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9.1Thermodynamics and Electrochemistry

Introduction to Entropy — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 15 1 quiz

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Lesson summary

9.1 Introduction to Entropy — AP CHEMISTRY notes: Explain entropy as disorder and predict how phase changes and reactions alter it.Entropy S — a measure of the number of ways particles and energy can be arranged; the disorder of a systemGreater disorder — higher entropy, and syst…

  • Order of entropy
  • Reverse changes
  • Gas formed
  • More product particles than reactant particles
  • Gas-phase reactions
  • Distribution

Section 1 of 8Entropy

Entropy S
  • Entropy S a measure of the number of ways particles and energy can be arranged; the disorder of a system
Greater disorder
  • Greater disorder higher entropy, and systems tend toward higher entropy
Units
  • J K-1 mol-1

Section 2 of 8Phase changes

1Melting
  • Melting particles become less ordered and freer to move, so entropy increases
2Order of entropy
  • Order of entropy gas > liquid > solid
3Reverse changes
  • Reverse changes freezing and condensation decrease entropy

Section 3 of 8Reactions

1Gas formed
  • CaCO3(s) → CaO(s) + CO2(g) increases entropy
2More product particles than reactant particles
  • More product particles than reactant particles entropy increases
3Gas-phase reactions
  • Gas-phase reactions entropy rises when moles of gas increase

Section 4 of 8Comparing reactions

ChangeΔS
Liquid reactant to gas productsPositive
Gaseous reactants to a solid productNegative
Gaseous reactants to liquid productsNegative, larger
All gases with equal molesAbout zero

Section 5 of 8Energy dispersal

Distribution
  • Distribution higher entropy means energy is spread over more arrangements
Preference
  • Preference systems tend toward states of higher entropy

Section 6 of 8More from the specification

Molecule
  • Two or more atoms held together by covalent bonds.
Element
  • A substance made of only one type of atom.
Standard entropy
  • Entropy of one mole of a substance in J K-1 mol-1.

Section 7 of 8Key terms

TermMeaning
EntropyMeasure of disorder.
DisorderNumber of arrangements of particles and energy.
Phase changeChange of state.
Spontaneous processProcess that is thermodynamically favoured.

Section 8 of 8Exam technique

Avoid these mistakes
Equating entropy with energy.
Saying solids have higher entropy than gases.
Thinking entropy never decreases for a system — it can decrease if surroundings increase more.

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