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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