Types of Redox Reactions: Oxidation, Reduction and Important Questions

Redox reactions are the chemical reactions in which the reactants undergo oxidation and reduction simultaneously. The term ‘redox’ is a short form of reduction-oxidation. There are many types of redox reactions. Here in this article, we will discuss the different types of redox reactions.

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Oxidation Reaction

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Oxidation reactions are the reactions in which oxygen is added to a substance or hydrogen is removed from a substance.

Some examples of oxidation reactions are:

2S(s) + O₂ (g) → SO₂ (g) 

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

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Reduction Reaction

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Reduction reactions are the reactions in which the addition of hydrogen or removal of oxygen takes place in a substance.

Some examples of reduction reactions are:

2CH₂CH₂ (g) + H₂ (g) → CH₃CH₃ (g)

2FeCl₃ (aq) + H₂ (g) → 2FeCl₂ (aq) + 2HCl (aq)

Also Read: Redox Reactions and Types of Redox Reactions

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Oxidizing and Reducing Agents

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Oxidizing Agent: The substance that gains electrons and is reduced to a low valency state is called Oxidising agent.

Reducing Agent: The substance that loses electrons and is oxidized to a higher valency state is called a reducing agent.

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Types of Redox Reactions

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Redox reactions can be mainly divided into four types-

• Combination Reactions
• Decomposition Reactions
• Displacement Reactions
• Disproportionate Reactions

Combination Reaction

This is a reaction, two different molecules reacts with each other to produce one single molecule. We can express the combination reaction as-

A + B → C

In a redox reaction, one of A and B or both of A and B should be present in their elementary form. The redox reactions which make use of dioxygen are referred to as combustion reactions. These reactions are the subdivision of combination reactions. Some important examples of combination reactions are given below:

C(s) + O₂ (g) → CO₂(g)

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O (l)

3Mg(s) + N₂(g) → Mg₃N₂(s)

Decomposition Reaction

Decomposition reactions are exactly the opposite of combination reactions. These types of reactions involve breaking down one molecule into two or more different molecules. As the combination reaction here also minimum one of the products should be present in its elementary form. Some examples of decomposition reaction are-

2NaH (s) → 2Na (s) + H₂(g)

2H₂O (l) → 2H₂ (g) + O₂(g)

2KClO₃ (s) → 2KCl (s) + 3O₂(g)

We have to take the heat in all of the above-mentioned reactions as a necessary condition. 

Exception

All decomposition reactions are not necessarily redox reactions. Like the decomposition of calcium carbonate can not be referred to as a redox reaction.

CaCO₃ (s) → CaO (s) + CO₂(g)

Displacement Reaction

In a displacement reaction, an ion(or atom) of a compound is replaced by another ion(or atom) of another compound. We can represent the displacement reaction as-

X + YZ → XZ + Y

Here we can see the Y atom of the YZ compound is replaced by the X atom and form XZ.

We can further divide the Displacement reaction into two parts.

1. Metal-displacement reactions
2. Non-metal displacement reactions

1. Metal-displacement reactions:

In this type of displacement reaction, one metal present in a combined state is displaced by another metal that is in an uncombined state. This reaction has a huge application in metallurgy where we can obtain metals from their ores. Some examples of these are mentioned below:

CuSO₄(aq) + Zn (s) → Cu(s) + ZnSO₄ (aq)

TiCl₄(l) + 2Mg(s) → Ti(s) + 2 MgCl₂ (s)

V₂O₅ (s) + 5Ca (s) → 2V (s) + 5CaO (s)

Cr₂O₃ (s) + 2 Al (s) → Al₂O₃ (s) + 2Cr(s)

2. Non-metal displacement reactions:

Here, in these types of reactions, a non-metal or metal is displaced by another non-metal present in the compound. Normally, Hydrogen undergoes displacement in these reactions, however, sometimes we can also see oxygens and halogens are also undergoing a displacement reaction.

According to the capability of reducing metal or non-metal, the following non-metal displacement reactions occurs:

• Hydrogen gets displaced by good reducing agents in cold water. Generally, all Alkaline metals and some of the alkaline-earth metals like Strontium, Calcium, and Barium are very good reducing agents.

2Na(s) +2H₂O(I) → 2NaOH(aq) +H₂ (g)

• Less active metals can’t react with hydrogen directly. They react with steam to produce hydrogen gas. Magnesium and iron are examples of less active metals.

Mg(s) +2H₂O(I) → 2Mg(OH)₂ (s)+H₂ (g)

2Fe(s) +3H₂O(I) → Fe₂O₃ (s)+3H₂ (g)

• When reacting in cold water, hydrogen can be displaced by some metals. Though hydrogen can be displaced by these metals from acids. 

Zn(s) +2HCl(aq) → ZnCl₂ (aq)+H₂ (g)

Fe(s) + 2HCl(I) → Fe₂Cl₃ (aq)+H₂ (g)

• Some metals like tin or cadmium do not react with steam to form hydrogen. However, they can displace hydrogen from acids to produce dihydrogen gas.

Cd(s) +2HCl(aq) → CdCl₂ (aq)+ H₂ (g)

• There are some metals like silver and gold which are less reactive. They don’t even react with dilute HCl.

Disproportionate Reaction

This is a special type of redox reaction (Oxidation-Reduction) where one metal undergoes both oxidation and reduction processes. This type of reaction can only occur if one of the elements in the reaction has at least three oxidation states. 

The reacting element has the intermediate oxygen state, the higher and lower oxygen states are present for both oxidation and reduction, respectively. The best example of a decomposition reaction is the decomposition of hydrogen peroxide where Oxygen experiences a disproportionate reaction.

2H₂O₂(aq) → 2H₂O (aq)+ O₂ (g)

Here we can see in peroxide the oxygen is in a -1 oxidation state, it undergoes both oxidation and reduction and changes its oxidation state from -1 to -2 in H₂O and -1 to O in O­­₂.

Compounds like Phosphorous, chlorine and sulphur show a disproportionation reaction in the alkaline medium.

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Things to Remember

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• In oxidation reactions, oxygen is added to a substance, or hydrogen is removed from a substance.
• In reduction reactions, the addition of hydrogen or removal of oxygen takes place in a substance.
• Redox reaction is a reaction in which oxidation and reduction take place simultaneously.
• Redox reactions are of four types: Combination Reaction, Decomposition Reaction, Displacement Reaction and Disproportionate Reaction.
• In a combination reaction, two molecules are combined to form one molecule.
• Decomposition reaction involves breaking down one molecule into two or more different molecules. 
• In a displacement reaction, an ion of a compound is replaced by another ion of another compound. 
• Disproportionate reactions are those reactions in which a substance undergoes oxidation and reduction simultaneously.

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Important Questions

Ques.1: What do you mean by a redox couple? (1 Mark)

Ques.2: Calculate the oxidation number of N in (NH₄)₂ SO₂? (1 Mark)

Ques.3: Calculate what is the oxidation number of oxygen in
(i) OF₂
(ii) O₂F₂ (1 Mark)

Ques.4:Consider the following species. Which of them do not show disproportionation reaction and explain? ClO^{2 –} , ClO^–, ClO^3 – and ClO^{4 –} 
Also, write the disproportionate reaction for each of the other remaining species. (2 Marks)

Ques.5: Why ClO^4- does not show disproportionation reaction whereas ClO^-, ClO^2-, ClO^3- shows? (2 Marks)

Ques.6: Name the different types of redox reaction (2 Marks)

Ques.7: All decomposition reactions are not redox reactions. Give reason. (2 Marks)

Ques.8: How would you know whether a redox reaction is taking place in an acidic/alkaline or neutral medium? (3 Marks)

Ques.9: How many millimoles of potassium dichromate are required to oxidize 24 cm³ of 0.5 M mohr’s salt solution in an acidic medium. (3 Marks)

Ques.10: Suggest which redox reaction falls under which category?
(a) N₂ (g) + O₂ (g) → 2 NO (g)
 (b) 2Pb(NO₃)₂(s) → 2PbO(s) + 2 NO₂ (g) + ½ O₂ (g)
(c) NaH(s) + H₂O(l) → NaOH(aq) + H₂ (g)
(d) 2NO₂(g) + 2OH^– (aq) → NO₂ ^– (aq) + NO₃ ^– (aq) + H₂O(l) (4 Marks)