Oxidation and Reduction Reactions

The chemical reactions which undergo both oxidation process and reduction process is usually known as the Redox reactions at the same time. We can say by seeing its name ‘Redox’, in which ‘Red’ means reduction and ‘ox’ means oxidation. It is the short form of Reduction and oxidation combined. Redox reactions are a kind of chemical reaction in which the reactants in the chemical reaction undergo some changes in oxidation states. Loss of electrons or increase in the oxidation state of an atom in a reaction is known as Oxidation. The gain of electrons or decrease in the oxidation state of an atom during the reaction is known as Reduction. In this reaction, the reducing substance is an oxidizing agent and oxidizing substance is usually known as a reducing agent. Let’s see the oxidation half and reduction half for a chemical reaction.

For a chemical reaction, A⁰ + B⁰ —-> A⁺ + B^–

Oxidation Half

Loss of electron
A⁰ —-> A⁺

Reduction Half

Gain of electron
B⁰ —-> B^–

Examples:

1) SnCl₂ + 2FeCl₃ —-> SnCl₄ + 2FeCl₂
Sn²⁺ —-> Sn⁴⁺ (Oxidation part)
Fe³⁺ —-> Fe²⁺(Reduction part)

2) 2KClO₃ —-> 2KCl + 3O₂
2KClO₃ —-> 2KCl (Reduction part)
2KClO₃ —-> 3O₂  (Oxidation part)

3) Cl2 + 2 OH^– —-> ClO^– + Cl^– + H₂O
Cl2 —-> ClO^– (Oxidation part)
Cl2 —-> Cl^–  (Reduction part)

Reductants and Oxidants

Oxidant

Oxidant, also known as an oxidizing agent, is a substance that can accept electrons.  

Important Oxidants

In the periodic table, molecules of electronegative elements (Oxygen) and halogens (Fluorine, Chlorine, Bromine, and Iodine) have high oxidation states. The following compounds have the highest Oxidation state, e.g: K₂Cr₂O₇, KMnO₄, HClO₄, H₂SO₄, and KClO₄.

Reductant

A reductant, which is also known as a reducing agent is a chemical substance that can donate electrons.

Important Reductants 

Metals such as sodium, Aluminum, and Zinc and non-metals like Calcium, Phosphorous, Sodium and Hydrogen gas are strong reducing agents. The following compounds have the lowest oxidation state, e.g: H₂C₂O₄, FeSO₄, HgCl₄, SnCl₂, H₂S, and SO₂.

Oxidation Number

Oxidation Number is the power that appears on an atom when all other atoms are removed as ions. Oxidation numbers can be negative or positive.

Determination of Oxidation Number

1.  The sum of the oxidation number of atoms in a neutral compound is equal to zero.
2.  The algebraic sum of the oxidation number of atoms in an ion is equal to the charge of that ion.
3.  The oxidation number of elements in their elementary state is equal to zero.
4.  The oxidation number of the element Fluorine is always equal to -1 in every compound.
5.  The oxidation number of Oxygen in every compound is -2, except in peroxides which are -1.
6.  The metal amalgam oxidation number is zero.
7.  The oxidation number of alkali metals is always +1 and the oxidation number of alkali earth metals is +2.
8.  For P-block elements, the highest oxidation number is always equal to their group number and their lowest oxidation number is equal to their group number minus 8.

Example: Find the oxidation number of the compound K₂Cr₂O₇.

K₂ = 2(+1) {Since oxidation number of potassium is +1}

Cr₂= 2(x)

O₇ = 7(-2) {Since oxidation number of oxygen is -2}

By adding them,

2+2(x)-14 = 0
2x – 12 = 0
2x = 12
x=12/2
x = +6

Therefore, oxidation number of chromium in K₂Cr₂O₇ is +6.

Types of Redox Reactions

There are four types of redox reactions. They are,

1. Decomposition Reaction
2. Combination Reaction
3. Displacement Reaction
4. Disproportionation Reaction

Decomposition Reaction

The reaction which results in the decomposition or breakdown of the compounds is called a decomposition reaction.

AB —–> A + B

Examples:

1) Na₂CO₃ —-> Na₂O + CO₂
In the above reaction, sodium carbonate is decomposed into Na₂O and CO₂.

2) H₂O —–> H₂ + O₂
In the above reaction, Water undergoes hydrolysis and decomposed into hydrogen gas and oxygen gas.

Combination Reaction

The reaction which results in the combining of the compounds is known as a combination reaction. A combination reaction is just the opposite to a decomposition reaction.

A + B —-> AB

Example: 4 Fe + 3O₂ —-> 2Fe₂O₃

In the above reaction, iron combines with oxygen and forms Fe2CO3. This is perfect example for combination reaction.

Displacement Reaction

The reaction in which displacing of the components happens is usually known as a displacement reaction.

A + BC —-> AB + C

Example: CuSO₄ + Zn —-> Cu + ZnSO₄

In above reaction, copper sulphate reacts with zinc and displacement happens which result in formation of copper and zinc sulphate.

Disproportionation Reaction

The reaction in which only a single reactant undergoes both oxidation and reduction is known as Disproportionation Reaction.

A + B —-> AB

Example: P₄ + 3NaOH + 3H₂O —-> 3NaH₂PO₂ + PH₃

In above reaction, phosporous undergoes both reduction and oxidation. So, this is an perfect example Disproportionation Reaction.

FAQs on Redox Reactions

Question 1: What is meant by redox reaction?

Answer:

Redox reaction is the combination both oxidation half and reduction half. Redox reactions are kind of chemical reactions in which the reactants in the chemical reaction undergoes some changes in oxidation states. Usually lose of electrons is called as oxidation. Gain of electrons is known as reduction. If these both reactions happens in the same reaction, then it is called as redox reaction.

Question 2: Give any five principles for finding the oxidation number of compounds in a reaction.

Answer:

1. Alkali metals, which are also known as 1st group elements have an oxidation number of +1 as they have one valency electron.
2. The oxidation number of elements in their standard form is zero.
3. Oxygen always posses an oxidation number of -2 in normal molecules and an oxidation number of -1 in their peroxide bonds.
4. Oxidation number of metal amalgams like Na-Hg is always zero.
5. Highest Oxidation number of p block elements is equal to their group number.

Question 3: Give two examples of redox reactions.

Answer: 

Examples of redox reaction:

1) 2KClO₃ —-> 2KCl + 3O₂
2KClO₃ —-> 2KCl (Reduction part)
2KClO₃ —-> 3O₂  (Oxidation part)

2) Cl₂ + 2 OH^– —-> ClO^– + Cl^– + H₂O
Cl₂ —-> ClO^– (Oxidation part)
Cl₂ —-> Cl^–  (Reduction part)

Question 4: How many types of redox reactions are there? What are they?

Answer:

There four types of redox reaction

• Displacement Reaction
• Decomposition Reaction
• Disproportionation Reaction
• Combination Reaction

Question 5: What is Oxidizing agent? Give some examples of oxidizing agents.

Answer:

Oxidizing agent: Oxidising agent is a substance that can accept electrons. Oxidizing agent is the reducing substance.

Examples:

Electronegative elements (Oxygen) and halogens (Flourine, Chlorine, Bromine and Iodine) have high oxidation state. The following compounds have the highest oxidation state, e.g. K₂Cr₂O₇, KMnO₄, HClO₄, H₂SO₄, KClO₃.