Lesson Note on Chemical Reactions: Factors Affecting Rate and Energy Changes for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: First Term
Week: 5
Age: 16 years
Duration: 45 minutes
Subject: Chemistry
Curriculum Theme: Chemical Principles and Reactions
Previous Lesson: Chemical Reactions: Meaning, Reactants, Products and Collision Theory.
Topic: CHEMICAL REACTIONS
Subject Matter: Factors affecting the rate of chemical reactions (nature of substance, concentration, pressure, temperature, catalyst), types of chemical reactions (endothermic reaction, exothermic reaction), energy relationships in endothermic and exothermic reactions (ΔG = ΔH – TΔS, graphical representations only).

Specific Objectives

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

Cognitive Domain:

• Define chemical reaction rate and list factors affecting it.
• Differentiate between endothermic and exothermic reactions.
• State the relationship between Gibbs free energy, enthalpy, and entropy.
• Interpret simple energy profile diagrams for chemical reactions.

Affective Domain:

• Appreciate the importance of reaction rates in industrial processes and daily life.
• Show interest in understanding energy changes during chemical reactions.
• Cooperate with peers during group discussions and experimental observations.

Psychomotor Domain:

• Observe and record the effects of various factors on reaction rate in simple experiments.
• Illustrate endothermic and exothermic reactions through simple experiments.
• Sketch and label energy profile diagrams for endothermic and exothermic reactions.

Social Domain:

• Collaborate effectively in groups to discuss and analyze experimental results.
• Communicate findings clearly during class discussions.
• Participate constructively in evaluating different types of chemical 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.
• https://www.chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/14%3A_Chemical_Kinetics/14.05%3A_Factors_Affecting_Reaction_Rate
• https://www.chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/05%3A_Thermochemistry/5.2%3A_Enthalpy_of_Reaction

Instructional Materials

The teacher will teach this lesson with the aid of:

• Hydrochloric acid (HCl)
• Copper(II) sulphate (CuSO₄)
• Ammonium chloride (NH₄Cl)
• Sodium chloride (NaCl)
• Ammonium hydroxide (NH₄OH)
• Heat source (e.g., Bunsen burner)
• Water
• Test-tubes
• Beakers
• Thermometer
• Stopwatch

Rationale for the Lesson

This lesson helps pupils understand how quickly chemical reactions occur and the energy changes involved. This knowledge is important for understanding processes like food spoilage, industrial production, and how our bodies function, enabling pupils to appreciate the chemistry around them.

Prerequisite/Previous Knowledge

Pupils are expected to have prior knowledge of basic chemical reactions, states of matter, and the concept of energy from previous lessons.

Lesson Content/Board Summary

CHEMICAL REACTIONS

I. Factors Affecting the Rate of Chemical Reactions

The rate of a chemical reaction is how fast reactants are converted into products. Several factors can influence this rate:

• Nature of Reactants: Different substances react at different rates due to their chemical properties, bond strength, and physical state. For example, ionic compounds often react faster than covalent compounds.
• Concentration: For reactants in solution, increasing concentration increases the number of particles per unit volume, leading to more frequent effective collisions and thus a faster reaction rate.
• Pressure: For gaseous reactants, increasing pressure increases the concentration of gas molecules, leading to more frequent effective collisions and a faster reaction rate.
• Temperature: Increasing temperature increases the kinetic energy of reactant molecules, causing them to move faster and collide more frequently and with greater energy. This increases the number of effective collisions, leading to a faster reaction rate.
• Catalyst: A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. It does this by providing an alternative reaction pathway with a lower activation energy.

II. Types of Chemical Reactions based on Energy Change

Chemical reactions involve energy changes, which can be classified as endothermic or exothermic:

• Endothermic Reactions:
  • These are reactions that absorb heat energy from their surroundings.
  • The temperature of the surroundings decreases.
  • The enthalpy change (ΔH) for endothermic reactions is positive (ΔH > 0).
  • Examples: Photosynthesis, dissolving ammonium chloride in water, melting ice.
• Exothermic Reactions:
  • These are reactions that release heat energy into their surroundings.
  • The temperature of the surroundings increases.
  • The enthalpy change (ΔH) for exothermic reactions is negative (ΔH < 0).
  • Examples: Combustion (burning), neutralization reactions, respiration, dissolving concentrated acids in water.

III. Energy Relationships in Endothermic and Exothermic Reactions

The spontaneity of a reaction and its energy changes are described by thermodynamic principles, including Gibbs free energy.

• Gibbs Free Energy Equation: The relationship between Gibbs free energy (ΔG), enthalpy change (ΔH), and entropy change (ΔS) at a given temperature (T) is given by:

  ΔG = ΔH – TΔS

  • ΔG (Gibbs Free Energy Change): Indicates the spontaneity of a reaction.
    • If ΔG < 0, the reaction is spontaneous.
    • If ΔG > 0, the reaction is non-spontaneous.
    • If ΔG = 0, the reaction is at equilibrium.
  • ΔH (Enthalpy Change): Represents the heat absorbed or released during a reaction at constant pressure.
  • T (Temperature): Absolute temperature in Kelvin.
  • ΔS (Entropy Change): Measures the change in disorder or randomness of a system.
• Graphical Representations (Energy Profile Diagrams):
  • Exothermic Reaction: The products have lower energy than the reactants, indicating a net release of energy. The diagram shows the energy of the reactants higher than the energy of the products. The activation energy barrier must be overcome for the reaction to proceed.
  • Endothermic Reaction: The products have higher energy than the reactants, indicating a net absorption of energy. The diagram shows the energy of the products higher than the energy of the reactants. The activation energy barrier must be overcome.
  • Both diagrams illustrate the activation energy (Ea), which is the minimum energy required for a reaction to occur.

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 asks pupils what they understand by a chemical reaction and gives examples of everyday reactions like rusting or cooking.
Pupils’ Activity: Pupils respond to the questions and share their understanding and examples of chemical reactions.
Learning Point: Pupils recall prior knowledge of chemical reactions and are introduced to the lesson topic.

Step 2: Factors Affecting Reaction Rate – Nature of Substance and Concentration/Pressure

Time: 10 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains how the nature of reactants affects reaction rate and demonstrates the effect of concentration using dilute and concentrated HCl reacting with a metal (e.g., magnesium ribbon). The teacher also explains the effect of pressure for gaseous reactions.
Pupils’ Activity: Pupils observe the demonstrations, ask questions, and take notes.
Learning Point: Pupils understand that the nature of reactants, concentration, and pressure influence reaction rates.

Step 3: Factors Affecting Reaction Rate – Temperature and Catalyst

Time: 7 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher demonstrates the effect of temperature by reacting a substance (e.g., effervescent tablet) in cold and hot water. The teacher then explains the role of a catalyst in speeding up reactions, using an example like the decomposition of hydrogen peroxide with manganese(IV) oxide.
Pupils’ Activity: Pupils observe the experiments, compare results, and listen to explanations.
Learning Point: Pupils learn how temperature and catalysts affect the speed of chemical reactions.

Step 4: Types of Chemical Reactions – Endothermic Reactions

Time: 7 minutes
Teaching Skill: Demonstration/Discussion
Teacher’s Activity: The teacher demonstrates an endothermic reaction by dissolving ammonium chloride (NH₄Cl) in water and noting the temperature drop. The teacher then defines endothermic reactions and provides other examples.
Pupils’ Activity: Pupils observe the temperature change, describe their observations, and contribute to the discussion.
Learning Point: Pupils understand and identify endothermic reactions.

Step 5: Types of Chemical Reactions – Exothermic Reactions

Time: 6 minutes
Teaching Skill: Demonstration/Discussion
Teacher’s Activity: The teacher demonstrates an exothermic reaction by adding a small amount of concentrated HCl to water and noting the temperature rise. The teacher then defines exothermic reactions and gives more examples.
Pupils’ Activity: Pupils observe the temperature change, describe their observations, and participate in the discussion.
Learning Point: Pupils understand and identify exothermic reactions.

Step 6: Energy Relationships in Endothermic and Exothermic Reactions

Time: 5 minutes
Teaching Skill: Explanation/Discussion
Teacher’s Activity: The teacher explains the Gibbs free energy equation (ΔG = ΔH – TΔS) and interprets simple energy profile diagrams for both endothermic and exothermic reactions, highlighting activation energy and enthalpy changes.
Pupils’ Activity: Pupils listen, ask questions, and interpret the graphical representations.
Learning Point: Pupils grasp the basic energy relationships in chemical reactions and can interpret energy diagrams.

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. List four factors that affect the rate of a chemical reaction.
2. Distinguish between endothermic and exothermic reactions with one example for each.
3. State the Gibbs free energy equation and explain what ΔG represents.
4. Draw a simple energy profile diagram for an exothermic reaction, labeling the reactants, products, and activation energy.

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
Teacher’s Activity: The teacher summarizes the key learning points of the lesson, emphasizing the importance of understanding reaction rates and energy changes in daily life and industry. The teacher assigns homework: research practical applications of catalysts.
Pupils’ Activity: Pupils listen to the summary, ask any final questions, and copy down the homework.
Learning Point: Pupils consolidate their understanding of the topic and prepare for further study.

Lesson Keywords

• Reaction Rate – The speed at which reactants are converted into products.
• Concentration – The amount of a substance in a given volume.
• Catalyst – A substance that speeds up a chemical reaction without being consumed.
• Endothermic Reaction – A chemical reaction that absorbs heat energy from its surroundings.
• Exothermic Reaction – A chemical reaction that releases heat energy into its surroundings.
• Enthalpy Change (ΔH) – The heat absorbed or released during a reaction at constant pressure.
• Gibbs Free Energy (ΔG) – A thermodynamic potential that measures the “useful” or process-initiating work obtainable from an isothermal, isobaric thermodynamic system.
• Entropy (ΔS) – A measure of the disorder or randomness of a system.
• Activation Energy (Ea) – The minimum energy required for a chemical reaction to occur.

Differentiation

For pupils who grasp concepts quickly, the teacher can encourage them to research advanced concepts like reaction orders or collision theory. For pupils needing more support, the teacher can provide simplified diagrams and offer one-on-one guidance during experimental observations and concept explanations.

Note for teachers using this lesson plan

Ensure all safety precautions are strictly followed during demonstrations involving chemicals and heat. Encourage active participation and critical thinking by asking open-ended questions. Emphasize the real-world applications of reaction kinetics and thermodynamics to make the lesson more relatable.