Answer
Step 3: Balance Oxygen by Adding H2O
In acidic redox reactions, after identifying and splitting the oxidation and reduction half-reactions and balancing all atoms except for hydrogen and oxygen, the next step is to balance the oxygen atoms by adding water molecules (H2O) to the side deficient in oxygen.
Example from Your Half-Reactions
Unbalanced Reduction Half-Reaction:
PbO2 + SO42− → PbSO4
PbO2 + SO42− → PbSO4
In this half-reaction:
- Left side has 2 oxygen atoms from PbO2 and 4 from SO42− → Total = 6 O atoms
- Right side has 4 O atoms from PbSO4
You are missing 2 oxygen atoms on the right side.
Balanced by adding H2O:
PbO2 + SO42− → PbSO4 + 2H2O
PbO2 + SO42− → PbSO4 + 2H2O
Remember: Water is only added to balance oxygen atoms. Hydrogen will be balanced in the next step by adding H+ ions (because the solution is acidic).
Continue With:
- Step 4: Balance hydrogen atoms by adding H+
- Step 5: Balance charges by adding electrons (e−)
- Step 6: Combine both half-reactions