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.
In either situation we first balance the redox equations in acid solution. The product is water.
Oxidation Half Equation (Acid Solution)
H₂ -----> H₂O
1. Add H₂O to balance O atoms
H₂O + H₂ -----> H₂O
2. Balance H atoms with H+
H₂O + H₂ -----> H₂O + 2H⁺
3. Balance charge with e⁻
There is 0 charge on LHS and +2 on RHS so add 2e to the RHS
H₂O + H₂ ----> H₂O + 2H⁺ + 2e⁻
4. Cancel H₂O
The oxidation half equation is balanced in acid solution: H₂ ----> 2H⁺ + 2e⁻
Now Balance in Alkaline Solution
1. Add the same amount of OH⁻ as H⁺ to both sides of the equation.
H₂ + 2 OH⁻ -----> 2 H⁺ + 2 OH⁻ + 2e⁻
2. OH⁻ and H⁺ combine to form H₂O
H₂ + 2 OH⁻ -----> 2H₂O + 2e⁻
The above oxidation half equation is balanced in alkaline solution!
Reduction Half Equation (Acid Solution)
O₂ ---------> H₂O
1. Balance the oxygen atoms with H₂O
O₂ ---------> 2 H₂O
2. Balance the H atoms with H⁺
O₂ + 4 H+ -----> 2 H₂O
3. Balance the charge using e⁻
O₂ + 4 H⁺ + 4e- --> 2 H₂O
The above reduction half equation is balanced in acid solution!
Now balance in alkaline solutiont
Start with the reduction equation in acid solution
O₂ + 4 H⁺ + 4e- --> 2 H₂O
1. Add the same amount of OH⁻ as H+ to both sides of the half equation
O₂+4H⁺ +4OH⁻+4e- > 2H₂O + 4OH⁻
2. OH⁻ and H⁺ form H₂O
O₂+4H₂O +4e- > 2H₂O + 4OH⁻
6. Cancel 2H₂O each side
O₂+2H₂O +4e- > 4OH⁻
The reduction half equation is balanced in alkaline solution!
Full Redox Equation
Acid Solution:
Combine the 2 half equations
H₂ ----> 2H⁺ + 2e⁻ (X2)
O₂ + 4H⁺ + 4e- --> 2 H₂O
1. Multiply so electrons balance..
2H₂ ----> 4H⁺ + 4e⁻
O₂ + 4 H⁺ + 4e- --> 2 H₂O
2. Cancel 4e⁻ and 4H+....
2 H₂ + O₂ -----> 2 H₂O (Full redox equation)
Alkaline Solution:
Combine the 2 half equations:
O₂ + 2H₂O + 4e⁻ ----> 4OH⁻
H₂ + 2OH⁻ -----> 2H₂O + 2e⁻ (×2)
1. Multiply so electrons balance..
O₂+2H₂O +4e- ----> 4OH⁻
2H₂ + 4OH⁻ -----> 4H₂O + 4e⁻
2. Cancel 4e⁻ and 4OH⁻ and 2H₂O....
2 H₂ + O₂ -----> 2 H₂O (Full redox equation)
Note: The full redox equation is the same in both acid and alkaline solution.
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