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Gibbs Free Energy and Thermodynamic Favorability

Gibbs free energy combines enthalpy and entropy into one favorability criterion: ΔG = ΔH − TΔS. A negative ΔG means the process is thermodynamically favored; a positive ΔG, not favored. Because of the −TΔS term, the sign of ΔS and the temperature together decide which side of the crossover a reaction is on.

UNIT 9 TOPIC 9.3 • GIBBS FREE ENERGY AND THERMODYNAMIC FAVORABILITY FAVORABILITY MAP AP Chemistry · Unit 9 · Applications of Thermodynamics Combines enthalpy and entropy to predict favorability at constant temperature and pressure. CORE EQUATION ΔG = ΔH − TΔS ΔG Gibbs free energy ΔH enthalpy change T temperature (K) ΔS entropy change WHAT IT MEANS ΔG tells you whether a process is thermodynamically favored (spontaneous) under the given conditions. ΔG < 0 → thermodynamically favored (spontaneous) KEY POINTS ΔH reflects enthalpic effects (bonds, attractions). ΔS reflects disorder / entropy (more disorder is +). T is always in kelvin (K). The larger the magnitude of −TΔS, the more thermodynamically favored the process. STANDARD ΔG OF REACTION ΔG°rxn = ΣΔG°f (products) − ΣΔG°f (reactants) Add standard free energies of formation (ΔG°f) to find ΔG°rxn for the whole reaction. ΔS > 0 entropy increases ΔS < 0 entropy decreases ΔH < 0 enthalpy decreases ΔH > 0 enthalpy increases I. ΔH < 0 , ΔS > 0 Favored at ALL temperatures ΔG < 0 for all T II. ΔH < 0 , ΔS < 0 Favored at LOW temperature only ΔG < 0 at low T III. ΔH > 0 , ΔS > 0 Favored at HIGH temperature only ΔG < 0 at high T IV. ΔH > 0 , ΔS < 0 NEVER favored at any temperature ΔG > 0 for all T SIGN OF ΔG ΔG < 0 → thermodynamically favored (spontaneous) ΔG = 0 → system at equilibrium ΔG > 0 → not favored (nonspontaneous) CED ANCHOR 9.3.A ΔG depends on the balance of enthalpy (ΔH) and entropy (ΔS), and how T scales the −TΔS term.
Gibbs free energy combines enthalpy and entropy: ΔG = ΔH − TΔS. A negative ΔG means the process is thermodynamically favored. Because of the temperature term, the sign of ΔS and the temperature together decide favorability at the crossover.
Favorability Map · Open the sandbox →

The trap misapplies the Gibbs relation — dropping the minus sign on the entropy term, or judging favorability from ΔH alone (an exothermic reaction is not automatically favored). Combine both terms with the correct signs and temperature.

The work

3 ways in · any order
Lesson
Gibbs Free Energy ›

ΔG = ΔH − TΔS decides favorability (negative = favored), with temperature and ΔS setting the crossover. The lesson applies the relation, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 9.3 misconception: the Gibbs relation misapplied — a lost minus sign or favorability judged from enthalpy alone.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first