Electrochemistry: Oxidation and Reduction as Electron Transfer (For CBSE, ICSE, IAS, NET, NRA 2022)

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Electrochemistry deals with the conversion of electrical energy into chemical energy and vice versa.

Oxidation and Reduction as Electron Transfer

  • Process in which an atom or ion loses one or more electron to other atom is called oxidation.
  • Process in which an atom or ion gains one or more electron is termed as reduction.
  • For example, formation of from and .

(loss of by ; oxidation)

(gain of by ; reduction)

  • Sodium undergoes oxidation and chlorine undergoes reduction.
  • A reductant is a species in a chemical reaction which loses its electron to another reactant.
  • Sodium helps chlorine to undergo reduction and therefore it is called a reducing agent and reductant .
  • An oxidant is a species which accepts electrons in a chemical reaction.
  • Chlorine, accepts electron, therefore it is an oxidizing agent.
  • Oxidation and Reduction processes occur simultaneously, so it is called oxidation-reduction reaction or Redox reaction.

Oxidation Number

  • Oxidation number apparent charge which an atom appears to have when each pair of electrons is counted with more electronegative atom.
  • The number indicate number of electrons that has been shifted from an atom towards a more electro- negative atom, in a heteronuclear covalent bond.
  • The sign for the atom shifting its electron away from itself and is given to more electro atom.

Rules for Assigning Oxidation Number

1. Oxidation number is taken as zero if atoms are present in elemental form. For example, .

2. The oxidation number of a monatomic ion is the same as the charge present on it. For example, will have oxidation number respectively.

3. The oxidation number of oxygen is in almost all the compounds except in peroxides e. g. where oxidation number is and super oxides where it is .

4. The oxidation number of is when combined with non-metal and is when combined with metal e. g. in the of is but in it is .

5. The Oxidation Number of alkali metal is in its compounds.

6. The sum of the oxidation numbers of all the atoms in a neutral compound is zero.

7. In a polyatomic ion, the sum of the oxidation numbers of all the atoms is equal to the charge on the ion. e. g. in , the sum of oxidation Number of carbon and oxygen is .

Balancing Redox Reaction

The redox reaction can be balanced by any of the following methods:

  • Oxidation number method
  • Ion Electron method

Balancing by Oxidation Number Method

Steps involve in balancing redox reaction by this method is as follows:

  • Write the skeletal equation of reaction i.e.. chemical equation without the stoichiometric coefficient.
  • Write the oxidation number of each atom above its symbol in the equation.
  • Identify the atoms undergoing change in oxidation number.
  • Calculate the increase or decrease in oxidation number per atom for the atom undergoing a change in oxidation number. If more than one atom is involved, multiply the increase or decrease in number with the number of atoms undergoing the change to determine the total change in oxidation number.
  • Equate the increase and decrease in oxidation number on the reactant side by multiplying the formulae of the oxidizing and reducing agents suitably.
  • Balance the equation with respect to all the atoms except hydrogen and oxygen.
  • Finally balance and also.
  • If the reaction is taking place in acidic medium balance the atoms by adding required number of molecule on the side where atoms are less in number. Balance the atoms by adding to the side deficient in atoms.
  • If the reaction is taking place in the basic medium by add required number of negative charges by adding required number of ions to the side deficient in the magnitude of charges, then add molecules to balance ions.

For example - When Phosphorus is treated with nitric acid, nitric oxide is formed.

The skeletal equation is

Write the oxidation number of each atom on the given skeletal equation

and are undergoing change in Oxidation Number.

Increase of

Decrease in

Equating the increase and decrease in Oxidation Number of and on the reactant side.

Balance the P and N atoms on both sides of the equation

and are already balanced in the equation.

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