Balancing REDOX reactions
There are 2 ways to balance redox reactions… by oxidation # and
by half reactions. We will use the half reaction method.
Why can’t we just balance normally?: because most redox
reactions happen in acidic/basic solutions
FIRST OF ALL… learning how to balance half-reactions properly:
Example #1 (acidic solution): Nitrous acid (HNO2) can be reduced
in an acidic solution to form nitrogen monoxide gas. What is the
reduction half-reaction of nitrous acid?
1. Write reactants and products and balance all atoms other
than oxygen and hydrogen
2. Balance oxygens with water molecules
3. Balance hydrogens with H+ ions
4. Balance charges by adding electrons to the more positive side
of the equation
Example #2 (basic solution): Copper metal can be oxidized in a
Basic solution to form copper (I) oxide. What is the half-reaction
for this process?
1. Assume that the solution is ACIDIC and follow the steps just
2. Once the equation is mass + charge balanced, add the same
number of OH- ions as there are H+ ions to both sides of the
3. Combine equal numbers of H+ and OH- to make H2O’s
4. Cancel any H2O’s (or other molecules) that appear on both sides
of the equation.
Now you try: chlorine is converted to perchlorate ions in an acidic
solution. Write a half-reaction equation and state if it is
oxidation or reduction.
Balancing redox equations using half reaction equations:
-All equations have a reduction half and an oxidation half. So we
combine half reactions!
Example #1: in a chemical analysis, a solution of dichromate ions is
reacted with an acidic solution of iron (II) ions. The product
formed are iron (III) and chromium (III) ions as shown by the
Fe2+ + Cr2O7 2- Fe3+ + Cr3+
Start by separating the above into 2 half-reactions, and treat as
we did before.
Example #1 continued