Class: Senior Secondary School 2 (SS2 / SSS2)
Term: Second Term
Week: 7
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Chemical Families
Previous Lesson: Halogens: Meaning, Configuration and Properties.
Topic: HALOGENS
Subject Matter: Compounds of halogens, uses of halogens (Cl2, Br2, I2), preparation of chlorine

Specific Objectives

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

Cognitive Domain:

Explain the gradation of properties of halogens down the group.
List common compounds of halogens.
State various uses of chlorine, bromine, and iodine.

Affective Domain:

Appreciate the importance of halogens in daily life.
Demonstrate a cautious attitude towards handling chemicals, especially during the preparation of chlorine.

Psychomotor Domain:

Describe the bleaching action of chlorine.
Observe and record the steps involved in the laboratory preparation of chlorine gas.

Participate actively in class discussions about the properties and uses of halogens.
Work collaboratively during observational activities.

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

Instructional Materials

The teacher will teach this lesson with the aid of:

Water
Apparatus and reagents for the preparation of chlorine gas (e.g., round-bottom flask, thistle funnel, delivery tube, gas jar, conc. HCl, MnO₂)
Samples of bromine and iodine (if available)
Litmus paper, coloured flower/cloth for bleaching test

Rationale for the Lesson

This lesson helps pupils understand the characteristics and trends of halogens as a chemical family. It enables them to identify the practical applications of these elements in various industries and daily life, including essential safety considerations during their preparation.

Prerequisite/Previous Knowledge

Pupils should have prior knowledge of the periodic table, groups and periods, general properties of non-metals, and basic chemical reactions.

Lesson Content/Board Summary

HALOGENS

Introduction to Halogens

Halogens are a group of highly reactive non-metallic elements found in Group 17 of the periodic table. The word “halogen” means “salt-former” because they readily react with metals to form a wide range of salts. The elements in this group include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At).

Gradation of Properties Down the Group

As one moves down Group 17 from fluorine to iodine, the properties of halogens show a gradual change:

Atomic Radius: Increases down the group due to the addition of new electron shells.
Electronegativity: Decreases down the group as the attraction of the nucleus for valence electrons lessens with increasing atomic size.
Oxidizing Power: Decreases down the group. Fluorine is the strongest oxidizing agent, while iodine is the weakest.
Physical State at Room Temperature: Fluorine (gas), Chlorine (gas), Bromine (liquid), Iodine (solid).
Melting and Boiling Points: Increase down the group due to increasing van der Waals forces.
Reactivity: Decreases down the group. Fluorine is the most reactive, and iodine is the least reactive.

Compounds of Halogens

Halogens form various compounds, including:

Halides: Formed with metals (e.g., NaCl, KBr, CaF₂) and non-metals (e.g., HCl, HBr, PCl₃).
Interhalogen Compounds: Formed between different halogens (e.g., BrF₃, ICl).
Oxyacids: Acids containing oxygen and a halogen (e.g., Hypochlorous acid (HClO), Chloric acid (HClO₃)).

Uses of Halogens (Cl₂, Br₂, I₂)

The following are some important uses of halogens:

Chlorine (Cl₂):
- Used in water purification to kill bacteria.
- Manufacture of polyvinyl chloride (PVC) plastics.
- Bleaching agent for textiles and paper pulp.
- Production of disinfectants and antiseptics.
Bromine (Br₂):
- Manufacture of photographic films (silver bromide).
- Production of dyes and pharmaceuticals.
- Used in fire retardants.
- As a sedative in medicine.
Iodine (I₂):
- Used as an antiseptic (tincture of iodine).
- Essential for the proper functioning of the thyroid gland (present in iodized salt).
- Used in photographic emulsions.
- In the manufacture of certain dyes.

Bleaching Action of Chlorine

Chlorine bleaches by oxidation in the presence of water. When chlorine dissolves in water, it forms hydrochloric acid (HCl) and hypochlorous acid (HClO). Hypochlorous acid is unstable and decomposes to release nascent (atomic) oxygen, which then oxidizes the coloured substance to a colourless one.

Cl₂(g) + H₂O(l) → HCl(aq) + HClO(aq)
HClO(aq) → HCl(aq) + [O]
Coloured substance + [O] → Colourless substance

Preparation of Chlorine Gas

Laboratory Preparation: Chlorine gas is commonly prepared in the laboratory by the oxidation of concentrated hydrochloric acid with manganese(IV) oxide (pyrolusite).

Equation: MnO₂(s) + 4HCl(aq) → MnCl₂(aq) + 2H₂O(l) + Cl₂(g)

Procedure:

Concentrated hydrochloric acid is added drop by drop to manganese(IV) oxide in a round-bottom flask.
The mixture is gently heated to speed up the reaction.
The chlorine gas produced is passed through water to remove hydrogen chloride gas and then through concentrated sulphuric acid to dry it.
The dry chlorine gas is collected by downward displacement of air (since it is denser than air).

Safety Precautions: Chlorine is a poisonous gas. The preparation should always be carried out in a fume cupboard, and inhalation of the gas must be avoided.

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 reviews their previous knowledge about the periodic table, specifically asking them to identify elements in Group 17.
Pupils’ Activity: Pupils respond to the greetings and identify elements like Fluorine, Chlorine, Bromine, and Iodine.
Learning Point: Pupils recall prior knowledge of the periodic table and Group 17 elements.

Step 2: Explanation of Halogens and Gradation of Properties

Time: 10 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains what halogens are and illustrates the gradation of properties (atomic radius, electronegativity, oxidizing power, physical state, melting/boiling points, reactivity) down the group with examples.
Pupils’ Activity: Pupils listen attentively, ask questions for clarification, and take notes on the trends.
Learning Point: Pupils understand the definition of halogens and the trends in their properties.

Step 3: Discussion on Compounds of Halogens

Time: 5 minutes
Teaching Skill: Discussion/Questioning
Teacher’s Activity: The teacher leads a discussion on the types of compounds formed by halogens, such as halides, interhalogen compounds, and oxyacids, providing examples.
Pupils’ Activity: Pupils contribute to the discussion by listing examples of halogen compounds they know.
Learning Point: Pupils identify and understand different types of halogen compounds.

Step 4: Explanation of Uses of Halogens (Cl₂, Br₂, I₂)

Time: 5 minutes
Teaching Skill: Lecture/Listing
Teacher’s Activity: The teacher explains and lists the various uses of chlorine, bromine, and iodine in industries and daily life.
Pupils’ Activity: Pupils listen and note down the uses of each halogen.
Learning Point: Pupils learn about the practical applications of halogens.

Step 5: Demonstration of Bleaching Action of Chlorine

Time: 5 minutes
Teaching Skill: Demonstration/Observation
Teacher’s Activity: The teacher demonstrates the bleaching action of chlorine on a piece of coloured cloth or litmus paper, explaining the chemical process involved.
Pupils’ Activity: Pupils observe the demonstration carefully and record their observations.
Learning Point: Pupils understand how chlorine acts as a bleaching agent.

Step 6: Demonstration of Preparation of Chlorine Gas

Time: 5 minutes
Teaching Skill: Demonstration/Safety Instruction
Teacher’s Activity: The teacher demonstrates the laboratory preparation of chlorine gas, highlighting the apparatus, reagents, steps, and crucial safety precautions.
Pupils’ Activity: Pupils observe the demonstration, noting the procedure and safety measures.
Learning Point: Pupils learn the method for preparing chlorine gas and the necessary safety measures.

Step 7: Evaluation/Review

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

State three trends in properties of halogens down Group 17.
List two common compounds formed by halogens.
Mention two uses each for chlorine and iodine.
Describe briefly how chlorine bleaches.
Write the balanced chemical equation for the laboratory preparation of chlorine gas.

Pupils’ Activity: Pupils answer orally and in writing.
Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 5 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the key points of the lesson and assigns homework: “Research and write down two industrial methods for the preparation of chlorine.”
Pupils’ Activity: Pupils listen to the summary and copy the homework.
Learning Point: Pupils consolidate their understanding and prepare for further learning.

Lesson Keywords

Halogens – Elements in Group 17 of the periodic table, known as salt-formers.
Gradation – A gradual change or progression in properties.
Electronegativity – The tendency of an atom to attract electrons in a chemical bond.
Oxidizing agent – A substance that causes another substance to lose electrons (get oxidized).
Bleaching action – The process by which a coloured substance is made colourless.
Nascent oxygen – Highly reactive atomic oxygen ([O]) produced during a chemical reaction.

Differentiation

The teacher will provide additional support to struggling learners by offering simplified explanations and visual aids, while advanced learners will be encouraged to research more complex halogen compounds and their reactions.

Note for teachers using this lesson plan

Ensure all safety precautions are strictly followed during the demonstration of chlorine preparation due to its toxic nature. Emphasize the real-world applications of halogens to make the lesson more relatable and engaging for the pupils.