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5.1.5Unit 5. Transition Metals and Organic Nitrogen Chemistry

Thermodynamics and Electrode Potential — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 11 of 43 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Feasibility and Free Energy

Positive E⦵cell
  • Positive E⦵cell A positive standard cell potential means the reaction is thermodynamically feasible as written under standard conditions.
Free energy
  • Free energy The standard free energy change is given by ΔG⦵ = -nFE⦵cell, where n is the number of electrons transferred.
Faraday constant
  • Faraday constant F is the charge on one mole of electrons, 96 500 C mol⁻¹, so ΔG⦵ is in J mol⁻¹ when E is in volts.
Sign
  • Sign A positive E⦵cell gives a negative ΔG⦵, which agrees with the idea that spontaneous reactions release free energy.

Part 2 of 5Equilibrium Link

Equilibrium Link
Relationship
  • Relationship ΔG⦵ = -RT ln K, so a large positive E⦵cell corresponds to a very large equilibrium constant.
Small voltages
  • Small voltages When E⦵cell is close to zero, K is near 1 and a mixture of reactants and products is present at equilibrium.
Direction
  • Direction A negative E⦵cell means K is below 1, so the reverse reaction is the favoured direction.

Part 3 of 5Why Predictions Can Fail

1
Conditions are rarely standard, and changing concentration or temperature shifts the electrode potentials from their E⦵ values.
2
A reaction may have a very high activation energy, so it is feasible but so slow that no change is observed.
3
Predictions apply only to the species written in the half-equations and ignore any competing reactions such as complex formation.
4
A reaction can be driven in the apparently unfavourable direction if a product is continuously removed, for example by precipitation.
5
Therefore E⦵ values predict whether a reaction can happen, but not how quickly it will take place.

Part 4 of 5Worked Example

Question Calculate ΔG⦵ for the cell reaction Zn + Cu²⁺ → Zn²⁺ + Cu, given E⦵cell = +1.10 V and F = 96 500 C mol⁻¹.

  1. n = 2 because two electrons are transferred per zinc atom.
  2. ΔG⦵ = -nFE⦵cell = -2 x 96 500 x 1.10 = -212 300 J mol⁻¹.
  3. Convert to kilojoules-212 kJ mol⁻¹ (3 s.f.).

Answer ΔG⦵ = -212 kJ mol⁻¹, so the reaction is feasible

Part 5 of 5Exam Checklist & Tips

5.1.5 – Key Points for the Exam

  • Faraday constant The charge carried by one mole of electrons, 96 500 C mol⁻¹.
  • ΔG⦵ Standard free energy change, negative for a feasible reaction.
  • Equilibrium constant Ratio of product to reactant concentrations at equilibrium.
  • Kinetic stability A feasible reaction that does not proceed because of a high activation energy.

Common Mistakes

  • Using kilojoules for ΔG in the equation without converting from joules.
  • Forgetting to include n, the number of electrons, in the calculation.
  • Saying a feasible reaction must be fast.
  • Treating a positive E⦵cell as proof that a reaction happens under non-standard conditions.

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