Illustrated
for the reaction: MnO4- + Br- --> Br2
+ MnO2
Write down the
overall equation. It is usually easiest to start with a net ionic equation.
Separate the
reaction into two half-reactions, one for oxidation and one for reduction.
Each half reaction shows the oxidation or reduction of one element. You do
not have to assign oxidation states unless you need to do this in order to
write the half-reactions.
MnO4-
--> MnO2
Br- --> Br2
Balance each
half-reaction as follows:
Balance all
elements other than oxygen and hydrogen.
MnO4--->
MnO2 2Br- --> Br2
Balance oxygen
by adding the appropriate number of water molecules (H2O) to
the side of the equation that needs more oxygens.
MnO4-
--> MnO2 + 2H2O
2Br- --> Br2
Balance hydrogen
by adding the appropriate number of hydrogen ions (H+) to the
side of the equation that needs more hydrogens.
4H+
+ MnO4- --> MnO2 + 2H2O
2Br- --> Br2
Balance the
charge by adding the appropriate number of electrons to the side of the
equation with the greater overall positive charge.
3e-
+ 4H+ + MnO4- --> MnO2
+ 2H2O
2Br- --> Br2 + 2e-
Multiply each
half-reaction by a whole number so that the electrons lost in one half-reaction
equal the number of electrons gained in the other half-reaction.
2(3e-
+ 4H+ + MnO4- --> MnO2
+ 2H2O) 3(2Br- --> Br2
+ 2e-)
Add the two
half reactions together, keeping all of the reactants together on the left
side of the reaction arrow and all of the products together on the right side
of the reaction arrow. The electrons will cancel so they are not shown in the final equation.
2(3e- +
4H+ + MnO4- --> MnO2 +
2H2O)
3(2Br- --> Br2 + 2e-)
-------------------------------------------------------------- 8H+ + 2MnO4-
+ 6Br- --> 2MnO2 + 4H2O + 3Br2
Cancel any substances that appear on both sides of the equation. Check
to make sure that the equation is balanced. If you removed spectator ions
before you wrote the half-reactions, put them back in and balance by inspection.