Curriculum

Science Grade XI Chemistry

0/9

Text lesson

Chapter 7: Redox Reactions

Grade 11 Science | Chapter 7

Redox Reactions

Countless reactions are transfers of electrons. This chapter defines oxidation and reduction, the rules for oxidation numbers, and how to identify and balance redox changes.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Giving and Taking Electrons

Oxidation and Reduction

Oxidation Numbers

Rules for Oxidation Numbers

Oxidising and Reducing Agents

Balancing Redox Reactions

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Giving and Taking Electrons

A huge range of reactions, from rusting to respiration, are redox reactions, in which electrons pass from one substance to another. This chapter defines oxidation and reduction in terms of electron transfer, introduces oxidation numbers to track that transfer, and shows how to identify the agents and balance the equation.

Core idea

Oxidation is the loss of electrons and reduction is the gain (OIL RIG). The two always occur together, and oxidation numbers let us track the electrons transferred.

2. Oxidation and Reduction

Oxidation is the loss of electrons and reduction is the gain of electrons, summarised by the phrase OIL RIG (oxidation is loss, reduction is gain). The two always happen together, since electrons lost by one substance are gained by another. A reaction in which both occur is a redox reaction.

Diagram 1 – Redox Is Electron Transfer

An electron transferred from one species to another, one oxidised and one reduced

Fig 1. One substance loses electrons (oxidation) and another gains them (reduction); the two always occur together.

3. Oxidation Numbers

The oxidation number is a charge assigned to an atom as if its bonds were fully ionic; it tracks how many electrons an atom has gained or lost. When the oxidation number of an atom rises, it has been oxidised; when it falls, it has been reduced. Oxidation numbers turn a redox reaction into a simple bookkeeping of electrons.

Diagram 2 – Oxidation Number Changes

A scale of oxidation numbers showing oxidation as a rise and reduction as a fall

Fig 2. A rise in oxidation number is oxidation; a fall is reduction.

4. Rules for Oxidation Numbers

Simple rules assign oxidation numbers. The oxidation number of a free element is zero; of a simple ion, its charge; of hydrogen, usually +1; and of oxygen, usually minus 2. The sum of the oxidation numbers in a neutral compound is zero, and in an ion it equals the ion’s charge. These rules let any atom’s oxidation number be found.

Atom

Usual Oxidation Number

Free element

Hydrogen (in compounds)

Oxygen (in compounds)

minus 2

Simple ion

its charge

5. Oxidising and Reducing Agents

The substance that gains electrons is reduced and is called the oxidising agent, because it oxidises the other. The substance that loses electrons is oxidised and is the reducing agent. So the oxidising agent is itself reduced, and the reducing agent is itself oxidised, which often confuses learners until the electron view is kept in mind.

6. Balancing Redox Reactions

A redox equation is balanced by ensuring that the electrons lost equal the electrons gained, as well as balancing atoms and charge. In the oxidation number method, the rise in oxidation number of the oxidised atom is matched against the fall of the reduced atom, and coefficients are chosen so the changes cancel. This guarantees that no electrons are created or lost.

7. Key Reasoning (Principles)

Principle 1: Oxidation and reduction occur together

Electrons lost by one substance are gained by another, so oxidation and reduction always happen at the same time in a redox reaction.

Principle 2: Oxidation number tracks electrons

A rise in oxidation number means electrons lost (oxidation); a fall means electrons gained (reduction), turning redox into electron bookkeeping.

Principle 3: Electrons lost equal electrons gained

Balancing a redox equation requires that the total electrons lost equal those gained, since electrons are conserved.

8. Worked Examples

Example 1

Q: What is oxidation in terms of electrons?

▶ Show Solution

Oxidation is the loss of electrons (the O and L of OIL RIG).

Answer: Loss of electrons.

Example 2

Q: What is reduction in terms of electrons?

▶ Show Solution

Reduction is the gain of electrons (the R and G of OIL RIG).

Answer: Gain of electrons.

Example 3

Q: What is the oxidation number of a free element such as O₂?

▶ Show Solution

A free element has oxidation number zero.

Answer: Zero.

Example 4

Q: Find the oxidation number of sulphur in H₂SO₄. (H +1, O minus 2)

▶ Show Solution

2(+1) + S + 4(minus 2) = 0, so 2 + S minus 8 = 0.

S = +6.

Answer: +6.

Example 5

Q: Find the oxidation number of manganese in KMnO₄. (K +1, O minus 2)

▶ Show Solution

+1 + Mn + 4(minus 2) = 0, so 1 + Mn minus 8 = 0.

Mn = +7.

Answer: +7.

Example 6

Q: If an atom’s oxidation number rises, has it been oxidised or reduced?

▶ Show Solution

A rise in oxidation number means electrons lost.

So it has been oxidised.

Answer: Oxidised.

Example 7

Q: The substance that gains electrons is the oxidising or reducing agent?

▶ Show Solution

Gaining electrons means it is reduced, so it is the oxidising agent.

Answer: The oxidising agent.

Example 8

Q: Find the oxidation number of carbon in CO₂. (O minus 2)

▶ Show Solution

C + 2(minus 2) = 0, so C minus 4 = 0.

C = +4.

Answer: +4.

Example 9

Q: What must be equal when a redox equation is balanced?

▶ Show Solution

The electrons lost must equal the electrons gained.

Answer: Electrons lost equal electrons gained.

Example 10

Q: Find the oxidation number of nitrogen in NO₃^–. (O minus 2, ion charge minus 1)

▶ Show Solution

N + 3(minus 2) = minus 1, so N minus 6 = minus 1.

N = +5.

Answer: +5.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. Oxidation is the: (a) gain of electrons (b) loss of electrons (c) gain of protons (d) loss of neutrons

2. Reduction is the: (a) loss of electrons (b) gain of electrons (c) gain of protons (d) loss of mass

3. The oxidation number of a free element is: (a) +1 (b) minus 1 (c) 0 (d) +2

4. Oxygen usually has an oxidation number of: (a) +2 (b) minus 2 (c) +1 (d) 0

5. The oxidising agent is itself: (a) oxidised (b) reduced (c) unchanged (d) destroyed

▶ Reveal Answers

1. (b) loss of electrons.

2. (b) gain of electrons.

3. (c) 0.

4. (b) minus 2.

5. (b) reduced.

Set B – Short Answer (Understanding)

1. State the meaning of OIL RIG.

2. Define oxidation number.

3. Why do oxidation and reduction always occur together?

4. What is an oxidising agent?

5. What must be equal when balancing a redox reaction?

▶ Reveal Answers

1. Oxidation Is Loss, Reduction Is Gain of electrons.

2. A charge assigned to an atom as if its bonds were fully ionic, tracking electrons gained or lost.

3. Because electrons lost by one substance are gained by another.

4. A substance that gains electrons and so oxidises another; it is itself reduced.

5. The number of electrons lost must equal the number gained.

Set C – Application and Reasoning

1. Find the oxidation number of sulphur in SO₂. (O minus 2)

2. Find the oxidation number of chlorine in NaCl. (Na +1)

3. If an atom’s oxidation number falls, has it been oxidised or reduced?

4. Find the oxidation number of nitrogen in NH₃. (H +1)

5. Why is the reducing agent itself oxidised?

▶ Reveal Answers

1. S + 2(minus 2) = 0, so S = +4.

2. Na is +1, so Cl is minus 1.

3. Reduced, since a fall means electrons gained.

4. N + 3(+1) = 0, so N = minus 3.

5. Because it loses electrons to the other substance, which is the definition of being oxidised.

Set D – Higher Order (Challenge)

1. Find the oxidation number of chromium in Cr₂O₇^2-. (O minus 2, charge minus 2)

2. Identify the oxidising and reducing agents when zinc displaces copper from copper sulphate.

3. Explain why combustion is a redox reaction.

4. Find the oxidation number of phosphorus in PO₄^3-. (O minus 2)

5. Explain why balancing electrons guarantees a balanced redox equation.

▶ Reveal Answers

1. 2Cr + 7(minus 2) = minus 2, so 2Cr minus 14 = minus 2, 2Cr = 12, Cr = +6.

2. Zinc loses electrons (reducing agent, oxidised); copper ions gain electrons (oxidising agent, reduced).

3. Because the fuel loses electrons to oxygen, which gains them, so both oxidation and reduction occur.

4. P + 4(minus 2) = minus 3, so P minus 8 = minus 3, P = +5.

5. Because electrons are conserved, matching electrons lost to electrons gained ensures charge and atoms also balance.

Chapter Summary

Oxidation and Reduction

Oxidation is loss of electrons; reduction is gain (OIL RIG).

Together

The two always occur together; electrons lost are electrons gained.

Oxidation Number

A charge tracking electrons; rises on oxidation, falls on reduction.

Rules

Free element 0, hydrogen +1, oxygen minus 2, ion equals its charge.

Agents

Oxidising agent is reduced; reducing agent is oxidised.

Balancing

Electrons lost equal electrons gained.

Quantity

Unit

Symbol

Oxidation

loss of electrons

–

Reduction

gain of electrons

–

Oxygen

minus 2

usual

8-Point Exam Quick-Check

Oxidation is loss of electrons; reduction is gain (OIL RIG).

Oxidation and reduction always occur together.

Oxidation number tracks electrons: rises on oxidation, falls on reduction.

Free element 0, hydrogen +1, oxygen minus 2, ion equals its charge.

Sum of oxidation numbers is zero in a compound, the charge in an ion.

The oxidising agent is reduced; the reducing agent is oxidised.

Balancing redox: electrons lost equal electrons gained.

Combustion and rusting are redox reactions.

School Revise Virtual Lab

Explore these ideas with interactive simulations and visual tools.

Open the Virtual Lab →

Class 11 Chemistry Chapter 7: Redox Reactions, Complete Notes and Practice

This revision guide follows the current NCERT Class 11 Chemistry syllabus and develops redox chemistry, covering oxidation and reduction as electron transfer, oxidation numbers and the rules for assigning them, oxidising and reducing agents, and balancing redox equations by the oxidation number method, with two diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.