redox reaction

Question

4. Balance the following redox reactions (using any method you prefer).

(8 K/U)

a) As2O3 (s) + NO3-(aq)  H3AsO4 (aq) + NO (g)

b) MnO (s) + PbO2 (s)  MnO4-(aq) + Pb2+(aq

Answer 1

Balancing redox reactions is quite involved, so I'll just show you the first one.

 

As₂O₃ → H₃AsO₄

As₂O₃ → 2H₃AsO₄             balance As

5H₂O + As₂O₃ → 2H₃AsO₄       balance O by adding H₂O to one side

5H₂O + As₂O₃ → 2H₃AsO₄ + 4H⁺    balance H by adding H⁺ to one side

5H₂O + As₂O₃ → 2H₃AsO₄ + 4 H⁺ + 4e^-     balance charge by adding electrons to one side

 

Now do the same for the other part of the reaction

NO₃^- → NO

NO₃^- → NO + 2H₂O

4H⁺ + NO₃^- → NO + 2H₂O

3e^- + 4H⁺ + NO₃^- → NO + 2H₂O

 

Now cancel the electrons by multiplying the first equation by 3 and the second equation by 4, then add them together

 

3As₂O₃ + 4NO₃^- + 7H₂O + 4H⁺ → 6H₃AsO₄ + 4NO