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# How to Balance Redox Equations in Alkaline (and Acid) Solutions?

The Fuel Cell of Hydrogen and Oxygen can take place in Acid or Alkaline Solution.

H₂ + 2 O₂ ------> 2 H₂O

In either situation we first balance the redox equations in acid solution

Oxidation Half Equation

H₂ -----> H₂O

1. Add H₂O to balance O

H₂O + H₂ -----> H₂O

2. Balance H with H+

H₂O + H₂ -----> H₂O + 2H⁺

3. Balance charge with e⁻

There is 0 on LHS and +2 on RHS so add 2e to the RHS

H₂O + H₂ ----> H₂O + 2H⁺ + 2e⁻

Cancel H₂O

The oxidation equation is balanced in acid solution

Now Balance in Alkaline Solution

3. Add same OH⁻ as H⁺ both sides

H₂ + 2 OH⁻ -----> 2 H⁺ + 2 OH⁻ + 2e⁻

4. OH⁻ and H⁺ form H₂O

H₂ + 2 OH⁻ -----> 2H₂O + 2e⁻

Reduction Half Equation

O₂ ---------> H₂O

1. Balance the oxygen atoms

O₂ ---------> 2 H₂O

2. Balance the H atoms with H⁺

O₂ + 4 H+ -----> 2 H₂O

3. Balance the charge using e⁻

O₂ + 4 H⁺ + 4 e- -----> 2 H₂O

Now balance in alkaline solution

4. Add OH⁻ to both sides

O₂+4H⁺ +4OH⁻+4e- > 2H₂O + 4OH⁻

5. OH⁻ and H⁺ form H₂O

O₂+4H₂O +4e- > 2H₂O + 4OH⁻

6. Cancel H₂O

O₂+2H₂O +4e- > 4OH⁻

Full Redox Equation

Combine the 2 half equations

O₂+2H₂O +4e- > 4OH⁻

H₂ + 2 OH⁻ -----> 2H₂O + 2e⁻ (×2)

1. Multiply so electrons cancel...

O₂+2H₂O +4e- > 4OH⁻

2H₂ + 4OH⁻ -----> 4H₂O + 4e⁻

2. Cancel e⁻ and OH⁻ and H₂O....

2 H₂ + O₂ -----> 2 H₂O

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