Difference Between Ion Electron Method and Oxidation Number Method

The key difference between ion electron method and oxidation number method is that, in ion electron method, the reaction is balanced depending on the charge of ions whereas, in oxidation number method, the reaction is balanced depending on the change in oxidation numbers of oxidants and reductants.

Both ion electron method and oxidation number method are important in balancing chemical equations. A balanced chemical equation is given for a particular chemical reaction and it helps us to determine how much of the reactant reacted to give a particular amount of the product, or the amount of reactants required to obtain a desired amount of the product.

CONTENTS

1. Overview and Key Difference
2. What is Ion Electron Method  
3. What is Oxidation Number Method
4. Side by Side Comparison – Ion Electron Method vs Oxidation Number Method in Tabular Form
5. Summary

What is Ion Electron Method?

Ion electron method is an analytical technique we can use to determine the stoichiometric relationship between reactants and products, using ionic half-reactions. Given the chemical equation for a particular chemical reaction, we can determine the two half-reactions of the chemical reaction and balance the number of electrons and ions in each half reaction to get completely balanced equations.

Let us consider an example to understand this method.

The reaction between permanganate ion and ferrous ion is as follows:

MnO₄^– + Fe²⁺   ⟶    Mn²⁺ + Fe³⁺ + 4H₂O

The two half-reactions are the conversion of permanganate ion into manganese(II) ion and ferrous ion into ferric ion. The ionic forms of these two half-reactions are as follows:

MnO₄^– ⟶    Mn²⁺

Fe²⁺   ⟶   Fe³⁺

Thereafter, we have to balance the number of oxygen atoms in each half-reaction. In the half-reaction where ferrous is converted into ferric ion, there are no oxygen atoms. Therefore, we have to balance the oxygen in the other half-reaction.

MnO₄^– ⟶    Mn²⁺ + 4O^2-

These four oxygen atoms come from the water molecule (not molecular oxygen because there is no gas production in this reaction). Then the correct half-reaction is:

MnO₄^– ⟶    Mn²⁺ + 4H₂O

In the above equation, there are no hydrogen atoms in the left side, but there are eight hydrogen atoms in the right side, so we have to add eight hydrogen atoms (in the form of hydrogen ions) to the left side.

MnO₄^– + 8H⁺   ⟶    Mn²⁺ + 4H₂O

In the above equation, the ionic charge of the left side is not equal to the right side. Therefore, we can add electrons to one of the two sides to balance the ionic charge. The charge in the left side is +7 and in the right side it is +2. Here, we have to add five electrons to the left side. Then the half-reaction is,

MnO₄^– + 8H⁺  +  5e^–  ⟶    Mn²⁺ + 4H₂O

When balancing the half-reaction of ferrous converting into ferric ion, the ionic charge converts from +2 to +3; here we need to add one electron to the right side as follows in order to balance the ionic charge.

Fe²⁺   ⟶   Fe³⁺ + e^–

Thereafter, we can add two equations together by balancing the number of electrons. We have to multiply the half-reaction with the conversion of ferrous into ferric by 5 to get five electrons and then by adding this modified half-reaction equation to the half-reaction with the conversion of permanganate into manganese(II) ion, the five electrons in each side cancels out. The following reaction is the result of this addition.

MnO₄^– + 8H⁺  +  5Fe²⁺ +  5e^–  ⟶    Mn²⁺ + 4H₂O  +  5Fe³⁺ +  5e^–

MnO₄^– + 8H⁺  + 5Fe²⁺  ⟶    Mn²⁺ + 4H₂O  + 5Fe³⁺

What is Oxidation Number Method?

Oxidation number method is an analytical technique we can use to determine the stoichiometric relationship between reactants and products, using the change in the oxidation of chemical elements when the reaction goes from reactants to products. In a redox reaction, there are two half-reactions: oxidation reaction and reduction reaction. For the same example as above, the reaction between permanganate and ferrous ions, oxidation reaction is the conversion of ferrous into ferric ion while reduction reaction is the conversion of permanganate ion into manganese(II) ion.

Oxidation: Fe²⁺   ⟶   Fe³⁺

Reduction: MnO₄^– ⟶    Mn²⁺

When balancing this type of reaction, first we need to determine the change in oxidation states of chemical elements. In the oxidation reaction, +2 of ferrous ion converts into +3 ferric ion. In the reduction reaction, +7 of manganese converts into +2. Therefore, we can balance the oxidation states of these by multiplying the half-reaction with the degree of increment/decrement of oxidation state in the other half-reaction. In the above example, change in oxidation state for oxidation reaction is 1 and change in oxidation state for reduction reaction is 5. Then, we have to multiply the oxidation reaction with 5 and the reduction reaction with 1.

5Fe²⁺   ⟶   5Fe³⁺

MnO₄^– ⟶    Mn²⁺

Thereafter, we can add these two half-reactions to get the complete reaction and then can balance the other elements (oxygen atoms) using water molecules and hydrogen ions to balance the ionic charge in both sides.

MnO₄^– + 8H⁺  + 5Fe²⁺  ⟶    Mn²⁺ + 4H₂O  + 5Fe³⁺

What is the Difference Between Ion Electron Method and Oxidation Number Method?

Ion electron method and oxidation number method are important in balancing chemical equations. The key difference between ion electron method and oxidation number method is that in ion electron method, the reaction is balanced depending on the charge of ions whereas, in oxidation number method, the reaction is balanced depending on the change in oxidation numbers of oxidants and reductants.

Below infographic summarizes the difference between ion electron method and oxidation number method.

Summary – Ion Electron Method vs Oxidation Number Method

The key difference between ion electron method and oxidation number method is that in ion electron method, the reaction is balanced depending on the charge of ions whereas, in oxidation number method, the reaction is balanced depending on the change in oxidation numbers of oxidants and reductants.

Reference:

1. “Balancing Equations .” org, 17 Apr. 2017, Available here.

Image Courtesy:

1. “Chemical reactions” By Daniele Pugliesi – Own work (CC BY-SA 3.0) via Commons Wikimedia