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4.4.1Additional lessons

Entropy (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 85 of 172

📚 IB Chemistry HL 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 4What Entropy Measures

Entropy helps explain why some endothermic processes still occur spontaneously.

Definition
  • Definition Entropy, S, measures how widely matter and energy are dispersed among the particles of a system and the ways they can be arranged.
Disorder picture
  • Disorder picture A more random, spread-out system has more possible arrangements, which corresponds to higher entropy.
Units
  • Units Standard molar entropy S⦵ is quoted in J K⁻¹ mol⁻¹, so it uses joules and not kilojoules.
Zero point
  • Zero point A perfect crystal at 0 K has zero entropy, so absolute entropy values for substances are always positive.

Part 2 of 4Ordering the States

Gases always have much higher entropy than liquids and solids.

1
A solid has particles fixed in a lattice and vibrating, so it has the lowest entropy of the three states.
2
On melting, particles can move past each other and entropy increases noticeably.
3
On vaporising, particles spread throughout the whole container, giving a very large rise in entropy.
4
Hence S(s) < S(l) << S(g) for the same substance under comparable conditions.
5
Raising temperature increases particle motion and raises the entropy of any substance.

Part 3 of 4Predicting the Sign of ΔS

Predicting the Sign of ΔS
Gas moles
  • Gas moles A reaction producing more moles of gas than it uses, such as CaCO₃(s) → CaO(s) + CO₂(g), has positive ΔS.
Fewer particles
  • Fewer particles A reaction such as N₂(g) + 3H₂(g) → 2NH₃(g) reduces the number of gas moles, so ΔS is negative.
Dissolving
  • Dissolving Dissolving a solid in water usually increases entropy because ions disperse in solution.
Mixing
  • Mixing Mixing two gases or liquids increases the number of arrangements, so entropy rises.

Part 4 of 4Exam Checklist & Tips

4.4.1 – Key Points for the Exam

  • Entropy A measure of the dispersal of energy and matter in a system.
  • Standard molar entropy Entropy of one mole of a substance under standard conditions, in J K⁻¹ mol⁻¹.
  • Microstate One specific arrangement of particles and energy within a system.
  • Second law of thermodynamics The total entropy of the universe increases in any spontaneous process.

Common Mistakes

  • Giving entropy values in kJ K⁻¹ mol⁻¹ without converting to J.
  • Saying that entropy always decreases when a solid dissolves.
  • Believing elements in their standard state have zero entropy; only ΔHf⦵ is zero for elements.
  • Ignoring the number of moles of gas when predicting the sign of ΔS.

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