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Learning objective

Use ΔG = ΔH - TΔS to calculate Gibbs free energy change.

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Topic

Thermodynamics (A-level only)

Subtopic

Entropy and Gibbs free energy (A-level only)

Aqa A Level ChemistryPhysical chemistry

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Quick explanation

Use ΔG = ΔH - TΔS to calculate Gibbs free energy change

  • This point belongs to Thermodynamics (A-level only), especially Entropy and Gibbs free energy (A-level only).
  • You need to be able to use ΔG = ΔH - TΔS to calculate Gibbs free energy change.
  • The key ideas to know are Gibbs free energy.
  • Use the linked flashcards and practice questions to check recall, then practise applying the idea in an exam-style answer.

Key concepts

Gibbs free energy

Why it matters

This objective helps connect Entropy and Gibbs free energy (A-level only) to exam-style questions, flashcards, and revision notes for Thermodynamics (A-level only).

Quick student answer

What is the Gibbs free energy change (ΔG) for a reaction at 298 K if the enthalpy change (ΔH) is -100 kJ/mol and the entropy change (ΔS) is 200 J/(mol·K)?

Direct answer

ΔG = ΔH - TΔS = -100 kJ/mol - (298 K × 0.2 kJ/(mol·K)) = -100 kJ/mol - 59.6 kJ/mol = -80 kJ/mol

Key terms

  • Spontaneous Reaction: A reaction that occurs without the need for external energy input, indicated by a negative ΔG.
  • Thermodynamic Equilibrium: A state in which the Gibbs free energy is at a minimum and the system is in balance, with no net change occurring.

Common trap

Incorrect Unit Conversion: Always ensure units are consistent; convert J to kJ by dividing by 1000.

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