Redox Reaction and Oxidation Number

Introduction

Redox reactions are oxidation-reduction chemical reactions in which the oxidation states of the reactants change. The word ‘redox’ is called the reduction-oxidation process. All redox reactions can be categorized into two types of reactions: reduction and oxidation.

The oxidation and reduction reactions always happen simultaneously in a redox reaction or Oxidation-Reduction reaction. The oxidizing agent is the substance being reduced in a chemical reaction whereas the reducing agent is the substance that is being oxidized.

Redox Reaction

A redox reaction is a chemical reaction in which electrons are transferred between reactant molecules that are involved. The significant changes in the oxidation states of the reacting species can be used to identify this electron transfer.

Redox Reactions Types

The following are examples of redox reactions:

Decomposition Reaction

The major breakdown of a compound into different compounds is what this reaction entails. These are some examples of these types of reactions:

2 NaH → 2 Na + H2

2 H2O → 2 H2 + O2

Na2CO3 → Na2O + CO2

All the reactions above lead to the breakdown of minor chemical compounds as, AB → A + B.

However, one exception proves that not all decomposition reactions are redox reactions.

For example, CaCO3 → CaO + CO2.

Combination Reaction

These reactions are the inverse of decomposition reactions in that they combine two compounds to form a single compound as, A + B → AB. 

Consider the following scenario:

H2 + Cl2 → 2 HCl

C + O2 → CO2

4 Fe + 3 O2 → 2 Fe2O3

Displacement Reaction

An atom or an ion in the compound is replaced by an atom or an ion of another element in this reaction. It can be written as X + YZ →  XZ + Y. Displacement reactions can also be divided into two types:

• Metal displacement Reaction
• Non-metal displacement Reaction
• Disproportionation Reactions

Disproportionation reactions are those that involve only one reactant being oxidized and reduced.

P4 + 3 NaOH + 3 H2O → 3 NaH2PO2 + PH3 is an example.

Oxidation and Reduction Reaction

Let’s look at oxidation and reduction reactions separately to understand redox reactions better.

What is the Oxidation Reaction, and how does it work?

According to one definition, adding oxygen or the more electronegative element to a substance or removing hydrogen or the more electropositive element from a sense is called an oxidation reaction.

What is Reduction Reaction, and how does it work?

Reduction reactions, like oxidation reactions, are defined as electron gains. Any substance that gains electrons during a chemical reaction gets reduced.

The reduction reaction is defined as adding hydrogen or a more electropositive element to a substance or removing a more electronegative element or oxygen.

Oxidizing and Reducing Agents

• An oxidizing agent is a substance (atom, ion, or molecule) that gains electrons and is thus reduced to a low valency state
• A reducing agent is a substance that loses electrons and thus oxidizes to a higher valency state

The Role of Oxidation Numbers in Oxidation and Reduction Reactions

Chemists ultimately extended the concept of oxidation and reduction to interactions that do not entail the transfer of an electron in the traditional sense.

Consider the reaction that follows:

CO(g) + H2O(g)  →  CO2(g) + H2(g)

The total number of electrons in each atom’s valence shell remains constant in the above reaction, as shown in the diagram below.

The oxidation state of these atoms changes during this reaction. Carbon’s oxidation state increases from +2 to +4, while hydrogen’s oxidation state decreases from +1 to 0.

As a result, the best way to define oxidation and reduction is as follows. When an atom’s oxidation number increases, it is oxidized. When an atom’s oxidation number decreases, it is reduced.

Balancing Redox Equations

Redox reactions, also known as oxidation and reduction reactions, entail the transfer of an electron between chemical species. The oxidation and reduction reactions equations should balance density and charge, which can be difficult by visual inspection alone. It is called the balancing redox equation.

When balancing redox reactions, keep the following in mind

• The equation is broken down into two halves, oxidation and reduction
• Adjusting coefficients and then adding H2O, H+, and e-  in this order balances the equation
• Aside from O and H, balance the atoms in the equation
• Add the required number of water (H2O) molecules into the other side to balance the Oxygen atoms
• Add H+ ions to balance the Hydrogen atoms (including those added in step 2)
• Total the charges on both sides. The more positive side must be equal by adding enough electrons (e-)
• Each side’s e- must be made equal; if they aren’t, they must be multiplied by appropriate integers to make them equal
• The half-equations are added together to form a single balanced equation, canceling the electrons. As much as possible, cancel out
• If the equation is balanced in a basic solution, the appropriate amount of OH- must be added to convert the remaining H+ into water molecules
• It’s now possible to check if the equation is balanced

Oxidation and Reduction example

To balance simple redox equations, follow these guidelines:

• For the reduced or oxidized species, write the oxidation and reduction half-reactions
• To make the half-reactions have the same number of electrons, multiply them by the appropriate number
• To cancel out the electrons, add the two equations together. It’s essential to have a balanced equation

The reaction of iron(III) sulphate with magnesium is illustrated below. 

Unbalanced Reaction: Mg(s) + Fe2(SO4)3(aq) → Fe(s) + MgSO4(aq)

This reaction is divided into two halves, one involving oxidation and reduction.

Reduction: Fe3+(aq) + 3 e– → Fe(s)

Oxidization: Mg(s) → Mg2+(aq) + 2e–

They can be balanced by ensuring that both half-reactions have the same number of electrons. Multiply the oxidation half-reaction by 3 and the reduction half-reaction by 2 to get 6e– for each half-reaction.

• 2 Fe3+(aq) + 6e– → 2 Fe(s)
• 3 Mg(s) → 3 Mg2+(aq) + 6e–

When these two half reactions are added together, we get the following balanced equation:

• 2 Fe3+(aq) + 3 Mg(s) → 2 Fe(s) + 3 Mg2+(aq)

Conclusion

Oxidation and reduction (redox reaction) are chemicals in which two chemicals exchange electrons. Any chemical process or reaction in which the oxidation number of a particle, molecule, atom, or ion modifies by gaining or losing an electron is known as an oxidation-reduction reaction. There are a few oxidation and reduction examples stated above.