College Board-Aligned Original Notes
AP Chemistry 9.5: Free energy of dissolution
Analyze dissolving using enthalpy, entropy, and free energy.
Aligned to Thermodynamics and Electrochemistry from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.
What To Know
- Dissolution can be favorable or unfavorable depending on delta H and delta S.
- Hydration of ions can release energy.
- Temperature can change solubility when entropy and enthalpy compete.
Detailed Notes
Free energy of dissolution is part of Unit 9: Thermodynamics and Electrochemistry. The main skill is to analyze dissolving using enthalpy, entropy, and free energy. Before answering, decide whether the prompt is asking for a particulate explanation, a mathematical setup, a graph interpretation, or a connection between more than one representation.
The first idea to keep straight is that dissolution can be favorable or unfavorable depending on delta H and delta S. In the same topic, remember that hydration of ions can release energy. A complete AP answer also uses the fact that temperature can change solubility when entropy and enthalpy compete. These ideas should be tied to specific particles, charges, attractions, energy changes, or measured quantities rather than stated as isolated facts.
For calculations or symbolic work, anchor the solution with delta Gdissolution = delta H - T delta S. Define what each quantity represents, substitute values with units, and check whether the sign, magnitude, charge balance, atom balance, or equilibrium direction makes chemical sense for this topic.
Some endothermic dissolving processes are favorable because entropy increases enough. In a free-response explanation, state the chemistry concept first, show the relevant equation or representation, and then explain how the evidence supports the conclusion for free energy of dissolution.
Key Vocabulary
Dissolution
The process of separating solute particles and dispersing them in solvent.
Hydration enthalpy
Energy released when ions become surrounded by water molecules.
Lattice enthalpy
Energy associated with separating ions from an ionic lattice.
Solubility favorability
The free-energy balance that determines whether dissolving is favorable.
Useful Relationships
Worked Study Approach
Some endothermic dissolving processes are favorable because entropy increases enough.
Common Mistakes
- Using a memorized rule without explaining the chemical reason behind it.
- Forgetting to conserve atoms, charge, energy, or units when the topic involves calculations.
- Mixing up particle-level explanations with macroscopic observations.
Quick Practice
How would you explain Free energy of dissolution in one sentence?
Use the focus statement above, then add one particle-level or mathematical detail.
What evidence would support an AP-style answer on this topic?
Use a balanced equation, diagram, graph, table, numerical setup, or particulate model depending on the prompt.