Question
Thu December 01, 2011 By: Vignesh Hariharasuthan

i cannot understand balancing redox reaction.how is it being done?

Expert Reply
Thu December 01, 2011

Two methods are used to balance chemical equations for redox processes.

(i) Oxidation Number Method (ii) Half Reaction Method

Both these methods can be used for balancing the redox reaction and the choice of their use rests with the individual using them.

(a) Oxidation Number Method: The oxidation number method involves the following steps:

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. If the reaction is carried out in acidic solution, use H+ ions in the equation; if in basic solution, use OH– ions.

Step 5: Make the numbers of hydrogen atoms in the expression on the two sides equal by adding water (H2O) molecules to the reactants or products. Now, also check the number of oxygen atoms. If there are the same number of oxygen atoms in the reactants and products, the equation then represents the

balanced redox reaction.

(b) Half Reaction Method: In this method, the two half equations are balanced separately and then added together to give balanced

equation. The following steps are involved in this task.

Step 1: Produce unbalanced equation for the reaction in ionic form.

Step 2: Separate the equation into half reactions.

Step 3: Balance the atoms other than O and H in each half reaction individually.

Step 4: For reactions occurring in acidic medium, add H2O to balance O atoms and H+ to balance H atoms.

Step 5: Add electrons to one side of the half reaction to balance the charges. If need be, make the number of electrons equal in the two half reactions by multiplying one or both half reactions by appropriate coefficients.

Step 6: We add the two half reactions to achieve the overall reaction and cancel the electrons on each side.

Step 7: Verify that the equation contains the same type and number of atoms and the same charges on both sides of the equation. This last check reveals that the equation is fully balanced with respect to number of atoms and the charges.

For the reaction in a basic medium, first balance the atoms as is done in acidic medium. Then for each H+ ion, add an equal number of OH– ions to both sides of the equation. Where H+ and OH– appear on the same side of the equation, combine these to give H2O.

You can also see our video on the balancing of redox reaction.

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