Mistake Master
Home› Unit 9 · Applications of Thermodynamics› 9.1· 9.2· 9.3· 9.4· 9.5· 9.6· 9.7· 9.8· 9.9· 9.10· 9.11

Free Energy of Dissolution

Whether something dissolves is a free-energy question, ΔG = ΔH − TΔS. The solution enthalpy is the sum of two parts: the endothermic cost of separating the lattice and the exothermic release of hydrating the ions. Even an endothermic dissolution can be favorable if the entropy gain makes ΔG negative.

UNIT 9 TOPIC 9.6 • FREE ENERGY OF DISSOLUTION DISSOLUTION AND ΔG ΔG = ΔH − TΔS decides whether dissolving is favorable. Gibbs free energy ΔG = ΔH − T ΔS ΔG < 0 favorable (spontaneous) ΔG = 0 equilibrium ΔG > 0 unfavorable (nonspontaneous) Dissolution of NaCl (s → aq) ionic solid (s) H₂O + − Na⁺ Cl⁻ hydrated ions (aq) grey ring = water shell Na⁺ Cl⁻ Temperature dependence of favorability Sign of ΔG = ΔH − TΔS determines whether the process is thermodynamically favorable. ΔH ΔS ΔG = ΔH − TΔS Favorable when ΔH < 0 ΔS > 0 (−) − T(+) → always < 0 ALL temperatures ΔH < 0 ΔS < 0 low T: |ΔH| > |TΔS| LOW temperature ΔH > 0 ΔS > 0 high T: TΔS > ΔH HIGH temperature ΔH > 0 ΔS < 0 (+) − T(−) → always > 0 NEVER favorable CED ANCHOR Endothermic dissolving (ΔH > 0) can still be spontaneous when TΔS > ΔH — entropy of mixing drives it. A large positive ΔS makes the −TΔS term outweigh a positive ΔH, giving ΔG < 0. AP Chemistry · Unit 9 · Applications of Thermodynamics
Whether a substance dissolves is a free-energy question: ΔG = ΔH − TΔS. The solution enthalpy combines the endothermic cost of separating the lattice and the exothermic release of hydrating the ions. An endothermic dissolution can still be favorable when the entropy gain drives ΔG negative.
Dissolution Thermo · Open the sandbox →

The traps misjudge the solution enthalpy — treating lattice separation as exothermic (it costs energy), or leaving out hydration — and rule out endothermic dissolving as impossible (it happens when entropy drives it). Add lattice and hydration, then weigh ΔH against TΔS.

The work

3 ways in · any order
Lesson
Free Energy of Dissolution ›

Dissolving is set by ΔG = ΔH − TΔS, with solution enthalpy = lattice cost + hydration release, so endothermic dissolving can still be favorable. The lesson reasons through both terms, then closes with a ten-scenario check.

Skill check · 10 scenarios
Open lesson →
Diagnostic
10-item topic check ›

Ten items spanning the Topic 9.6 misconceptions: solution enthalpy misjudged (lattice cost and hydration), and endothermic dissolution wrongly ruled out.

Not started · 10 items · ~15 min
Start diagnostic →
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