Gibbs Free Energy & Equilibrium

Curriculum Area

Subject: A-Level Chemistry
Exam Board: AQA / OCR / Edexcel (UK Specifications)
Topic: Thermodynamics – Gibbs Free Energy, Equilibrium & Feasibility
Level: Year 12 & 13 (A-Level)

Lesson Objectives

By the end of the lesson, students will be able to:

• Explain the concept of Gibbs Free Energy (ΔG) and its relation to feasibility.
• Use the Gibbs Free Energy equation: ΔG = ΔH - TΔS to determine reaction feasibility.
• Understand the link between ΔG, equilibrium, and spontaneity.
• Apply knowledge to A-Level exam-style questions.
• Engage in hands-on activities that reinforce concepts.

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Lesson Breakdown (90 mins)

Starter (10 mins) – Feasibility in Real Life

• Engagement Question: "Why do some reactions happen naturally, but others need energy?"
• Real-World Examples: Ice melting, spontaneous combustion, rusting.
• Quick Class Discussion: What makes a reaction feasible?

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Concept Understanding (20 mins) – Gibbs Free Energy

Teacher Explanation:

• Introduce Gibbs Free Energy equation:
  ΔG = ΔH - TΔS
  • ΔG < 0 → Feasible Reaction
  • ΔG = 0 → Equilibrium
  • ΔG > 0 → Non-Feasible Reaction
• Explain the role of entropy (ΔS) and enthalpy (ΔH).
• Link feasibility to temperature (T) changes.

Class Activity: Whiteboard Challenge (10 mins)

• Teacher writes ΔH and ΔS values for different reactions.
• Students work in pairs to calculate ΔG and determine feasibility.
• Answers discussed as a group.

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Investigation (25 mins) – Hands-on Experiment

Practical: Predicting Feasibility with Temperature
Objective: Testing feasibility using the dissolution of ammonium nitrate.

Materials:

• Ammonium nitrate
• Water
• Thermometer
• Beaker
• Stirring rod

Method:

1. Measure initial water temperature.
2. Dissolve ammonium nitrate and stir.
3. Measure final temperature.

Class Discussion:

• Was the reaction feasible at room temperature?
• If the temperature were different, how might ΔG change?
• Are all endothermic reactions feasible?

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Higher Order Thinking (15 mins) – Link to Equilibrium

• Point out that when ΔG = 0, the reaction is at equilibrium.
• Interactive Group Challenge: Given a reaction at dynamic equilibrium, students predict how ΔG will shift if conditions change (Le Chatelier’s Principle integration).
• Mini Quiz: 5 short-answer questions testing Gibbs Free Energy, Equilibrium, and Feasibility understanding.

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Plenary (10 mins) – Exam-Style Question & Reflection

• Exam Question: Students attempt an A-Level past paper question on ΔG.
• Peer Marking: Students swap and give constructive feedback.
• Reflection: "One key takeaway from today’s lesson?"

Differentiation:

• Support provided through scaffolded questions, visual aids, and structured worksheets.
• Challenge via synoptic links to entropy, enthalpy, and equilibrium.

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Assessment & Homework

• Exit Ticket: Define feasibility in 10 words or less.
• Homework: Research a real-world reaction where feasibility changes with temperature.

Resources

• Worksheets (calculations & past paper questions)
• Coloured cards for interactive learning
• AQA / OCR / Edexcel revision guides

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This lesson is designed to be engaging, hands-on, and accessible to all students, including those with learning difficulties. The interactive components ensure strong understanding while catering to different learning styles.

Teacher Tips:

• Encourage discussion & participation – real-world examples make ΔG relatable.
• Keep the experiment simple but thought-provoking.
• Use visuals, diagrams, and scaffolded tasks to support learners.

This structured yet dynamic approach will make Gibbs Free Energy memorable and practical! 🚀