Physical chemistryLesson 2 of 2 in the course Thermodynamics

Gibbs Free Energy: Spontaneity, Reaction Calculations, and Equilibrium

5 flashcards · 5 quiz questions · 5 match pairs · 4 fill-in sentences · English

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Summary

This lesson introduces Gibbs free energy as a system property that helps predict spontaneity at constant temperature and pressure. It also explains how free energy relates to the maximum useful work a process can provide.

Here, TT is the absolute temperature in kelvins, ΔH\Delta H is the system’s enthalpy change, and ΔS\Delta S is its entropy change. Because GG is a state function, its change depends only on the initial and final states, not on the path between them.

All flashcards

Standard free-energy change from enthalpy and entropy
At a specified temperature, calculate it as ΔG∘=ΔH∘−TΔS∘\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ.
Standard free energy of formation
The free-energy change when one mole of a substance forms from its elements in their standard states.
Free-energy changes for combined reactions
Add the reaction free-energy changes; reversing a reaction reverses the sign, and scaling it scales the change.
Nonstandard-condition free-energy change
Calculate it as ΔG=ΔG∘+RTln⁡Q\Delta G = \Delta G^\circ + RT\ln Q.
Equilibrium constant and standard free energy
They are related by ΔG∘=−RTln⁡K\Delta G^\circ = -RT\ln K.

Quiz questions

  1. At constant temperature and pressure, what does ΔG<0\Delta G < 0 indicate for a process as written?Answer: The forward direction is thermodynamically favored.
  2. How is ΔG∘\Delta G^\circ calculated from standard free energies of formation for a balanced reaction?Answer: Sum the stoichiometric product values and subtract the reactant sum.
  3. For a process with ΔH<0\Delta H < 0 and ΔS>0\Delta S > 0, what does ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta S predict?Answer: The forward process is spontaneous at every temperature in this sign-based analysis.
  4. For a reaction mixture with Q<KQ < K, which direction is favored as the system moves toward equilibrium?Answer: The forward direction.
  5. If K>1K > 1 at a specified temperature, what does this indicate about the equilibrium mixture?Answer: Products are favored relative to reactants.

Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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