up of impure metal and a thin strip of pure metal acts as cathode. A salt solution of the metal is generally used as electrolyte. The copper obtained by this process in 99.9% pure.

In Electroplating Objects

Electroplating is the process of electrolysis in which the desired metal is deposited on the another material to provide shiny appearance and prevent it from corrosion and scratch.

e.g., chromium plating is done on many object such as car parts, etc., Jewellery makers electroplate gold on silver or copper or nickel ornaments by placing these metals in a solution having a salt of gold and by passing an electric current. Tin cans used for storing food are made by electroplating tin onto iron.

In Electrotyping

It is used in printing industries for making blocks, graphics, etc. e.g., in the large printing press, a thin layer of copper is coated on the printing paper by using copper voltmeter and replacing cathode by printing paper because on copper coated papers, excellent prints appear.

In Electrometallurgy

Electrometallurgy is the process of extraction of certain metals like calcium, aluminum, magnesium, etc from respective compounds.

In manufacturing of Compounds 

Chemical compounds like chloroform, ethane acetylene, drugs are manufactured by the process of electrolysis.

In the evaluation of equivalent weight of Metals

The equivalent weight of the metals is calculated by using the formula w ∝ E.

In Electrolytic Capacitor

Here, aluminum metal is used for making both the electrodes. The mixture of boric acid, glycerine, and aqueous ammonia is used as the electrolyte. On passing electric current, a layer of aluminum hydroxide deposits on the anode which acts as dielectric for the electrodes

 

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