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By Oxidation Number Method


It involves the following steps
  1. Write the skeleton equation representing the actual chemical change.
  2. Assign oxidation numbers to the atoms that undergo change.
  3. Determine the change in oxidation number of the atoms involved.
  4. Multiply the ions in the equation by suitable coefficient so that the total increase in oxidation number is equal to the total decrease in oxidation number of the ions. 
Example
Balance the following redox reactions
  1. Zn + HNO3Zn(NO3)2 + NO + H2O
  2. KMnO4 + FeSO4 + H2SO4 K2SO4 + MnSO4 + Fe2(SO4)3 + H2O
Solution
  1. (i) Write the skeleton equation with O.N. of each element at its top.


    ii) Locate the atoms showing O.N. changes (oxidant and reductant)
    (Decrease of 3) 2 (Oxidant)

    +5                +2                 +2

    Zn0 + HNO3  Zn(NO3)2 + NO + H2O

    (increase of 2) 3(Reductant)

    iii) Balance the changes in oxidation number by multiplying the oxidant and reductant by suitable numbers.
    3Zn + 2HNO3Zn(NO3)2 + NO + H2O

    iv) Balance the equation atomically. Also balance HNO3 with NO only. (Except 'O' and 'H' atoms)
    3Zn + 2HNO33Zn(NO3)2 + 2NO + H2O

    v) Balance the NO3- ions by adding 6 molecules of HNO3 on the reactant side (the only source of NO3- ions).

    vi) Balance the 'O' atoms by adding three molecules of water on the product side.
    3Zn + 8HNO33Zn(NO3)2 + 2NO + 3H2O + H2O
    3Zn + 8HNO33Zn(NO3)2 + 2NO + 4H2O

  2. (i) Write the skeleton equation with O.N. of each element at its top.


    (ii) Write the O.N. change of oxidant and reductant


    (iii) Balance the O.N. change by multiplying the oxidant and reductant by suitable numbers
    2KMnO4 + 10FeSO4 + H2SO4 K2SO4 + MnSO4 + 5Fe2(SO4)3 + H2O

    iv) Balance the equation atomically.
    2KMnO4 + 10FeSO4 + H2SO4K2SO4 +2MnSO4 + 5Fe2(SO4)3 + H2O

    v) Balance SO42- ions by adding 7 molecules of H2SO4 on the reactant side (or make the no. of molecules of H2SO4 to be 8).
    2KMnO4 + 10FeSO4 + 8H2SO4K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + H2O

    vi) Balance 'O' atoms by making the no. of molecules of water to be 8.
    2KMnO4 + 10FeSO4 + 8H2SO4K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + 8H2O




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