Calculation of change in oxidation number in a redox reaction

How to calculate the change in oxidation number in a redox reaction?

Following are the steps to follow when you calculate change in oxidation number in a redox reaction. Once you practice two to three times than that becomes super easy to calculate the change in O.N.

Take an example of the following reaction (First and easy method).

H₂S + Br₂ -----> HBr + S

Step 1: Write the equation in the ionic form and do not balance it.

H⁺+S^2-+ Br₂ -----> H⁺+Br^-+S

Step 2: Ignore common ions on both side

S^2- + Br₂ -----> Br^- + S

Step 3: Write the O.N of elements at the top now

S^2- + Br₂⁰ -----> Br^- + S⁰

Step 4: Find the O.N change of any element by:

For example, O.N change of S = [0 – (--2)] X 1 = +2

O.N change of Br = [–1 – 0] X 2 = –2