Using the change-in-oxidation-number method, balance the redox reaction below. Show your work; partial credit will be given.
NO2– + MnO4– → NO3– + Mn2+
Figure out which atoms’ oxidation numbers are changing, and by how much. Balance the equation for charge. Add H2O and H+ to balance for atoms
http://www.chemteam.info/Redox/Redox.html
To balance the redox reaction using the change-in-oxidation-number method, follow these steps:
1. Identify the atoms whose oxidation numbers are changing. In this reaction, we can see that the oxidation number of nitrogen (N) is changing from +3 to +5, and the oxidation number of manganese (Mn) is changing from +7 to +2.
2. Write down the unbalanced equation, assigning oxidation numbers to the atoms involved.
NO2– + MnO4– → NO3– + Mn2+
(+3) (+7) (+5) (+2)
3. Balance the equation for the atoms other than hydrogen (H) and oxygen (O). In this case, we don't have any atoms other than H and O, so we can move on to the next step.
4. Balance the equation for oxygen by adding H2O molecules. Since there are four oxygen atoms on the reactant side (from MnO4–), we need to add four water molecules (H2O) to the product side.
NO2– + MnO4– → NO3– + Mn2+ + 4H2O
(+3) (+7) (+5) (+2)
5. Balance the equation for hydrogen by adding H+ ions to the opposite side of the equation. Since there are eight hydrogen atoms on the product side (from H2O), we need to add eight H+ ions to the reactant side.
NO2– + MnO4– + 8H+ → NO3– + Mn2+ + 4H2O
(+3) (+7) (+5) (+2)
6. Balance the equation for charge by adding electrons (e–) as needed. In this reaction, the total charge on the reactant side is -1 (from NO2–), and the total charge on the product side is 0. Since the reactant side has a net negative charge, we need to add electrons to balance the charges. In this case, we need to add five electrons (5e–) to the left side of the equation.
NO2– + MnO4– + 8H+ + 5e– → NO3– + Mn2+ + 4H2O
(+3) (+7) (+5) (+2)
7. Check the balance of atoms, charges, and oxidation numbers. The equation is now balanced with respect to atoms, charges, and oxidation numbers:
2NO2– + MnO4– + 4H+ + 5e– → 2NO3– + Mn2+ + 4H2O
(+3) (+7) (+5) (+2)
That's the balanced redox reaction using the change-in-oxidation-number method.