Le Chatelier shortcuts the algebra for stresses, and solubility products apply the machinery to salts. The tool below drills the three numbers.
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Unit 7 three numbers, from your own notes
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Dynamic means busy: rates match, amounts rest
Equilibrium is not a stop: forward and reverse reactions run at equal rates, so concentrations hold while molecules keep trading places. The definition is tested directly, and the standard trap is treating equilibrium amounts as equal amounts: K decides where the mixture rests, and rest can sit far to either side. Write rates equal, amounts constant, and the concept questions fall.
K sizes the mix; the expression has rules
The equilibrium constant multiplies product concentrations over reactants, each raised to its coefficient, with pure solids and liquids omitted because their concentrations do not change. Large K rests among products, small K among reactants. Kp uses partial pressures with its own conversion. The graded details are the omissions and the powers, since the expression's shape decides every later step.
Q points the direction before any algebra
The reaction quotient Q uses the K expression on any mixture, resting or not: Q below K and the reaction runs forward, Q above K and it reverses, Q equal and it rests. This single comparison answers a whole question family, which way will this mixture move, without an ICE table. Compute Q first, always; the direction organises everything after it.
ICE tables and the small x shortcut
Initial, change, equilibrium: the ICE table writes the stoichiometry into one unknown x, and K's equation solves it. When K is small against the initial concentrations, x barely dents them, and the approximation drops the subtraction, checked afterward by the five percent rule. The exam grades the setup, signs on the change row following the direction Q chose, more than the algebra, so build the table cleanly.
Le Chatelier and Ksp: stress and salts
Le Chatelier predicts the shove: add a species and the system shifts away, compress and it favours fewer gas moles, heat and it runs the endothermic way, temperature alone rewriting K itself. Solubility product applies the machinery to sparingly soluble salts, and the common ion effect is Q logic in disguise: shared ions raise Q, so less salt dissolves. Say which tool you are using and the answer follows.
What to photograph for Equilibrium
Your ICE tables and shift predictions. Related: Unit 6, photo to quiz and pricing.
Sources used on this page
- College Board, AP Chemistry
- Chemical equilibrium
- Reaction quotient
- Le Chatelier's principle
- Equilibrium constant
- 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
| Situation | The tool | The verdict logic |
|---|---|---|
| Which way will it move? | Q against K | Below forward, above reverse |
| Where will it rest? | ICE plus K | Solve the change x |
| Species added | Le Chatelier | Shift away from the addition |
| Volume halved | Gas mole count | Toward fewer gas moles |
| Temperature raised | Reaction's sign | Endothermic direction, new K |
| Common ion present | Q reasoning | Solubility suppressed |
What does AP Chemistry Unit 7 cover?
Equilibrium: the dynamic definition, K and Q, ICE tables, Le Chatelier's principle, and solubility equilibria with the common ion effect.
What is the fastest way to find a reaction's direction?
Compute Q with the K expression and compare: below K runs forward, above K reverses, equal rests.
Why do solids and liquids leave the K expression?
Their concentrations are fixed properties of the pure phase, so they cannot vary with the mixture and are folded into K.
When is the small x approximation safe?
When K is small against initial concentrations; check afterward that x stays within about five percent of what it was subtracted from.
Which stress actually changes K?
Temperature only: concentration and pressure shifts move the mixture along the same K, heat rewrites the constant itself.
Can I build questions from my own Unit 7 notes?
Yes. Upload the chapter as a PDF or photograph the pages, and every question comes from those pages rather than from the wider syllabus.
Last updated: 2026-08-15