Difference Between Oxidation Number Method and Half Reaction Method

Key Difference – Oxidation Number Method vs Half Reaction Method
 

Oxidation number method and half-reaction method are two methods used to balance a chemical equation of a redox reaction. A redox reaction is a chemical reaction that is composed of two parallel chemical reactions; oxidation reaction and reduction reaction. These are known as half reactions of a redox reaction. Therefore, a redox reaction takes place in a reaction mixture where a substance undergoes oxidation and another substance (or the same substance) undergoes reduction. Oxidations and reductions change the oxidation number or the oxidation state of a chemical element. Either this change in oxidation number or the half-reactions occur in redox reaction can be used to balance the overall chemical equation of the redox reaction. The key difference between oxidation number method and half-reaction method is that oxidation number method uses the change on oxidation number of chemical species in the reaction mixture whereas half-reaction method uses the method of balancing the two parallel half reactions followed by their addition with each other.

CONTENTS

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

What is Oxidation Number Method?

Oxidation number method is a method of balancing a chemical equation of a redox reaction using the oxidation numbers of chemical species in the reaction mixture. The oxidation number of a chemical element is the degree of oxidation of that element. The oxidation number is sometimes called oxidation state, and it can be either a positive value, a negative value or it can be zero. When the oxidation number of an atom is increased, we call that atom has oxidized; in contrast, when the oxidation number has decreased, that atom is reduced.

Ex: The reaction between zinc (Zn) and hydrochloric acid (HCl) gives Zinc chloride (ZnCl₂) and hydrogen gas (H₂). This reaction is a redox reaction in which zinc undergo oxidation and hydrogen atom undergo reduction where the oxidation number of chlorine is not changed. Zn atom has oxidized to Zn²⁺ whereas H⁺ ion has reduced to H₂.

Zn   +    HCl→ZnCl₂   +   H₂

Oxidation number method can be used to balance the above equation. First, the oxidation numbers of all the atoms should be indicated.

Zn = 0

H in HCl = +1

Znin ZnCl₂ = +2

H in H₂ = 0

Then should determine the changes that have taken place in these oxidation numbers. Zn has oxidized to Zn²⁺ while H⁺ has been reduced to H₂. After recognizing these changes, increase or decrease of oxidation number per atom should be indicated. Let us call this factor as “ON factor” (the increase or decrease per atom). After determining the ON factor, oxidizing atom should be multiplied by the ON factor of reducing atom and vice versa.

ON factor of Zn =2

ON factor of H = 1

When multiplied, {Zn x 1} + {HCl x 2}      →      {ZnCl₂ x 1} + {H₂}

This gives the balanced redox reaction: Zn + 2HCl       →       ZnCl₂ + H₂

What is Half Reaction Method?

Half-reaction method is a method of balancing a redox reaction using the two parallel half reactions; oxidation half-reaction and reduction half-reaction. In redox reactions, one reactant acts as the oxidizing agent that oxidizes the other reactant, while reducing itself.

Ex: For the reaction between Zinc (Zn) and hydrochloric acid (HCl), zinc act as the reducing agent whereas hydrogen in HCl is the oxidizing agent. Then the balanced two half-reactions can be written as:

Oxidation: Zn     → Zn⁺²+   2e

Reduction: 2HCl+  2e→   H₂     +   2Cl^–

Then we can simply add the half-reactions to get the balanced redox reactions. But before adding them should check whether the number of electrons on both sides is equal (only then the electrons in both sides can be canceled to get the net equation). If the electrons are not equal, then the whole equation (of one half-reaction) should be multiplied by a suitable value until it equals the number of electrons in the other half reaction.

The balanced redox reaction: Zn + 2HCl    →    ZnCl₂ + H₂

What is the Difference Between Oxidation Number Method and Half Reaction Method?

Summary – Oxidation Number Method vs Half Reaction Method 

A redox reaction is a common type of reaction in which one reactant act as an oxidizing agent whereas another reactant acts a reducing agent. There are two major ways to balance a redox reaction; oxidation number method and half reaction method. The difference between oxidation number method and half-reaction method is that oxidation number method uses the change on oxidation number of chemical species in the reaction mixture whereas half-reaction method uses the method of balancing the two parallel half reactions followed by their addition with each other.

Reference:

1.“Balancing Redox Reactions.” Balancing Redox Reactions, Ion Electron Method, Oxidation Number Method | Chemistry@TutorVista.com. Available here
2.“Oxidation numbers method of balancing redox reaction equations.” Chemical calculator software for pH calculation, concentration conversion solution preparation and chemical equation balancing. Available here

Image Courtesy:

1.’Zn reaction with HCl’By Chemicalinterest – Own work, (Public Domain) via Commons Wikimedia