Change in oxidation number in a redox reaction and balancing

K. Pratap
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The change in oxidation number and balancing a redox reaction

Calculating the change in oxidation number:

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++S2-+ Br2 -----> H++Br-+S

Step 2: Ignore common ions on both side

S2- + Br2 -----> Br- + S

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

S2- + Br20 -----> Br- + S0

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

Change in oxidation number of element in a redox reaction

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

O.N change of Br = [–1 – 0] X 2 = –2

  Balancing the change in oxidation number:

We must be clear that increase in oxidation number is called the oxidation and decrease in oxidation number is the reduction. The molecule in which the oxidation  number of an element increases is the reducing agent or reductant and whose oxidation number decreases is the oxidising agent  or oxidant. Consider the example of:

NH3 + O2  ------> NO + H2O

change in oxidation number and how to balance it

Now to balance the change in oxidation number,

1. Multiply the molecule of oxidizing agent (Oon the reactant side), with the change in oxidation number in the oxidation process.

 i.e., Obecomes 5O2

 2. Multiply the molecule of reducing agent (NH3  on the reactant side), with the change in oxidation number in the reduction process.

i.e.,   NHbecomes 4NH3

3. The reaction is written including point 1 and 2 together as:

4 NH3 + 5 O2  ------> NO + H2O

4. Now keep the reactant side unaltered and make the number of atoms of different elements (whose O.N has changed) on the product side equal to the number of atoms of the same elements in the reactant side by multiplying suitable integer. 

5. Thus the reaction with balanced O.N change is:

4 NH3 + 5 O2  ------> 4 NO + 6 H2O

Balancing a chemical equation by oxidation number change method

Before balancing a redox reaction, we should have an idea of the type of redox reactions:

1. Reactions involving neural medium

2. Reactions involving acidic medium

3. Reactions involving alkaline medium

4. Disproportionation reactions

5. Comproportionation reaction


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