SAT / PSAT
SAT / PSAT Prep
History & Social Science
AP World History AP US History AP European History AP Human Geography AP US Government & Politics AP Psychology AP Macroeconomics AP Microeconomics
English
AP English Language & Composition AP English Literature & Composition
Math & Computer Science
AP Calculus AB/BC AP Precalculus AP Statistics AP Computer Science A AP Computer Science Principles
Sciences
AP Biology AP Chemistry AP Environmental Science AP Physics 1 AP Physics 2
World Languages & Arts
AP Spanish Language AP Art History AP Music Theory Start studying →
⚖️ Unit 7 · Equilibrium 🏠 Unit Hub 🗂 Flashcards 🗺 Cheat Sheet ⭐ Essentials 🎨 Visual Review 📝 MC Practice ✍️ SAQ Practice

AP Chemistry Unit 7 Essentials

The must-know terms and big ideas for Unit 7: Equilibrium. Every vocabulary word and concept you need to master.

← Back to Unit 7 hub
Big Idea 1
Equilibrium is dynamic, not static — and Q vs. K tells you everything
At equilibrium, the forward and reverse reactions haven't stopped — they're occurring at exactly equal rates, so concentrations stop changing even though the reaction is still happening at the molecular level. Comparing the reaction quotient (Q) to the equilibrium constant (K) at any moment tells you exactly which direction the reaction will shift: Q < K means the forward reaction proceeds (not enough product yet), Q > K means the reverse reaction proceeds (too much product), and Q = K means the system is already at equilibrium.
Dynamic Equilibrium Q vs. K Reaction Quotient
Big Idea 2
A system at equilibrium fights back against change — but only partially
Le Châtelier's principle states that when a stress (a change in concentration, volume/pressure, or temperature) is applied to a system at equilibrium, the system shifts in the direction that partially counteracts that stress, re-establishing equilibrium at a new position. This same logic — comparing Q to K after the stress is applied — explains every Le Châtelier shift, and it extends directly to solubility equilibria, where the common-ion effect is just Le Châtelier's principle applied to a slightly soluble solid's dissolution equilibrium.
Le Châtelier's Principle Solubility Equilibria Common-Ion Effect
🔍
Dynamic equilibrium
The state of a reversible reaction in which the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant over time (though the reaction continues to occur in both directions).
Equilibrium Basics
Reversible reaction
A reaction that can proceed in both the forward and reverse directions under the same conditions.
Equilibrium Basics
Reaction quotient (Q)
An expression with the same form as the equilibrium constant, but calculated using concentrations (or pressures) at any point in time, not necessarily at equilibrium.
Q vs. K
Equilibrium constant (K)
The value of the reaction quotient specifically when a system is at equilibrium; Kc uses molar concentrations, Kp uses partial pressures.
Q vs. K
Q < K
Indicates the system has too few products relative to equilibrium; the forward reaction proceeds (net reaction shifts toward products).
Q vs. K
Q > K
Indicates the system has too many products relative to equilibrium; the reverse reaction proceeds (net reaction shifts toward reactants).
Q vs. K
Magnitude of K
A large K (K >> 1) indicates a reaction that favors products at equilibrium; a small K (K << 1) indicates a reaction that favors reactants.
Equilibrium Constant
Homogeneous vs. heterogeneous equilibrium
A homogeneous equilibrium has all species in the same phase; a heterogeneous equilibrium involves species in different phases (pure solids and liquids are omitted from the K expression).
Equilibrium Constant
ICE table
A table tracking Initial, Change, and Equilibrium concentrations of reactants and products, used to solve for unknown equilibrium values.
Calculations
Le Châtelier's principle
States that when a system at equilibrium experiences a stress (a change in concentration, volume/pressure, or temperature), it shifts in the direction that partially counteracts that stress.
Le Châtelier's Principle
Stress (in equilibrium)
A disturbance to a system at equilibrium — typically a change in concentration, volume, pressure, or temperature — that shifts the position of equilibrium.
Le Châtelier's Principle
Solubility product constant (Ksp)
The equilibrium constant for the dissolution of a slightly soluble ionic compound into its constituent ions.
Solubility Equilibria
Molar solubility
The number of moles of a solid that can dissolve per liter of solution before the solution becomes saturated; calculated from Ksp.
Solubility Equilibria
Common-ion effect
The decrease in molar solubility of a slightly soluble ionic compound when a solution already contains one of the ions involved in its dissolution equilibrium.
Common-Ion Effect
Free energy of dissolution
The Gibbs free energy change associated with a solid dissolving into solution; relates to Ksp the same way ΔG relates to any equilibrium constant (ΔG° = −RT ln K).
Free Energy