Redox Reactions Class 11 Notes

Last Updated: March 15, 2026

Quick Recall Box

• Redox reactions involve the transfer of electrons between species.
• Oxidation and reduction occur simultaneously in a redox reaction.
• The species that loses electrons is said to be oxidized, while the species that gains electrons is said to be reduced.
• Redox reactions can be balanced using the half-equation method.
• Redox reactions are important in various biological and industrial processes.

Table of Contents

1. Introduction to Redox Reactions
2. Why Redox Reactions Matter
3. Ayush's Note
4. Core Concepts
5. Shortcut Formula/Trick
6. Trap Questions/Exceptions
7. Practice MCQs
8. Related Notes Links

Introduction to Redox Reactions

Redox reactions are chemical reactions that involve the transfer of electrons between species. This transfer of electrons results in a change in the oxidation state of the species involved. Redox reactions are also known as oxidation-reduction reactions.

Redox reactions are characterized by the presence of an oxidizing agent and a reducing agent. The oxidizing agent is the species that gains electrons, while the reducing agent is the species that loses electrons. The oxidizing agent is said to be reduced, while the reducing agent is said to be oxidized.

Why Redox Reactions Matter

Redox reactions are important in various biological and industrial processes. In biology, redox reactions are involved in the production of energy in cells, the transport of electrons in the electron transport chain, and the synthesis of ATP. In industry, redox reactions are used in the production of chemicals, the extraction of metals, and the generation of electricity.

For example, in the human body, redox reactions are involved in the production of energy in cells. The electron transport chain, which is a series of redox reactions, generates ATP, which is the energy currency of the cell. In industry, redox reactions are used in the production of chemicals, such as the production of chlorine and sodium hydroxide through the electrolysis of sodium chloride.

3 questions in JEE Mains 2025 Session 1 came from this topic.

Ayush's Note

Core Concepts

Definition of Redox Reactions

Redox reactions are chemical reactions that involve the transfer of electrons between species. Redox reaction is defined as a chemical reaction in which a change in the oxidation state of a species occurs.

Half-Equation Method

The half-equation method is a method used to balance redox reactions. In this method, the reaction is divided into two half-equations: the oxidation half-equation and the reduction half-equation. The oxidation half-equation involves the loss of electrons, while the reduction half-equation involves the gain of electrons.

Oxidation Numbers

Oxidation numbers are used to keep track of the electrons in a redox reaction. The oxidation number of an element is the charge that the element would have if the electrons in the bond were assigned to the more electronegative atom.

The formula to calculate the oxidation number is: $\text{Oxidation Number} = \text{Number of electrons gained or lost}$

For example, in the reaction: $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$ The oxidation number of iron increases from +2 to +3, indicating that iron has lost an electron.

Shortcut Formula/Trick

To balance a redox reaction, we can use the following shortcut formula: $\text{Redox Reaction} = \text{Oxidation Half-Equation} + \text{Reduction Half-Equation}$ This formula helps us to balance the reaction by combining the oxidation and reduction half-equations.

Trap Questions/Exceptions

Wrong Answer: $\text{Fe}^{2+}$ is the reducing agent in the reaction $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$.

Right Answer: $\text{Fe}^{2+}$ is the oxidizing agent in the reaction $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$, but it is being oxidized, so it is the species being oxidized.

Why Students Get it Wrong: Students often get confused between the terms oxidation and reduction. They think that the species that loses electrons is the reducing agent, but it is actually the oxidizing agent that gains electrons.

Practice MCQs

Easy

1. What is the oxidation number of iron in $\text{Fe}^{2+}$? a) +1 b) +2 c) +3 d) +4

Solution: b) +2

2. Which of the following is an example of a redox reaction? a) $\text{NaCl} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{HCl}$ b) $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$ c) $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ d) $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$

Solution: b) $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$

Medium

3. Balance the following redox reaction: $\text{MnO}_4^- + \text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + \text{Fe}^{3+}$ a) $\text{MnO}_4^- + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}$ b) $\text{MnO}_4^- + 3\text{Fe}^{2+} + 4\text{H}^+ \rightarrow \text{Mn}^{2+} + 3\text{Fe}^{3+} + 2\text{H}_2\text{O}$ c) $\text{MnO}_4^- + 2\text{Fe}^{2+} + 3\text{H}^+ \rightarrow \text{Mn}^{2+} + 2\text{Fe}^{3+} + \text{H}_2\text{O}$ d) $\text{MnO}_4^- + \text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + \text{Fe}^{3+}$

Solution: a) $\text{MnO}_4^- + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}$

4. What is the reducing agent in the reaction $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$? a) $\text{Fe}^{2+}$ b) $\text{Fe}^{3+}$ c) $\text{e}^-$ d) None of the above

Solution: a) $\text{Fe}^{2+}$

Hard

5. Balance the following redox reaction: $\text{Cr}_2\text{O}_7^{2-} + \text{Fe}^{2+} \rightarrow \text{Cr}^{3+} + \text{Fe}^{3+}$ a) $\text{Cr}_2\text{O}_7^{2-} + 6\text{Fe}^{2+} + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$ b) $\text{Cr}_2\text{O}_7^{2-} + 3\text{Fe}^{2+} + 4\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 3\text{Fe}^{3+} + 2\text{H}_2\text{O}$ c) $\text{Cr}_2\text{O}_7^{2-} + 2\text{Fe}^{2+} + 3\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 2\text{Fe}^{3+} + \text{H}_2\text{O}$ d) $\text{Cr}_2\text{O}_7^{2-} + \text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + \text{Fe}^{3+}$

Solution: a) $\text{Cr}_2\text{O}_7^{2-} + 6\text{Fe}^{2+} + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$

Related Notes Links

• Chemical Bonding Class 11 Notes
• Electrochemistry Class 12 Notes

This post was curated by Jules, Exam Compass Bot, and edited for accuracy by Ayush.