Lesson Note on Redox Reactions: Meaning, Definitions and Oxidation Numbers for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS 2)
Term: Third Term
Week: 3
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Chemical Reactions
Previous Lesson: Sulphur: Compounds, Contact Process and Uses of H2SO4.
Topic: OXIDATION-REDUCTION (REDOX) REACTIONS
Subject Matter: Meaning of redox reactions (reactions involving oxidation and reduction together), oxidation definition, reduction definition, redox reactions, oxidation numbers of central elements in some compounds.

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

• Define oxidation, reduction, and redox reactions.
• State the rules for assigning oxidation numbers to elements in compounds and ions.
• Calculate the oxidation number of a central element in a given compound or ion.

Affective Domain:

• Appreciate the importance of redox reactions in everyday life.
• Develop an interest in calculating oxidation numbers.

Psychomotor Domain:

• Write down the definitions of oxidation and reduction accurately.
• Solve problems involving the calculation of oxidation numbers correctly.

Social Domain:

• Participate actively in class discussions on redox reactions.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• New School Chemistry for Senior Secondary Schools by Osei Yaw Ababio
• Essentials of Chemistry for Senior Secondary Schools by O.A. Oyewole

Instructional Materials

The teacher will teach this lesson with the aid of:

• Textbooks
• Whiteboard/Blackboard
• Flip-charts
• Chart showing common oxidation number rules

Rationale for the Lesson

This lesson helps pupils understand fundamental chemical processes where electrons are transferred, which is important for understanding energy production in batteries, corrosion, and many industrial chemical processes. It also builds foundational knowledge for advanced topics in electrochemistry.

Prerequisite/Previous Knowledge

Pupils have basic knowledge of atoms, ions, chemical bonding, and simple chemical reactions.

Lesson Content/Board Summary

OXIDATION-REDUCTION (REDOX) REACTIONS

Meaning of Oxidation

Oxidation is a chemical process involving:

• Loss of electrons by an atom or ion.
• Gain of oxygen by an atom or compound.
• Loss of hydrogen by an atom or compound.
• An increase in the oxidation number of an element.

Meaning of Reduction

Reduction is a chemical process involving:

• Gain of electrons by an atom or ion.
• Loss of oxygen by an atom or compound.
• Gain of hydrogen by an atom or compound.
• A decrease in the oxidation number of an element.

Redox Reactions

Redox reactions (oxidation-reduction reactions) are chemical reactions where both oxidation and reduction occur simultaneously. One substance loses electrons (is oxidized) while another gains electrons (is reduced). In a redox reaction, the total number of electrons lost equals the total number of electrons gained.

Rules for Assigning Oxidation Numbers

Oxidation number (or oxidation state) is the hypothetical charge an atom would have if electrons were transferred completely according to a set of rules. It helps in identifying which species are oxidized or reduced.

The following are rules for assigning oxidation numbers:

• The oxidation number of an uncombined element is zero (e.g., O₂ = 0, Na = 0, H₂ = 0).
• The oxidation number of a monatomic ion is equal to its charge (e.g., Na⁺ = +1, Cl⁻ = -1, O²⁻ = -2).
• The sum of the oxidation numbers of all atoms in a neutral compound is zero.
• The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge on the ion.
• Group 1 metals (Li, Na, K) usually have an oxidation number of +1 in compounds.
• Group 2 metals (Mg, Ca, Ba) usually have an oxidation number of +2 in compounds.
• Oxygen usually has an oxidation number of -2, except in peroxides (e.g., H₂O₂, where it is -1) and when combined with fluorine (e.g., OF₂, where it is +2).
• Hydrogen usually has an oxidation number of +1 in compounds, except in metal hydrides (e.g., NaH, where it is -1).
• Fluorine always has an oxidation number of -1 in compounds. Other halogens (Cl, Br, I) usually have -1, except when combined with oxygen or a more electronegative halogen.

Calculation of Oxidation Numbers

Example 1: Calculate the oxidation number of Sulfur (S) in H₂SO₄.

Step 1: Assign known oxidation numbers.

• Hydrogen (H) = +1
• Oxygen (O) = -2

Step 2: Set up the equation for the sum of oxidation numbers.

2(Oxidation number of H) + 1(Oxidation number of S) + 4(Oxidation number of O) = 0 (since H₂SO₄ is a neutral compound)

Step 3: Substitute known values and solve for S.

2(+1) + S + 4(-2) = 0

2 + S – 8 = 0

S – 6 = 0

S = +6

The oxidation number of Sulfur in H₂SO₄ is +6.

Example 2: Calculate the oxidation number of Nitrogen (N) in HNO₃.

Step 1: Assign known oxidation numbers.

• Hydrogen (H) = +1
• Oxygen (O) = -2

Step 2: Set up the equation.

1(Oxidation number of H) + 1(Oxidation number of N) + 3(Oxidation number of O) = 0

Step 3: Substitute and solve for N.

1(+1) + N + 3(-2) = 0

1 + N – 6 = 0

N – 5 = 0

N = +5

The oxidation number of Nitrogen in HNO₃ is +5.

Example 3: Calculate the oxidation number of Chlorine (Cl) in KClO₃.

Step 1: Assign known oxidation numbers.

• Potassium (K) = +1 (Group 1 metal)
• Oxygen (O) = -2

Step 2: Set up the equation.

1(Oxidation number of K) + 1(Oxidation number of Cl) + 3(Oxidation number of O) = 0

Step 3: Substitute and solve for Cl.

1(+1) + Cl + 3(-2) = 0

1 + Cl – 6 = 0

Cl – 5 = 0

Cl = +5

The oxidation number of Chlorine in KClO₃ is +5.

Example 4: Calculate the oxidation number of Chromium (Cr) in the dichromate ion, Cr₂O₇²⁻.

Step 1: Assign known oxidation numbers.

• Oxygen (O) = -2

Step 2: Set up the equation for the sum of oxidation numbers, which equals the charge of the ion.

2(Oxidation number of Cr) + 7(Oxidation number of O) = -2 (charge of the ion)

Step 3: Substitute and solve for Cr.

2Cr + 7(-2) = -2

2Cr – 14 = -2

2Cr = -2 + 14

2Cr = 12

Cr = +6

The oxidation number of Chromium in Cr₂O₇²⁻ is +6.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks questions about simple chemical reactions they have learned previously. The teacher then introduces the topic: Oxidation-Reduction (Redox) Reactions.
Pupils’ Activity: Pupils respond to the questions and listen attentively to the introduction of the new topic.
Learning Point: Pupils are prepared for the new lesson and recall prior knowledge.

Step 2: Definition of Oxidation

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher guides pupils to define oxidation in terms of electron transfer, oxygen gain/loss, hydrogen gain/loss, and change in oxidation number, writing the definitions on the board.
Pupils’ Activity: Pupils define oxidation based on the teacher’s guidance and copy the definitions into their notes.
Learning Point: Pupils understand and define oxidation in various contexts.

Step 3: Definition of Reduction

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher guides pupils to define reduction using the same parameters as oxidation (electron transfer, oxygen, hydrogen, oxidation number), emphasizing that it is the opposite of oxidation.
Pupils’ Activity: Pupils define reduction and note the differences and similarities with oxidation.
Learning Point: Pupils understand and define reduction.

Step 4: Redox Reactions

Time: 5 minutes
Teaching Skill: Elucidation
Teacher’s Activity: The teacher explains that oxidation and reduction always occur together in a chemical reaction, hence the term “redox reaction.” The teacher provides simple examples to illustrate this concept.
Pupils’ Activity: Pupils listen and grasp the concept that oxidation and reduction are simultaneous processes.
Learning Point: Pupils understand the nature of redox reactions.

Step 5: Rules for Assigning Oxidation Numbers

Time: 7 minutes
Teaching Skill: Listing/Explanation
Teacher’s Activity: The teacher presents and explains the general rules for assigning oxidation numbers to elements in compounds and ions, using a chart or writing them clearly on the board.
Pupils’ Activity: Pupils listen carefully, ask questions for clarification, and copy the rules into their notes.
Learning Point: Pupils learn the fundamental rules for determining oxidation numbers.

Step 6: Calculation of Oxidation Numbers

Time: 7 minutes
Teaching Skill: Demonstration/Practice
Teacher’s Activity: The teacher demonstrates how to calculate the oxidation number of central elements in compounds like H₂SO₄, HNO₃, and KClO₃, and in an ion like Cr₂O₇²⁻, following the step-by-step method on the board.
Pupils’ Activity: Pupils observe the examples, ask questions, and attempt to solve similar problems provided by the teacher.
Learning Point: Pupils develop the skill of calculating oxidation numbers.

Step 7: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define oxidation and reduction.
2. State any three rules for assigning oxidation numbers.
3. Calculate the oxidation number of Nitrogen in N₂O₅.
4. Calculate the oxidation number of Manganese in MnO₄⁻.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the definitions of oxidation, reduction, redox reactions, and the importance of oxidation numbers. The teacher then gives homework.
Pupils’ Activity: Pupils listen to the summary and copy down the assigned homework.
Learning Point: Pupils consolidate their learning and prepare for further practice.

Lesson Keywords

• Oxidation – Loss of electrons or increase in oxidation number.
• Reduction – Gain of electrons or decrease in oxidation number.
• Redox Reaction – A chemical reaction involving both oxidation and reduction simultaneously.
• Oxidation Number – The charge an atom would have if electrons were transferred completely.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide more complex compounds for oxidation number calculations (e.g., polyatomic ions with multiple central atoms or organic compounds). For pupils who need more support, the teacher will provide simpler compounds and revisit the rules with additional examples.

Note for teachers using this lesson plan

Teachers should emphasize that oxidation and reduction are complementary processes. Encouraging pupils to practice calculating oxidation numbers with various compounds is essential for mastery. Real-life examples of redox reactions (e.g., rusting, batteries, photosynthesis) can be used to make the lesson more relatable.