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Balancing by Oxidation Number Method and Its Example for JEE

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Last updated date: 19th Jul 2024
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Redox Reactions

Redox reactions are oxidation-reduction processes in which the oxidation states of the reactants change and both oxidation, and reduction occur simultaneously. Redox reactions can be divided into two types: reduction and oxidation reactions. During these chemical processes, the substance that causes oxidation is called an oxidizing agent; the substance that causes reduction is called the reducing agent. The oxidizing agent gets reduced, while the reducing agent gets oxidized during redox reactions.


Balancing of Redox Reactions

In chemical reactions, a balanced chemical equation accurately describes the quantities of reactants and products. In a typical chemical reaction, the Law of Conservation of Mass states that no mass is generated or destroyed. This indicates that both sides of a chemical equation must have the same number of atoms of each element. In addition, the sum of the charges on one side of the equation must equal the sum of the charges on the other side of the equation. The equation is considered to be balanced when these two conditions are met. Hence, balancing redox reactions are of greatest significance.


How to Balance Redox Reactions?

The redox reaction can be balanced in two ways. The change in oxidation numbers of the oxidizing and reducing agents is one technique, while the other is based on separating the redox reaction into two half processes, one of reduction and the other of oxidation. Hence, there are two methods for balancing redox reactions. One is the oxidation number method, and the other is the half reaction method.


Oxidation Number Method

The oxidation number approach is based on the difference between the oxidizing and reducing agents oxidation numbers. It is important, as with any reaction, to write accurate compositions and formulas. When writing oxidation-reduction reactions, it is important to express the compositions and formulae of the substances and products involved in the chemical process correctly. The following are utilized in the oxidation method for balancing redox equations:

  • Step 1

Correctly express the redox reaction formula for reactants and products in the chemical reaction.


  • Step 2

Determine the elements that undergo a change in oxidation number in the given reaction by assigning oxidation numbers to the individual elements present in the reaction.


  • Step 3

To calculate the oxidation number for a molecule or ion in a chemical reaction, look at the number of atoms involved. If the numbers are not equal, you can multiply them together to get a number that makes them equal overall. If two substances only undergo oxidation or reduction, then this suggests a problem with the chemical reaction. This means that one of the formulas for the reactants or products may be incorrect. It may also mean that the assignment of oxidation states is incorrect.


  • Step 4

If the process takes place in water, keep in mind that it involves ions. As a result, add H+ or OH ions to the appropriate side of the reaction. The ionic charges of the reactant and products will be equivalent in the end. If the reaction occurs in an acidic solution, H+ ions should be included in the chemical equation. In the same way, if the reaction occurs in a basic solution, add OH ions to the chemical equation.


  • Step 5

The number of hydogen atoms on each side of the equation must be equalised by adding water molecules or H2O molecules. It is also important to double-check the oxygen atoms in the equation. If there are equal number of oxygen atoms present on both the reactant and the product side, the reaction is called a balanced reaction.


By solving a problem using the oxidation number approach, the learners can get a better understanding of the procedures involved in balancing redox reactions.


Balancing Redox Reactions Examples: In An Acidic Solution, Potassium Dichromate(Vi) (K2cr2o7) Reacts With Sodium Sulphite (Na2so3) To Form Sulfate Ions and Chromium(Iii) Ions. Let Us Write a Balanced Redox Reaction Equation for this Reaction:

Step 1

Write the basic ionic form of the chemical reaction:

  • $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+\mathrm{SO}_{3}^{2-} \rightarrow \mathrm{Cr}^{+3}+\mathrm{SO}_{4}{ }^{2-}$


Step 2

Assign oxidation state to each species:


(image will be uploaded soon)


Image: Assigning oxidation state to each species


Step 3

After calculation of the oxidation number, increase and decrease as is required to make either side of the equation equal.


The reaction takes place in an acidic medium, as we can see from the equation. Furthermore, the ionic charges on both sides of the equation are not the same. As a result, we will add 8H+ to equalize the ionic charges.

  • $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+3 \mathrm{SO}_{3}^{2-}+8 \mathrm{H}^{+} \rightarrow 2 \mathrm{Cr}^{+3}+3 \mathrm{SO}_{4}{ }^{2-}$


Step 4

To make the equation a balanced redox reaction, we will calculate the appropriate amount of water molecules and add it to the right side of the equation. To balance the equation in the preceding equation, we must add four water molecules to the right side.

  • $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+3 \mathrm{SO}_{3}^{2-}+8 \mathrm{H}^{+} \rightarrow 2 \mathrm{Cr}^{+3}+3 \mathrm{SO}_{4}^{2-}+4 \mathrm{H}_{2} \mathrm{O}$

The equation is balanced.


Summary

In chemical reactions, a balanced chemical equation is important in describing the quantities of reactants and products accurately. According to the Law of Conservation of Mass, no mass can be generated or destroyed. This indicates that both sides of a chemical equation must have the same amount of atoms of each element. Hence, balancing a chemical equation is required in order to study chemical reactions. In this article, we have learnt how balancing can easily be done by the oxidation number method. The oxidation number approach is based on the difference between the oxidizing and reducing agents's oxidation numbers. The change in the oxidation state of elements is made equal by multiplying with a suitable factor, and the ions and water is added to balance it further. We also looked at many redox examples.

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FAQs on Balancing by Oxidation Number Method and Its Example for JEE

1. Define the ion-electron method for balancing redox chemical reactions.

Ion Electron Method

Divide the entire reaction into two halves, one for oxidation and the other for reduction. Other than the 'O' and 'H' atoms, balance first. In an acidic or neutral medium, add H2O molecules to balance oxygen atoms and H+ ions to balance hydrogen atoms. The oxygen atom is balanced in an alkaline medium by adding an H2O molecule and an equal number of hydrogen atoms. If H+ atoms are still imbalanced, OH- is added on the other side. The addition of electrons balances the charges on either sides. Multiply by a large enough integer to cancel the number of electrons. Add both half-reactions together, subtract similar terms, and write the final equation.

2. What is redox titration?

A redox reaction between the titrant and the analyte is used to determine the concentration of a given analyte in a laboratory setting. A potentiometer or a redox indicator may be required for these types of titrations.


The titrant and the analyte undergo an oxidation-reduction reaction, which is used in redox titration. One of the most common laboratory methods for determining the quantity of unknown analytes is redox titration. The titration of potassium permanganate (KMnO4) against oxalic acid (C2H2O4) is an example of redox titration.