Redox reactions and their different types

Introduction

In a chemical reaction, oxidation and reduction always go side-by-side. The reactions in which oxidation and reduction occur simultaneously are called “redox reactions.” There are different types of redox reactions. In this article, we will see all of its varieties, along with some examples.

Chemical reactions, which involve the transfer of electrons from one chemical substance to another are called “oxidation-reduction” or “redox reactions.” In such reactions, one element gets reduced, and the other gets oxidized. Such reactions involve two half-reactions, one involving loss of electron/s (oxidation) and the additional involving gain of electron/s (reduction).

Types of Redox Reactions

The following are the five types of redox reactions:

1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
   1. Metal displacement
   2. Non-metal displacement
4. Disproportionate and Oxidation-reduction
5. Comproportionation 

Type 1: Combination reaction

A compound is formed by a chemical combination of two or more compounds or elements. The combination of elements or compounds with oxygen is called combustion. Such combinations that involve an oxidation state change are called “redox reactions.” 

The combination reactions generally resemble the equation,

 X + Y ⟶ Z. 

Here is an example: 

C(s) + O2(g) ⟶ CO2(g)

In the example above, the reactants C and O2 have an oxidation state of zero because they are in their molecular forms. On the product side, the oxidation state of carbon is changed to +4, and for oxygen, it is -2. We can see that simultaneous oxidation and reduction occur in one reaction. Hence, it is a combination redox reaction.

Other examples include:

H2 + Cl2 ⟶ 2HCl

3Mg + N2 ⟶ Mg3N2

Type 2: Decomposition reaction

Decomposition is the reverse process of a combination reaction. A decomposition reaction involves the breakdown of a compound into two or more components. The product side contains at least one component in its elemental state. 

The general equation for the decomposition reaction looks like 

AB ⟶ A + B. 

2H2O (g) ⟶ 2H2 (g) + O2 (g)

The reaction above falls under the category of “decomposition reactions,” where two molecules of H2O are decomposed into H2 and O2. As a result, the oxidation state of hydrogen changed from +1 to 0. Hence, it is an example of reduction, while the oxidation state of oxygen changed from -2 to 0, which is oxidation. 

Other examples include:

2NaH (s) ⟶ 2Na (s) + H2 (g)

Note: All decomposition reactions are not redox reactions.

For example, CaCO3 (s) ⟶ CaO (s) + CO2 (g)

In the reaction above, CaCO3 decomposes into CaO and CO2, but the oxidation state of calcium (Ca), carbon (C), and oxygen (O) has not changed.

Type 3: Displacement reaction

A displacement reaction is one of the types of redox reactions in which another atom or ion displaces an atom or ion in a compound. The general equation for displacement reactions is A + BC ⟶ AC + B. Displacement reactions are further divided into two categories.

Metal displacement

In these reactions, metal in a compound is replaced by another in an uncombined state. It is found that metals with a strong reducing character can displace other metals with a weaker reducing character.

For example: CuSO4 + Zn (s) ⟶ ZnSO4 (aq) + Cu (s)

In the example above, copper is replaced by zinc to give products such as ZnSO4 and Cu. These types of reactions are important in metallurgical extraction.

Non-metal displacement

Non-metal displacement reactions generally involve redox reactions wherein hydrogen is displaced.

Alkali and alkaline earth metals are highly electropositive; they displace hydrogen from cold water. 

For example:

2Na(s) + 2H2O (l) ⟶ 2NaOH (aq) + H2 (g)

Less reactive metals like iron and magnesium displace hydrogen gas in reaction with steam. 

For example:

Mg (s) + 2H2O (g) ⟶ Mg(OH)2 (s) + H2 (g)

The metals which cannot displace hydrogen in reaction with water or steam might displace hydrogen by acid treatment. 

For example:

Zn (s) + 2HCl (aq) ⟶ ZnCl2 (aq) + H2 (g)

The noble gases like Ag and Au do not displace hydrogen even after being treated with acids. The rate of evolution of H2 depends on the reactivity of the metal element.

Type 4: Disproportionation reaction

There are circumstances when the same compound may act simultaneously as an oxidising agent and a reducing agent. Part of it gets oxidised to a higher oxidation state, and the rest gets reduced to a lower form of oxidation. 

Such reactions in which a substance simultaneously undergoes oxidation and reduction are called “disproportionation reactions.” That substance is called disproportionate. Let’s see some examples to understand this better.

2H2O2 ⟶ 2H2O + O2

In the example above, the oxygen’s oxidation number changed from -1 (in peroxide) to -2 (in H2O) and 0 (in O2). As a result, the same element gets oxidised and reduced in this reaction. Hence, oxygen is disproportionate. Therefore, this reaction is called a “disproportionate reaction.”

More examples of disproportionate reaction include:

4KClO3 ⟶ 3KClO4 + KCl

Here, the oxidation state of chlorine changed from +5 in KClO3 to +7 in KClO4 and -1 in KCl. Therefore, this reaction falls in the category of disproportionate reaction.

You might have noticed three different oxidation states of elements in disproportionate reactions. From this, we can understand that if a component on the reactant’s side is at its highest or lowest oxidation state, it will not undergo a disproportionate reaction because, at the highest oxidation state, it can only decrease on the product side and cannot increase beyond its highest number.

For example: 

The oxidation state of chlorine lies between -1 to +7. Thus ClO4- does not undergo a disproportionation reaction because chlorine is at its highest oxidation state, i.e., +7.

Type 5: Comproportionation reaction

This type of redox reaction is the reverse process of the disproportionation reaction. 

For example:

10Br- + 2BrO3- + 12H+ ⟶ 6Br2 + 6H2O

Conclusion

In this article, we learned what redox reactions are and also discussed the five different types of redox reactions. The five main types of redox reactions are:

1. In a combination reaction, two compounds are combined to give a product.
2. In a decomposition reaction, the breakdown of the reactant occurs to give different products. 
3. An atom or ion gets displaced by another element in a displacement reaction. Displacement redox reactions are classified into two categories, metal displacement, and non-metal displacement reactions. 
4. Disproportionate reactions happen by the oxidation and reduction of the same element.
5. Lastly, a comproportionation redox reaction is the reverse of a disproportionate redox reaction. 

You are more likely to come across the first four types of redox reactions because the last type, comproportionation redox reaction, is not very popular.

On the reactants side, the oxidation state of bromine (Br) is -1 (in Br-) and +5 (in 2BrO3-)