Class 11 Chemistry Redox Reactions Rules to assign and calculate oxidation number

Rules to assign and calculate oxidation number

  1. The oxidation number of atoms in their elemental state is taken as zero.
  2. The oxidation number of mono-atomic atoms like Na+ etc is taken as 1.
  3. The oxidation number of Hydrogen is +1 when present with non metals and -1 when present with metals.
  4. The oxidation number of oxygen is -2 in most of the compounds but in peroxides it is -1.
  5. The metals always have oxidation number in positive and non metal in negative when present together in ionic compounds.
  6. In compounds that have two atoms with different electro negativities, the oxidation number of more electronegative is taken as –ve and for less electronegative it is taken as positive. For example: In OF2 the oxidation number of oxygen will be in positive and oxidation of fluorine will be in negative.
  7. In neutral compounds, the sum of all oxidation numbers is equal to zero.
  8. In complex ions, the sum of oxidation states of all the atoms is equal to the charge present on the complex.

Oxidation: It is defined as increase in oxidation number of an element.

   Mg  +        H2 -->       Mg2+ H2

Magnesium Hydrogen  MagnesiumHydride

In this, Magnesium oxidation number is increased from 0 to +2.Therefore, oxidation occurs.

Reduction: It is defined as decrease in oxidation number of an element.

For Example:- 

Fe3+ + e-  --> Fe2+

Ferric Ion         Ferrous Ion

In this oxidation number of iron is reduced .Therefore, reduction occurs.

Redox reaction in terms of oxidation number

In this, Zinc is undergoing oxidation and act as reducing agent whereas Hydrogen is undergoing reduction and act as an oxidizing agent.

Zn0 +       H+Cl-               --> Zn2+ + 0H2

Zinc  HydrogenChloride  Zinc Chloride  Hydrogen Gas

Problem 1:-

Q: Find the oxidation number of N in NO32- ?

Answer 1:-


Problem 2:-

Q: Find the oxidation number of S in H2SO4 ?

Answer 2:-


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