Balancing redox reaction – Oxidation number and Ion-electron method

Balancing redox reaction:

Redox reaction can be balanced by either of the following two methods:

• Oxidation number method
• Ion-electron method

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Oxidation number method to balance redox reaction

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The oxidation number of an oxidizing agent decreases and that of the reducing agent increases in a redox process. This change in oxidation number should be balanced to balance a redox reaction by the oxidation number method.

Balancing of redox reaction by oxidation number method involves the following steps:

Step I : Identifying the oxidizing and reducing agents.

Oxidizing and reducing agents are identified by observing the change in oxidation number of main elements present in oxidizing and reducing agents on reactant and product sides. Eg.

Here, Zn is oxidized and HNO₃ is reduced to Zn(NO₃)₂ and N₂O respectively.

So, Zn is reducing agent and HNO₃ is oxidizing agent.

Step II : Calculating the change in oxidation number.

The total change in oxidation number in oxidation and also in reduction is calculated after equalizing the number of atoms undergoing O.N. change on both sides. Eg.

Oxidation: total change in O.N. = 2-0 = 2

Reduction: Total change in O.N. = 1-5 = 4×2 =8

Step III : Balancing of oxidizing and reducing agents.

Number of atoms undergoing oxidation number change are balanced by criss-cross multiplication with change in oxidation number on reactant as well as product side. Eg.

Step IV : Balancing of other elements.

All the other atoms or radicals are balanced by hit and trial method.

• Balancing the equation with respect to all other atoms except hydrogen and oxygen.

• Finally, balancing hydrogen and oxygen.

It is balanced chemical equation.

• Note:

• For reactions taking place in acidic solutions, add H⁺ ions to the side deficient in hydrogen atoms.

• For reactions taking place in basic solutions, add H₂O molecules to the side deficient in hydrogen atoms and simultaneously add equal number to OH^– ions on the other side of the equation.

More examples:

Example – 1

Step I : Identifying the oxidizing and reducing agents.

Step II : Calculating the change in oxidation number.

Step III : Balancing of oxidizing and reducing agents.

Step IV : Balancing of other elements.

• Balancing the equation with respect to all other atoms except hydrogen and oxygen.

• Finally, balancing hydrogen and oxygen.

It is balanced chemical equation.

Example – 2

Step I : Identifying the oxidizing and reducing agents.

Step II : Calculating the change in oxidation number.

Step III : Balancing of oxidizing and reducing agents.

Step IV : Balancing of other elements.

It is balanced chemical equation.

Example-3 :

Step I : Identifying the oxidizing and reducing agents.

Here, Cl₂ undergoes self oxidation and reduction (i.e. disproportionation reaction). So, Cl₂ is written twice to calculate the change in oxidation number.

Step II : Calculating the change in oxidation number.

Step III : Balancing of oxidizing and reducing agents.

Step IV : Balancing of other elements.

It is balanced chemical equation.

Example-4 :

Step I : Identifying the oxidizing and reducing agents.

Step II : Calculating the change in oxidation number.

Here, more than two elements have changed in the oxidation number.

Change in O.N. of As = +2 X 2 = +4

Change in O.N. of S = +2 X 3 = +6

Change in O.N. of N = – 3

Thus, total increase in O.N. = 4+6=10

Total decrease in O.N. = 3

Step III : Balancing of oxidizing and reducing agents.

Step IV : Balancing of other elements.

It is balanced chemical equation.

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Ion-Electron method [ Half reaction method] to balance reaction

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This method is based on the fact that number of electrons lost during oxidation half reaction of redox reaction is equal to number of electrons gained during reduction half reaction.

Following steps are involved in balancing redox rection by ion-electron method:

• Write the given equation and identify the oxidized and reduced species by analyzing change in oxidation number.
• Write the oxidation and reduction half reactions (ionic if required)
• Balance two half reactions separately.

1. Balance main element (i.e. atom whose O.N. is changed) by hit and trial.
2. Balance oxygen by adding H₂O.
3. Balance hydrogen by adding H⁺.
4. Balance charge by adding electrons.
5. In basic medium, OH^– ions are added on either side to cancel out H⁺ ions.

• Add two half reactions by making electrons gained equal to electrons lost in two half reactions.
• Finally, convert the balanced ionic equation to balanced molecular equation (if required).

Some Examples:

Example-1 :

Counting of oxidation number:

Identification and balancing of two half reactions:

Oxidation half reaction:

Balancing charge by adding electrons:

This is balanced oxidation half reaction.

Reduction half reaction:

Balancing main element by hit and trial:

Balancing oxygen by adding H₂O:

Balancing hydrogen by adding H⁺:

Balancing charge by adding electrons:

This is balanced reduction half reaction.

Adding two half reactions:

Balanced two half reactions are added after multiplying oxidation half by 4 to cancel electrons.

This is balanced ionic equation. To convert it into molecular form, 8 NO₃^– ions are added on both side of equation.

This is balanced molecular equation.

Example-2:

Counting of oxidation number:

Identification and balancing of two half reactions:

Oxidation half reaction:

Balancing charge by adding electrons:

This is balanced oxidation half reaction.

Reduction half reaction:

Balancing main element by hit and trial:

Balancing oxygen by adding H₂O:

Balancing hydrogen by adding H⁺:

Balancing charge by adding electrons:

This is balanced reduction half reaction.

Adding two half reactions:

Balanced two half reactions are added after multiplying oxidation half by 6 to cancel electrons.

This is balanced ionic equation. To convert it into molecular form, suitable number of K⁺ and SO₄^2- have to be added on both side of equation.

This is balanced molecular equation.

Example -3:

Counting of oxidation number:

Identification and balancing of two half reactions:

Oxidation half reaction:

Balancing main element by hit and trial:

Balancing charge by adding electrons:

This is balanced oxidation half reaction.

Reduction half reaction:

Balancing oxygen by adding H₂O:

Balancing hydrogen by adding H⁺:

Balancing charge by adding electrons:

This is balanced reduction half reaction.

Adding two half reactions:

This is balanced equation.

Example-4: [ Basic medium ]

Counting of oxidation number: Here, chlorine is oxidized as well as reduced.

Identification and balancing of two half reactions:

Oxidation half reaction:

Balancing oxygen by adding H₂O:

Balancing hydrogen by adding H⁺.

The reaction is carried out in basic medium, so, all H⁺ ions must be neutralized by adding same number of OH^– ions on both sides.

Balancing charge by adding electrons:

This is balanced oxidation half reaction.

Reduction half reaction:

Balancing oxygen by adding H₂O:

Balancing hydrogen by adding H⁺:

The reaction is carried out in basic medium, so, all H⁺ ions must be neutralized by adding same number of OH^– ions on both sides.

Balancing charge by adding electrons:

This is balanced reduction half reaction.

Adding two half reactions:

This is balanced ionic equation. In order to convert it into molecular form, 8K⁺ ions are added on both sides of equation.

This is balanced molecular equation.

Example-5:

Counting of oxidation number: Here, chlorine is oxidized as well as reduced.

Identification and balancing of two half reactions:

Oxidation half reaction:

Balancing main element by hit and trial:

Balancing oxygen by adding H₂O:

Balancing hydrogen by adding H⁺:

The reaction is carried out in basic medium, so, all H⁺ ions must be neutralized by adding same number of OH^– ions on both sides.

Balancing charge by adding electrons:

This is balanced oxidation half reaction.

Reduction half reaction:

Balancing main element by hit and trial:

Balancing charge by adding electrons:

This is balanced reduction half reaction.

Adding two half reactions:

This is balanced ionic equation. In order to convert it into molecular form, 6 Na⁺ ions are added on both sides of equation.

This is balanced molecular equation.

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Self Practice 

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Balance the following equations by the oxidation number method or ion-electron method.

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References

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• https://chemicalnote.com/oxidation-and-reduction-oxidants-and-reductants-and-redox-reaction/
• Atkins, Peter, Paula, de Julio, Atkin’s Physical Chemistry, Seventh Edition, Oxford University Press, (Printed in India, 2002).
• Gurtu, J.N., Snehi, H., Advanced Physical Chemistry, Seventh Edition, Pragati Prakashan India, 2000.
• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.