Lesson Note on Chemical Reactions: Chemical Equilibrium and Le Chatelier’s Principle for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: First Term
Week: 6
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Chemical Interactions
Previous Lesson: Chemical Reactions: Factors Affecting Rate and Energy Changes.
Topic: CHEMICAL REACTIONS
Subject Matter: Chemical equilibrium (introduction using simple equations, Le Chatellier’s principles), factors affecting equilibrium of chemical reactions (concentration, temperature, pressure)

Specific Objectives

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

Cognitive Domain:

• Define chemical equilibrium.
• State Le Chatellier’s principle.
• Identify the factors that affect the position of chemical equilibrium.

Affective Domain:

• Appreciate the dynamic nature of chemical equilibrium.
• Recognise the importance of chemical equilibrium in industrial processes.

Psychomotor Domain:

• Predict the shift in equilibrium when conditions change.
• Write and balance simple equations for chemical reactions at equilibrium.

Social Domain:

• Collaborate effectively in discussing concepts of chemical equilibrium.

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:

• Chart showing examples of reversible chemical reactions.
• Common chemical equations written on the board or chart.
• Simple apparatus for demonstrating colour changes in equilibrium reactions (e.g., cobalt chloride solution).

Rationale for the Lesson

Understanding chemical equilibrium helps pupils to explain why certain reactions do not go to completion and how reaction conditions can be manipulated. This knowledge is important for understanding industrial processes and predicting chemical behaviour in various systems.

Prerequisite/Previous Knowledge

Pupils should have prior knowledge of basic chemical reactions, reaction rates, and how to balance chemical equations.

Lesson Content/Board Summary

CHEMICAL EQUILIBRIUM

Introduction to Chemical Equilibrium

Chemical equilibrium is a state in a reversible chemical reaction where the rate of the forward reaction is equal to the rate of the backward (reverse) reaction. At equilibrium, the concentrations of reactants and products remain constant, but the reactions continue to occur (dynamic equilibrium).

Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Le Chatellier’s Principle

Le Chatellier’s principle states that if a change of condition (such as temperature, pressure, or concentration) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress or counteracts the change.

Factors Affecting Chemical Equilibrium

The position of chemical equilibrium can be shifted by changes in:

• Concentration: If the concentration of a reactant is increased, the equilibrium shifts to the right (forward reaction) to consume the added reactant. If the concentration of a product is increased, the equilibrium shifts to the left (reverse reaction) to consume the added product. Similarly, decreasing concentration shifts the equilibrium to produce more of that substance.
• Temperature:
  • For an exothermic reaction (releases heat, ΔH is negative), increasing the temperature shifts the equilibrium to the left (reverse reaction) to absorb the added heat.
  • For an endothermic reaction (absorbs heat, ΔH is positive), increasing the temperature shifts the equilibrium to the right (forward reaction) to absorb the added heat.
• Pressure: Pressure changes mainly affect reactions involving gases.
  • Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas to reduce the pressure.
  • Decreasing the pressure shifts the equilibrium towards the side with more moles of gas to increase the pressure.
  • If the number of moles of gas is the same on both sides of the equation, pressure has no effect on the equilibrium position.

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, reviews the previous lesson on types of chemical reactions, and introduces the concept of chemical equilibrium as a special type of reversible reaction.
Pupils’ Activity: Pupils respond to greetings, recall previous knowledge, and listen attentively to the introduction of the new topic.
Learning Point: Pupils are prepared for the new lesson and connect it to prior knowledge.

Step 2: Explanation of Chemical Equilibrium

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines chemical equilibrium, explaining it as a dynamic state where forward and reverse reaction rates are equal, and shows a general reversible reaction equation.
Pupils’ Activity: Pupils listen, ask questions for clarification, and copy the definition and example into their notes.
Learning Point: Pupils understand the definition and characteristics of chemical equilibrium.

Step 3: Introduction to Le Chatellier’s Principle

Time: 8 minutes
Teaching Skill: Principle Elucidation
Teacher’s Activity: The teacher states Le Chatellier’s principle and explains its meaning using simple, relatable examples of how a system responds to stress to restore balance.
Pupils’ Activity: Pupils listen carefully, state the principle, and discuss its implications with the teacher.
Learning Point: Pupils grasp Le Chatellier’s principle and its general application.

Step 4: Effect of Concentration on Equilibrium

Time: 7 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains how changes in the concentration of reactants or products affect the equilibrium position, providing an example like the reaction of iron(III) ions with thiocyanate ions.
Pupils’ Activity: Pupils observe the explanation and examples, and predict equilibrium shifts based on concentration changes.
Learning Point: Pupils learn how concentration changes shift the equilibrium.

Step 5: Effect of Temperature on Equilibrium

Time: 6 minutes
Teaching Skill: Explanation/Comparison
Teacher’s Activity: The teacher explains how temperature changes affect equilibrium, distinguishing between exothermic and endothermic reactions and providing examples for each.
Pupils’ Activity: Pupils listen, take notes, and differentiate the effect of temperature on different types of reactions.
Learning Point: Pupils understand the role of temperature in shifting equilibrium for exothermic and endothermic reactions.

Step 6: Effect of Pressure on Equilibrium

Time: 5 minutes
Teaching Skill: Explanation/Application
Teacher’s Activity: The teacher explains the effect of pressure changes on gaseous reactions at equilibrium, highlighting the importance of the number of moles of gas on each side of the equation.
Pupils’ Activity: Pupils listen, analyze examples, and deduce the direction of equilibrium shift based on pressure changes and mole count.
Learning Point: Pupils learn how pressure affects equilibrium in gaseous systems.

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 chemical equilibrium.
2. State Le Chatellier’s principle.
3. List three factors that can affect the position of chemical equilibrium.
4. For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = -92 kJ/mol), what happens to the equilibrium if the temperature is increased?

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 on chemical equilibrium, Le Chatellier’s principle, and the factors affecting equilibrium. The teacher then assigns homework related to predicting equilibrium shifts in various reactions.
Pupils’ Activity: Pupils listen to the summary and copy down the homework assignment.
Learning Point: Pupils consolidate their understanding and prepare for further practice.

Lesson Keywords

• Equilibrium – A state where opposing forces or influences are balanced.
• Le Chatellier’s Principle – States how a system at equilibrium responds to changes in conditions.
• Concentration – The amount of a substance in a given volume.
• Temperature – A measure of the average kinetic energy of the particles in a substance.
• Pressure – Force exerted per unit area, particularly important for gases.
• Reversible reaction – A chemical reaction where reactants form products that, in turn, react to form the original reactants.

Differentiation

For pupils who are struggling, the teacher will provide simplified examples and extra guidance during group activities. More advanced pupils will be challenged with complex equilibrium problems and encouraged to research real-world applications of Le Chatellier’s principle.

Note for teachers using this lesson plan

Encourage pupils to think critically about how changes in conditions affect reaction systems. Practical demonstrations, even simple ones (e.g., colour changes), can significantly enhance understanding. Emphasize the dynamic nature of equilibrium, rather than a static state.