Checking the half-reaction:
Now work on the reduction. It is necessary to convert the four oxygen atoms in the permanganate into four water molecules. To do this, add eight H
+ to convert the oxygen into four water molecules:
Then add five electrons to the left side to balance the charge:
Make sure to check the half-reaction:
Collecting what we have so far:
The least common multiple for the electrons is 30, so multiply the oxidation half-reaction by five, the reduction half-reaction by six, combine, and simplify:
Checking each side of the equation:
This is the balanced equation in acidic solution.
Check your learning
Balance the following equation in acidic solution:
Answer:
Balancing basic oxidation-reduction reactions
Balance the following reaction equation in basic solution:
Solution
This is an oxidation-reduction reaction, so start by collecting the species given into an unbalanced oxidation half-reaction and an unbalanced reduction half-reaction
Starting with the oxidation half-reaction, we can balance the chromium
In acidic solution, we can use or generate hydrogen ions (H
+ ). Adding one water molecule to the left side provides the necessary oxygen; the “left over” hydrogen appears as five H
+ on the right side:
The left side of the equation has a total charge of [0], and the right side a total charge of [−2 + 5
(+1) = +3]. The difference is three, adding three electrons to the right side produces a mass- and charge-balanced oxidation half-reaction (in acidic solution):
Checking the half-reaction:
Now work on the reduction. It is necessary to convert the four O atoms in the MnO
4
− minus the two O atoms in MnO
2 into two water molecules. To do this, add four H
+ to convert the oxygen into two water molecules: