26. Balance the following ionic equations.

(a) Cr2O72-+H++I-Cr3++I2+H2O

(b) Cr2O72-+Fe2++H+Cr3++Fe3++H2O

(c) MnO4-+SO32-+H+Mn2++SO42-+H2O

(d) MnO4-+H++Br-Mn2++Br2+H2O


(a) Write the O.N. of all atoms above their respective symbols.
O.N. decreases by, 3 per Cr-atom
Divide the given equation into two half reactions
Reduction half reaction : Cr2O7Cr3+
Oxidation half reaction : I-I2
To balance reduction half reaction.
Cr2O72-+14H++6e-2Cr3++7H2O
To balance oxidation half reaction.
2I-I2+2e-
To balance reaction by electrons gained and lost
Cr2O72-+14H++6e-2Cr3++7H2O6I-3I2+6e-Cr2O72-+14H++6I-2Cr3++3I2+7H2O
This gives the final balanced ionic equations.
(b) Write the skeletal equation of the given reaction
Cr2O72-(aq)+Fe2+(aq)Cr3+(aq)+Fe3+(aq)
Write the O.N. of all the elements above their respective symbols.
Divide the given equation into two half reactions
Oxidation half reaction : Fe2+(aq)Fe3+(aq)
Reduction half reaction : Cr2O72-(aq)Cr3+(aq)
To balance oxidation half reaction
Fe2+(aq)Fe3+(aq)+e-
To balance reduction half reaction
Cr2O72-(aq)+6e-2Cr3+(aq)
Balance charge by adding H+ ions.
Cr2O72-(aq)+14H++6e-2Cr3+(aq)
Balance O atoms by adding H2O molecules
Cr2O72-(aq)+14H++6e-2Cr3+(aq)+7H2O(l)
To balance the reaction
6Fe2+(aq)6Fe3+(aq)+6e-Cr2O72-(aq)+14H+(aq)+6e-2Cr3+(aq)+7H2O(l)Cr2O72-(aq)+6Fe2+(aq)+14H+(aq)2Cr3+(aq)+7H2O(l)+6Fe3+(aq)
(c) Write the O.N. of all atoms above their respective symbols.
Divide the skeleton equation into two half-reactions.
Reduction half reaction : MnO4-Mn2+
Oxidation half reaction : SO32-SO42-
To balance reduction half reaction
MnO4-+8H++5e-Mn2++4H2O
To balance oxidation half reaction
SO32-SO42-+2e-
Balance charge by adding H+ ions.
SO32-SO42-+2H++2e-
Balance O-atoms by adding H2O
SO32-+H2OSO42-+2H++2e-
To balance the reaction
2MnO4-+16H++10e-2Mn2++8H2O5SO32-+5H2O5SO42-+10H++10e-2MnO4-+5SO32-+6H+2Mn2++5SO42-+3H2O
This represents the correct balance redox equation.
(d) Write the O.N. of all the atoms above their respective symbols.
Divide skeleton equation into two half reactions
Reduction half reaction : MnO4-Mn2+
Oxidation half reaction : Br-+Br2
To balance half reaction
MnO4-+8H++5e-Mn2++4H2O
To balance oxidation half reaction
2Br-Br2+2e-
To balance the reaction
2MnO4-+16H++10e-2Mn2++8H2O10Br-5Br2+10e-2MnO4-+10Br-+16H+2Mn2++5Br2+8H2O
This represents the correct balance ionic equation.