Chemical Equilibrium and Le Chatelier’s Principle for SS 1

Class: SS 1
Term: Second Term
Week: 9
Age: 15 years
Duration: 60 minutes
Subject: Chemistry
Curriculum Theme: The chemical world
Focal competence: Identifying reactions in equilibrium
Key competencies/values: Critical Thinking; Communication
Skills:

• Writing and balancing reversible chemical equations
• Predicting the effects of applied stress (e.g. increased temperature, pressure, concentration, etc.) on systems in equilibrium

Previous Lesson: Chemical Bonds, Types, Properties and IUPAC Naming
Topic: Chemical Equilibrium
Subject Matter: Meaning of Chemical Equilibrium, Chemical equilibrium, Equilibrium equations Le Chatelier’s, Principle

Specific Objectives

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

Cognitive Domain

• Define Chemical Equilibrium.
• Explain chemical equilibrium as a dynamic process.
• Write balanced equations for simple equilibrium reactions.
• State the characteristics of equilibrium systems.

Affective Domain

• Appreciate the industrial applications of Le Chatelier’s Principle.
• Develop critical thinking skills in predicting changes in equilibrium.

Psychomotor Domain

• Write and balance reversible chemical equations correctly.
• Predict the effects of changes in concentration, temperature, and pressure on equilibrium systems.

Social Domain

• Collaborate effectively in group and pair activities to understand and apply equilibrium concepts.

Reference Materials

The following resources were used in planning this lesson:

• 2025 New Revised Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• A suitable Chemistry textbook for SS 1
• The HeadTeacher Scheme of work For The New Revised Senior Secondary Education Curriculum (SSEC)

Instructional Materials

The teacher will teach this lesson with the aid of:

• Ice cubes
• CD and DVD players (for educational videos)
• Source of heat (e.g., Bunsen burner)
• Rope (for demonstrating opposing forces)
• Charts showing examples of reversible reactions and equilibrium systems

Rationale for the Lesson

Understanding chemical equilibrium is fundamental to comprehending how many chemical reactions proceed and reach a steady state. Le Chatelier’s Principle provides a powerful tool for predicting and controlling reaction outcomes, which is crucial in industrial processes and biological systems. This lesson helps students develop analytical skills and appreciate the dynamic nature of chemical changes around them.

Prerequisite/Previous Knowledge

Students should have a basic understanding of chemical reactions, reactants, products, reaction rates, and the concept of balanced chemical equations.

Lesson Content/Board Summary

Chemical Equilibrium

Meaning of Chemical Equilibrium

Chemical equilibrium is a state in a reversible chemical reaction where the rate of the forward reaction (reactants forming products) is equal to the rate of the reverse reaction (products forming reactants). At equilibrium, the concentrations of reactants and products remain constant, although the reactions are still occurring.

Characteristics of Chemical Equilibrium

Equilibrium systems exhibit several key characteristics:

1. Dynamic Nature: Equilibrium is not static; both forward and reverse reactions continue to occur at equal rates.
2. Constant Concentrations: At equilibrium, the concentrations of reactants and products remain constant over time, assuming no external changes.
3. Reversibility: Equilibrium can only be established in reversible reactions, which proceed in both forward and reverse directions.
4. Closed System: Equilibrium can only be attained in a closed system, where no reactants or products can escape or be added.
5. Attainable from Either Direction: Equilibrium can be reached whether starting with reactants or products.
6. Temperature Dependence: The position of equilibrium is dependent on temperature.

Reversible Reactions and Equilibrium Equations

A reversible reaction is one where the products can react to reform the original reactants. These reactions are represented by a double arrow ((rightleftharpoons)) between reactants and products.

Example:

( text{N}_2(g) + 3text{H}_2(g) rightleftharpoons 2text{NH}_3(g) )

In this equation:

1. (text{N}_2(g)) and (text{H}_2(g)) are reactants.
2. (text{NH}_3(g)) is the product.
3. The double arrow indicates that ammonia can decompose back into nitrogen and hydrogen.

Le Chatelier’s Principle

Le Chatelier’s Principle states that if a change of condition (stress) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress and re-establishes a new equilibrium.

Common stresses include changes in concentration, temperature, and pressure.

Effect of Concentration

If the concentration of a reactant or product is changed, the equilibrium will shift to counteract that change:

1. Adding a reactant: The equilibrium shifts to the right (towards products) to consume the added reactant.
2. Removing a reactant: The equilibrium shifts to the left (towards reactants) to replenish the removed reactant.
3. Adding a product: The equilibrium shifts to the left (towards reactants) to consume the added product.
4. Removing a product: The equilibrium shifts to the right (towards products) to replenish the removed product.

Example: (text{Fe}^{3+}(aq) + text{SCN}^-(aq) rightleftharpoons text{FeSCN}^{2+}(aq))

If more (text{Fe}^{3+}) is added, the equilibrium shifts to the right, increasing the concentration of (text{FeSCN}^{2+}).

Effect of Temperature

Temperature changes affect the equilibrium position depending on whether the reaction is exothermic or endothermic:

1. Exothermic reaction ((Delta H < 0)): Heat is a product.
   • Increasing temperature: Equilibrium shifts to the left (towards reactants) to absorb the added heat.
   • Decreasing temperature: Equilibrium shifts to the right (towards products) to produce more heat.
2. Endothermic reaction ((Delta H > 0)): Heat is a reactant.
   • Increasing temperature: Equilibrium shifts to the right (towards products) to consume the added heat.
   • Decreasing temperature: Equilibrium shifts to the left (towards reactants) to produce more heat.

Example: (text{N}_2(g) + 3text{H}_2(g) rightleftharpoons 2text{NH}_3(g) quad Delta H = -92 text{ kJ/mol}) (Exothermic)

Increasing temperature shifts the equilibrium to the left, reducing ammonia yield.

Effect of Pressure

Pressure changes primarily affect reactions involving gases. The system shifts to reduce or increase the total number of gas moles:

1. Increasing pressure: Equilibrium shifts to the side with fewer moles of gas to reduce the pressure.
2. Decreasing pressure: Equilibrium shifts to the side with more moles of gas to increase the pressure.
3. No change in moles: If the number of moles of gas on both sides of the equation is equal, a change in pressure has no effect on the equilibrium position.

Example: (text{N}_2(g) + 3text{H}_2(g) rightleftharpoons 2text{NH}_3(g))

Reactant side has (1+3=4) moles of gas. Product side has (2) moles of gas.

Increasing pressure shifts the equilibrium to the right (fewer moles of gas), favouring ammonia production.

Teaching Methods/Instructional Techniques

Discussion, Explanation, Question and Answer, Pair Work, Group Work, Demonstration, Guided Practice

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Engaging/Recalling

Teacher’s Activity: The teacher greets the students and reviews previous knowledge by asking questions about reversible reactions and reaction rates. The teacher then introduces the concept of chemical equilibrium with a simple analogy, like a tug-of-war where forces are balanced but still active (using the rope). The teacher states the topic for the day.

Pupils’ Activity: Pupils respond to questions and listen attentively to the introduction and analogy.

Learning Point: Introduction to equilibrium

Step 2: Meaning and Characteristics of Chemical Equilibrium

Time: 10 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. The teacher uses examples like the melting and freezing of ice in a closed container (using ice cubes) to illustrate dynamic equilibrium. The teacher then explains the key characteristics of equilibrium systems.

Pupils’ Activity: Pupils listen, ask questions for clarification, and take initial notes on the definition and characteristics.

Learning Point: Equilibrium definition, characteristics

Step 3: Reversible Reactions and Equilibrium Equations

Time: 10 minutes

Teaching Skill: Demonstration/Modelling

Teacher’s Activity: The teacher displays charts of various reversible reactions and explains how to write balanced chemical equations for them, emphasising the use of the double arrow ((rightleftharpoons)). The teacher models writing a few examples on the board, such as the Haber process or the reaction between hydrogen and iodine.

Pupils’ Activity: Pupils observe the charts, participate in discussing the equations, and practice writing simple reversible equations in their notebooks.

Learning Point: Writing equilibrium equations

Step 4: Introduction to Le Chatelier’s Principle

Time: 5 minutes

Teaching Skill: Explanation

Teacher’s Activity: The teacher introduces Le Chatelier’s Principle, stating its definition clearly. The teacher explains that the principle helps predict how an equilibrium system responds to external changes or “stress.”

Pupils’ Activity: Pupils listen and write down the statement of Le Chatelier’s Principle.

Learning Point: Le Chatelier’s Principle statement

Step 5: Effects of Concentration and Temperature on Equilibrium

Time: 10 minutes

Teaching Skill: Guided Discussion/Application

Teacher’s Activity: The teacher guides students to work in pairs (Activity 2) to describe and explain the effects of changing concentration and temperature on equilibrium using charts and equations. The teacher provides examples of exothermic and endothermic reactions and asks students to predict the shift in equilibrium for each scenario. The teacher uses a source of heat to demonstrate a simple reversible reaction (e.g., cobalt chloride hydration/dehydration if available) to show temperature effect.

Pupils’ Activity: Pupils discuss in pairs, predict equilibrium shifts, and observe the demonstration, explaining their observations.

Learning Point: Concentration, temperature effects

Step 6: Effect of Pressure on Equilibrium and Applications

Time: 5 minutes

Teaching Skill: Explanation/Problem Solving

Teacher’s Activity: The teacher explains the effect of pressure changes on gaseous equilibrium systems, focusing on the number of moles of gas. The teacher provides examples and guides students to predict shifts. The teacher then introduces the application of Le Chatelier’s Principle in chemical industries (e.g., Haber process for ammonia, Contact process for sulfuric acid).

Pupils’ Activity: Pupils listen, ask questions, and practice predicting pressure effects. They also note down industrial applications.

Learning Point: Pressure effects, applications

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. What is chemical equilibrium?
2. State two characteristics of a system in chemical equilibrium.
3. Write a balanced equation for a simple reversible reaction.
4. State Le Chatelier’s Principle.
5. How would increasing the temperature affect an exothermic reaction at equilibrium?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Equilibrium concepts understanding

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on chemical equilibrium and Le Chatelier’s Principle into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson notes

Step 9: Conclusion

Time: 5 minutes

Teaching Skill: Summarising

Teacher’s Activity: The teacher briefly summarises the main points of the lesson, reiterating the definition of chemical equilibrium and the importance of Le Chatelier’s Principle in predicting and controlling chemical reactions. The teacher encourages students to think about everyday examples of equilibrium.

Pupils’ Activity: Pupils listen to the summary and prepare for the next lesson.

Learning Point: Equilibrium concepts reinforced

Continuous Assessment/Further Study

Type: Homework/Group Work

Instruction: Answer the following questions and complete the group activity:

1. Define chemical equilibrium and give two examples of reversible reactions.
2. List three characteristics of a system at equilibrium.
3. For the reaction: (text{PCl}_5(g) rightleftharpoons text{PCl}_3(g) + text{Cl}_2(g) quad Delta H = +87.9 text{ kJ/mol}). Predict the effect of:
   1. Increasing the concentration of (text{PCl}_3).
   2. Decreasing the temperature.
   3. Increasing the pressure.
4. In groups, use the internet to research and design illustrative posters on the application of Le Chatelier’s Principle in the chemical industry (e.g., Haber process, Contact process). Present your findings next week.

Lesson Keywords

• Chemical Equilibrium – A state where the rates of forward and reverse reactions are equal, and concentrations of reactants and products remain constant.
• Reversible Reaction – A chemical reaction that can proceed in both forward and reverse directions.
• Forward Reaction – The reaction where reactants form products.
• Reverse Reaction – The reaction where products reform reactants.
• Le Chatelier’s Principle – States that a system at equilibrium will shift to relieve any applied stress.
• Stress – A change in conditions (concentration, temperature, pressure) applied to an equilibrium system.
• Exothermic Reaction – A reaction that releases heat ((Delta H < 0)).
• Endothermic Reaction – A reaction that absorbs heat ((Delta H > 0)).

Differentiation

Support: Provide simplified charts and worked examples for students who struggle with predicting equilibrium shifts. Offer sentence starters for explanations and allow peer tutoring. Focus on understanding the basic definitions and one effect of Le Chatelier’s Principle at a time.

Extension: Challenge advanced students to research more complex industrial applications of Le Chatelier’s Principle or to consider the effect of catalysts on equilibrium (no effect on position, only on rate of attainment).

Suggested Lesson Videos

For further understanding, students can search on YouTube for:
chemical equilibrium le chatelier principle SS1 chemistry