Writing Half Equations: Aqueous Solutions
To write a balanced oxidation or reduction reaction for a species in aqueous solution:
- Write a skeletal equation for the oxidation or reduction equation based on the information provided.
- Balance the half-reaction equation according to the following sequence:
- Balance all atoms other than H and O by inspection
- Balance O atoms by adding H2O to the appropriate side
- Balance the H atoms.
The way this is done depends on whether the solution is acidic or basic:
- Acidic Solution:
add the appropriate number of H+ to the side deficient in H
- Basic Solution:
add one H2O molecule to the side deficient in H
add one OH- ion to the opposite side, for each H atom needed.
You may need to cancel out H2O molecules duplicated on both sides of the equation at this point.
- Balance the charge by adding electrons (e) to the side deficient in negative charge.
Examples of Writing Half-equations for Aqueous Solutions
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