Is it necessary to break the equation into half reactions in the oxidation number method?

1 Answer
Ernest Z.
Apr 28, 2015

No, you don't use half-reactions in the oxidation number method.

Here's how you use the oxidation number to balance the following equation.

MnO−4+HSO−3→Mn2++SO2−4+H2O

Step 1. Identify the atoms that change oxidation number

Left hand side: Mn=+7; O=−2; H=+1; S=+4
Right hand side: Mn=+2; S=+6; O=−2; H=+1

The changes in oxidation number are:
Mn: +7 → +2; Change = -5
S: +4 → +6; Change = +2

Step 2. Equalize the changes in oxidation number

You need 5 atoms of S for every 2 atoms of Mn. This gives us total changes of +10 and -10.

Step 3. Insert coefficients to get these numbers

2MnO−4+5HSO−3→2Mn2++5SO2−4

Step 4. Balance O by adding H2O molecules to the appropriate side

2MnO−4+5HSO−3→2Mn2++5SO2−4+3H2O

Step 5. Balance H by adding H⁺ ions to the appropriate side

2MnO−4+5HSO−3+1H+→2Mn2++5SO2−4+3H2O

And the equation is balanced.

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