Balanicng Redox Reactions

Question

Complete and balance the following redox reaction in basic solution. Be sure to include the proper phases for all species within the reaction.

Cr2O7 2-(aq)+I2(s) --> Cr3+(aq)+IO3 -(aq)

Answer 1

Cr₂O₇^2- ==> Cr³⁺ unbalanced reduction reaction

Cr₂O₇^2- ==> 2Cr³⁺ + 7H₂O .. balanced for Cr and O

Cr₂O₇^2- + 14H₂O ==> 2Cr³⁺ + 7H₂O + 14OH^- .. balanced for Cr, O and H by adding base (OH^-)

Cr₂O₇^2- + 14H₂O + 6e- ==> 2Cr³⁺ + 7H₂O + 14OH^- .. balanced for mass and charge = balanced eq.

I₂ ==> IO₃^- unbalanced oxidation reaction

I₂ + 6H₂O ==> 2 IO₃^- .. balanced for I and O

I₂ + 6H₂O + 12 OH^- ==> 2 IO₃^- + 12H₂O .. balanced for I, O and H by adding base (OH^-)

I₂ + 6H₂O + 12 OH^- ==> 2 IO₃^- + 12H₂O + 10e- .. balanced for mass and charge = balanced eq.

In order to equalize the electrons, multiply reduction reaction by 5 and oxidation reaction by 3. Then add the two reactions and combine/cancel like terms to end up with the final balanced redox equation.

5Cr₂O₇^2- + 70H₂O + 30e- ==> 10Cr³⁺ + 35H₂O + 70 OH^-

3 I₂ + 18H₂O + 36 OH^- ==> 6 IO₃^- + 36H₂O + 30e-

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5Cr₂O₇^2- + 17H₂O + 3 I₂ ==> 10Cr³⁺ + 34 OH^- + 6 IO₃^- ... BALANCED REDOX EQ.