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9.5Thermodynamics and Electrochemistry

Free Energy and Equilibrium — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 7 parts Lesson 9 of 15 1 quiz

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Lesson summary

9.5 Free Energy and Equilibrium — AP CHEMISTRY notes: Relate ΔG° to the equilibrium constant and to free-energy diagrams.ΔG°KResultNegativeK > 1Products favoured at equilibriumZeroK = 1Reactants and products comparablePositiveK < 1Reactants favoured at equilibriumRelationship — Δ…

  • Rearranged
  • Calculation
  • Conclusion
  • Favoured reaction
  • Equilibrium position
  • Relationship — ΔG° = −RT ln K

Section 1 of 7Sign of ΔG° and K

ΔG°KResult
NegativeK > 1Products favoured at equilibrium
ZeroK = 1Reactants and products comparable
PositiveK < 1Reactants favoured at equilibrium

Section 2 of 7Equation

Relationship
  • ΔG° = −RT ln K
Rearranged
  • K = e−ΔG°/RT
Constants
  • R = 8.314 J mol-1 K-1 and T in kelvin

Section 3 of 7Pattern: K = 45 at 298 K

Calculation
  • ΔG° = −(8.314)(298) ln 45 = −9431 J mol-1 = −9.43 kJ mol-1
Conclusion
  • Conclusion the forward reaction is favoured

Section 4 of 7Free-energy diagram

Minimum
  • Minimum the lowest point on the free-energy curve is the equilibrium position
Favoured reaction
  • Favoured reaction minimum lies close to products
Unfavoured reaction
  • Unfavoured reaction minimum lies close to reactants
ΔG
  • ΔG the slope shows which direction the reaction proceeds toward the minimum

Section 5 of 7More from the specification

1Precipitation of insoluble crystalline solid
  • Precipitation of insoluble crystalline solid mixing silver nitrate and sodium chloride solutions immediately yields silver chloride as an insoluble white solid precipitate.
2Endothermic
  • A reaction that takes in thermal energy from the surroundings.
3Molecule
  • Two or more atoms held together by covalent bonds.
4Enthalpy change
  • The thermal energy transferred during a reaction, ΔH.
5Kinetic control
  • Rate determined by the activation barrier rather than favourability.
6Activation energy
  • The minimum energy colliding particles need to react.
7Thermodynamically favoured
  • Reaction with ΔG° < 0.
8Coupling reaction
  • Electrophilic substitution of a diazonium ion into an activated ring.
9Temperature
  • A measure of the average kinetic energy of the particles.
10Dissolution
  • Process of a solute dissolving in a solvent.
11Entropy
  • A measure of disorder, related to the number of ways of arranging particles and energy.
12Exothermic
  • A reaction that transfers thermal energy to the surroundings.

Section 6 of 7Key terms

TermMeaning
ΔG°Standard free-energy change.
KEquilibrium constant.
Equilibrium positionPoint of minimum free energy.
RGas constant, 8.314 J mol-1 K-1.

Section 7 of 7Exam technique

Avoid these mistakes
Using kJ with R in J without converting.
Using K with ΔG° at a different temperature.
Saying ΔG° = 0 means no reaction occurs — it means equilibrium.

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