The verdict wires outward: negative delta G means K above one and positive cell voltage. The tool below drills the connections.
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Unit 9 verdict machinery, from your own notes
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Entropy counts arrangements, and signs are predictable
Entropy measures dispersal: more gas moles, dissolved particles, higher temperature, larger molecules all raise it, and the exam expects sign predictions from the equation alone, gas count first. The second law keeps the universe's total rising for spontaneous change, system and surroundings together, which is why an ordering process can still happen when it exports enough heat.
Gibbs free energy signs the verdict
Delta G equals delta H minus T delta S, and the four sign cases are the unit's core table: favourable both ways, unfavourable both ways, and the two temperature dependent mixes where T flips the verdict, ice melting the classic. Spontaneous means thermodynamically favoured, not fast, kinetics kept its own unit, and delta G's size caps the useful work the change can fund.
The verdict wires to K
Delta G and the equilibrium constant tell one story in two dialects: negative standard delta G pairs with K above one, products favoured at rest; positive pairs with K below one. The relation is monotone, more negative, larger K, and questions test the translation both directions plus the Q connection: away from equilibrium, the instantaneous delta G drives toward it, zero exactly at rest, which ties this unit back to the equilibrium machinery.
Galvanic cells cash the verdict as voltage
A galvanic cell splits a favourable redox pair so electrons travel the wire: oxidation at the anode, reduction at the cathode, the salt bridge closing the circuit. Standard potential reads cathode minus anode off the reduction table, positive for working cells, and the memory anchors spell it out: reduction at the cathode, oxidation at the anode. Electrons flow anode to cathode; current's convention runs opposite, a planted trap.
Electrolysis pays uphill, and Faraday keeps the books
Electrolysis drives unfavourable reactions with an external source, plating metals and splitting compounds; the cathode still reduces, but now by payment. Faraday's arithmetic converts current times time to charge, charge to moles of electrons, and moles of electrons through the half reaction's coefficient to grams plated. Write the chain in units and the calculation grades itself.
What to photograph for Thermodynamics and Electrochemistry
Your sign tables and cell diagrams. Related: Unit 8, photo to quiz and pricing.
Sources used on this page
- College Board, AP Chemistry
- Entropy
- Gibbs free energy
- Galvanic cell
- Electrolysis
- Active recall
- Spaced repetition
- Testing effect
- Forgetting curve
- Generation effect
- Judgment of learning
- Metacognition
- Desirable difficulty
- Distributed practice
- Formative assessment
- Flashcard
- Cloze test
- Multiple choice
- Test (assessment)
- Educational assessment
- Advanced Placement
- Curriculum
- Study skills
- Study guide
- Note-taking
- Overlearning
- Instructional scaffolding
- Item analysis
- Mastery learning
| Question | The tool | The verdict logic |
|---|---|---|
| Will entropy rise? | Count gas moles first | Dispersal predicts the sign |
| Will it go? | Gibbs sign table | Four cases, T can flip two |
| Where does it rest? | Delta G to K | Negative pairs with K above one |
| What voltage? | Cathode minus anode | Positive means it runs |
| Which electrode? | Red cat, an ox | Reduction at cathode, always |
| How much plated? | Faraday chain | Current, charge, moles, grams |
What does AP Chemistry Unit 9 cover?
Spontaneity and its wiring: entropy, Gibbs free energy, the link to equilibrium constants, galvanic cells, electrolysis and Faraday's arithmetic.
Does spontaneous mean fast?
No: thermodynamics favours, kinetics schedules. A favourable reaction can sit for ages behind its activation energy.
When does temperature flip a verdict?
In the mixed sign cases: enthalpy and entropy pulling opposite ways, with T delta S outweighing delta H past a crossover temperature.
How do I compute a standard cell potential?
Cathode's reduction potential minus the anode's, both read as reductions from the table; positive means the cell runs as written.
How does Faraday's law turn current into mass?
Chain the units: current times time gives charge, charge over Faraday's constant gives electron moles, coefficients convert to metal moles and grams.
Can I build questions from my own Unit 9 notes?
Yes. Whatever you upload sets the boundary. Photograph the pages or send the chapter as a PDF and the questions stay inside it.
Last updated: 2026-08-15