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Redox Reactions

Question
CBSEENCH11006784

Balance the following equation in basic medium by ion-electron method and oxidation number method and identify the oxidising agent and the reducing agent.
straight N subscript 2 straight H subscript 4 left parenthesis straight l right parenthesis space plus space ClO subscript 3 superscript minus left parenthesis aq right parenthesis space space rightwards arrow space space space NO left parenthesis straight g right parenthesis space plus space Cl to the power of minus left parenthesis straight g right parenthesis

Solution

(A) Ion electron method:
1. Write the oxidation and reduction half reaction by observing the changes in oxidation number.
           Oxidation half reaction:
              -2           +2
             straight N subscript 2 straight H subscript 4 left parenthesis straight l right parenthesis space space rightwards arrow space space space NO left parenthesis straight g right parenthesis 
                                    +5             -1
Reduction half reaction:    ClO subscript 3 superscript minus left parenthesis aq right parenthesis space space space rightwards arrow space space space Cl to the power of minus
2. Balancing the oxidation half reaction:
  (i) Balancing N atoms by multiplying NO by 2
                       straight N subscript 2 straight H subscript 4 left parenthesis straight g right parenthesis space space space rightwards arrow space space space space 2 NO left parenthesis straight g right parenthesis
(ii)  Add eight electron towards R.H.S. to balance the charges
                              straight N subscript 2 straight H subscript 4 left parenthesis straight g right parenthesis space space space rightwards arrow space space space space 2 NO left parenthesis straight g right parenthesis space plus space 8 straight e to the power of minus
(iii) Add eight OH- ions towards LHS to balance the charge. 
                straight N subscript 2 straight H subscript 4 left parenthesis straight g right parenthesis space plus space 8 OH to the power of minus left parenthesis aq right parenthesis space space rightwards arrow space space 2 NO left parenthesis straight g right parenthesis space plus space 8 straight e to the power of minus
(iv) Balance O atoms by adding six H2O molecules towards R.H.S.
                 straight N subscript 2 straight H subscript 4 left parenthesis straight g right parenthesis space plus space 8 OH to the power of minus left parenthesis aq right parenthesis space space rightwards arrow space space space 2 NO left parenthesis straight g right parenthesis space plus space 6 straight H subscript 2 straight O left parenthesis straight l right parenthesis space plus space 8 straight e to the power of minus
                                   [Balanced oxidation half reactions]
3. Balancing the reduction half reaction:
(i) Add 6 electrons towards LHS to balance the charges on Cl atom
               ClO subscript 3 superscript minus left parenthesis aq right parenthesis space plus space 6 straight e to the power of minus space space rightwards arrow space space space Cl to the power of minus left parenthesis aq right parenthesis
(ii) Add six OH- ions towards RHS to balance the charges
                     ClO subscript 3 superscript minus left parenthesis aq right parenthesis space plus space 6 straight e to the power of minus space space rightwards arrow space space space Cl to the power of minus left parenthesis aq right parenthesis space plus space 6 OH to the power of minus left parenthesis aq right parenthesis
(iii) Balance O atoms by adding three H2O molecules towards LHS.
ClO subscript 3 superscript minus left parenthesis aq right parenthesis space plus space 3 straight H subscript 2 straight O left parenthesis straight l right parenthesis space plus space 6 straight e to the power of minus space space space rightwards arrow space space space Cl to the power of minus left parenthesis aq right parenthesis space plus space 6 OH to the power of minus left parenthesis aq right parenthesis
space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space open square brackets Balanced space reduction space half space reaction close square brackets
(iv) Multiplying balanced oxidation half reaction by 3 and reduction half reaction by 4 to equate the electrons and add both the half reactions.
3 straight N subscript 2 straight H subscript 4 left parenthesis straight l right parenthesis space plus space 4 ClO subscript 3 superscript minus left parenthesis aq right parenthesis space rightwards arrow space space 6 NO left parenthesis straight g right parenthesis space plus space 4 Cl to the power of minus left parenthesis aq right parenthesis space plus space 6 straight H subscript 2 straight O left parenthesis straight l right parenthesis
This is the balanced redox equation.

(b) Oxidation number method
(i) The skeleton equation along with oxidation number of each atom is
-2 + 1             +5                +2 -2           -1
straight N subscript 2 straight H subscript 4 left parenthesis straight l right parenthesis space plus space ClO subscript 3 superscript minus left parenthesis aq right parenthesis space space space rightwards arrow space space space space straight N space straight O left parenthesis straight g right parenthesis space plus space Cl to the power of minus left parenthesis aq right parenthesis
(ii) The oxidation number of N increase by 4 per N atom while that of CI decreases by 6 per atom.

Therefore, straight N subscript 2 straight H subscript 4 acts as the reducing agent while ClO subscript 3 superscript minus acts as the oxidising agent.
(iii) Equalise the increase/decrease in O.N. by multiplying straight N subscript 2 straight H subscript 4 space by space 3 and ClO subscript 3 superscript minus space by space 4.
(iv) To balance N and Cl atoms, multiplying NO by 6 and Cl- by 4.
3 straight N subscript 2 straight H subscript 4 left parenthesis straight l right parenthesis space plus space 4 ClO subscript 3 superscript minus left parenthesis aq right parenthesis space space rightwards arrow space space space 6 NO left parenthesis straight g right parenthesis space plus space 4 Cl to the power of minus left parenthesis aq right parenthesis
(v) Balance O atoms, by adding six H2O molecules towards R.H.S.
           3 straight N subscript 2 straight H subscript 4 left parenthesis straight l right parenthesis space plus space 4 ClO subscript 3 superscript minus left parenthesis aq right parenthesis space space rightwards arrow space space space 6 NO left parenthesis straight g right parenthesis space plus space 4 Cl to the power of minus left parenthesis aq right parenthesis space plus space 6 straight H subscript 2 straight O left parenthesis straight l right parenthesis
which is the balanced equation.