Question

For a particular redox reaction MnO2 is oxidized to MnO4– and Cu2 is reduced to Cu . Complete and balance the equation for this reaction in basic solution. Phases are optional.

This is what I keep getting: http://tinypic.com/r/sq1hc3/9

Answer 1

Step 1

Write the unbalanced equation

MnO₂ + Cu²⁺ MnO₄^- + Cu

Step 2

Identify and combine these redox couples into two half-reactions

Oxidation

MnO₂ MnO₄^-

Reduction

Cu²⁺ Cu

Step 3

Balance the atoms in the half reaction.

a) Balance all atoms except hydrogen and oxygen.

Oxidation

MnO₂ MnO₄^-

Reduction

Cu²⁺ Cu

b) Balance the oxygen atoms.

Oxidation

MnO₂ + 2H₂O MnO₄^-

Reduction

Cu²⁺ Cu

c) Balance the hydrogen atoms.

Oxidation

MnO₂ + 2H₂O MnO₄^- + 4H⁺

Reduction

Cu²⁺ Cu

d) For reactions in a basic medium, add one OH- ion to each side for every H+ ion present

Oxidation

MnO₂ + 2H₂O + 4OH^- MnO₄^- + 4H₂O

Reduction

Cu²⁺ Cu

Step 4

Balance the charge

Oxidation

MnO₂ + 2H₂O + 4OH^- MnO₄^- + 4H₂O + 3e^-

Reduction

Cu²⁺ + 2e^- Cu

Step 5

Make electron gain equivalent to electron lost.

Oxidation

2MnO₂ + 4H₂O + 8OH^- 2MnO₄^- + 8H₂O + 6e^-

Reduction

3Cu²⁺ + 6e^- 3Cu

Step 6

Add the half-reactions together.

2MnO₂ + 3Cu²⁺ + 4H₂O + 6e^- + 8OH^- 2MnO₄^- + 3Cu + 8H₂O + 6e^-

Step 7

Simplify the equation.

2MnO₂ + 3Cu²⁺ + 8OH^- 2MnO₄^- + 3Cu + 4H₂O