CBSE Class 11-science Answered

(a) The steps to balance a chemical equation using redox reactions are as follows:

(*Note: All the steps are same in both the acidic and basic medium however only one difference is done that in acidic medium, H⁺ ions are used while in basic medium OH^- ions are used.)

Step 1:

Write the correct formula for each reactant and product.

Step 2:

 Identify atoms which undergo change in oxidation number in the reaction by assigning the oxidation number to all elements in the reaction.

Step 3:

Calculate the increase or decrease in the oxidation number per atom and for the entire molecule/ion in which it occurs. If these are not equal then multiply by suitable coefficients so that these become equal.

Step 4:

Ascertain the involvement of ions if the reaction is taking place in water, add H⁺ or OH^- ions to the expression on the appropriate side so that the total ionic charges of reactants and products are equal. Use, H⁺ ions when reaction in acidic medium and OH^- ions when reaction in basic medium.

Step 5:

Equalize the number of H-atoms on both the sides by adding H₂O molecules to reactants or product wherever necessary and also check the no. of oxygen atom and balance it.

For example: In acidic medium reaction: Cr ₂ O ₇ ^2- + NO ₂ ^- → Cr ³⁺ + NO ₃ ^- (acid)

=> The oxidation number is assigned as:

  Answered by Hanisha Vyas | 10 Dec, 2013, 11:42: AM