College Board-Aligned Original Notes
AP Chemistry 9.4: Free energy and equilibrium
Connect delta G, Q, K, and equilibrium position.
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
- At equilibrium, delta G = 0.
- Standard free energy relates to the equilibrium constant.
- Q compared with K predicts reaction direction.
Detailed Notes
Free energy and equilibrium is part of Unit 9: Thermodynamics and Electrochemistry. The main skill is to connect delta G, Q, K, and equilibrium position. 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 at equilibrium, delta G = 0. In the same topic, remember that standard free energy relates to the equilibrium constant. A complete AP answer also uses the fact that q compared with K predicts reaction direction. 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 G = delta G standard + RT ln Q; delta G standard = -RT ln K. 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.
If Q < K, the reaction proceeds forward to reach equilibrium. 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 and equilibrium.
Key Vocabulary
Standard free energy
The free energy change when reactants and products are in standard states.
Equilibrium linkage
The relationship between delta G standard and the equilibrium constant.
Nonstandard free energy
The free energy change under actual concentrations or pressures.
Driving force
The tendency of a system to move toward lower free energy.
Useful Relationships
Worked Study Approach
If Q < K, the reaction proceeds forward to reach equilibrium.
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 and equilibrium 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.