# How do you balance redox equations by oxidation number method?

##### 1 Answer
Mar 11, 2014

You follow a series of steps in order.

For example, balance the equation

HNO₃ + H₃AsO₃(aq) → NO(g) + H₃AsO₄(aq) + H₂O(l)

1. Identify the oxidation number of every atom.
Left hand side: H= +1; N= +5; O = -2; As = +3
Right hand side: N = +2; O = -2; H = +1; As = +5

2. Determine the change in oxidation number for each atom that changes.
N: +5 → +2; Change = -3
As: +3 → +5; Change = +2

3. Make the total increase in oxidation number equal to the total decrease in oxidation number.
We need 2 atoms of N for every 3 atoms of As. This gives us total changes of -6 and +6.

4. Place these numbers as coefficients in front of the formulas containing those atoms.
2HNO₃ + 3H₃AsO₃(aq) → 2NO(g) + 3H₃AsO₄(aq) + H₂O(l)

5. Balance all remaining atoms other than H and O.
Done.

6. Balance O.
Done.

7. Balance H.
Done.

Now try to balance the equations in the link below (answers included).

http://bowvalleycollege.ca/Documents/Learning%20Resource%20Services/Library%20Learning%20Commons/E-Resources/Study%20guides/chemistry30%20ox_num_method.pdf