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
If you'd like my help with A Level Chemistry book a free consulation here...
#FuelCells
