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Learning objective

Calculate entropy changes from standard entropy data.

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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

Calculate entropy changes from standard entropy data

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

Key concepts

entropy

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 standard entropy change (ΔS°) for the reaction: 2H₂(g) + O₂(g) → 2H₂O(g) given the standard molar entropies: S°(H₂) = 130.6 J K⁻¹ mol⁻¹, S°(O₂) = 205.0 J K⁻¹ mol⁻¹, and S°(H₂O) = 188.8 J K⁻¹ mol⁻¹?

Direct answer

−228.6 J K⁻¹

Key terms

  • Standard Entropy (S°): The entropy of a substance at a standard state (1 bar pressure and a specified temperature, usually 298 K).
  • Gibbs Free Energy (G): A thermodynamic quantity that combines enthalpy and entropy to determine the spontaneity of a process.

Common trap

Incorrectly calculating ΔS°: Always ensure to account for the coefficients from the balanced equation when summing the standard entropies.

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