Unit Overview
Equilibrium Overview
AP Chemistry· 5 min read 📊 7-9% of overall AP Chemistry exam score
1. Unit at a Glance
This unit builds conceptually from foundational definitions to increasingly complex quantitative calculations and real-world applications. We start with what dynamic equilibrium actually is, then introduce the core metrics Q (reaction quotient) and K (equilibrium constant) that you will use for every type of equilibrium problem.
After mastering general equilibrium rules and Le Châtelier’s principle, we apply these concepts to the specific case of solubility equilibria, a frequently tested topic, before closing by connecting equilibrium back to the free energy concepts you learned in the previous unit.
Below are all sub-topics in this unit, ordered for logical learning:
AP Chemistry Introduction to equilibrium
Learn the definition of dynamic equilibrium and how it differs from static equilibrium and completion reactions.
★⏱ 4 min
AP Chemistry Magnitude of K
Interpret what the size of K tells you about the relative amounts of reactants and products at equilibrium.
★⏱ 3 min
AP Chemistry Reaction quotient Q
Understand how Q is calculated and how it compares to the equilibrium constant K.
★★⏱ 4 min
AP Chemistry Direction of reversible reactions
Use Q and K to predict which direction a reaction will shift to reach equilibrium.
★★⏱ 4 min
AP Chemistry Calculating the equilibrium constant K
Calculate K from known initial and equilibrium concentrations of reactants and products.
★★★⏱ 5 min
AP Chemistry Calculating equilibrium concentrations
Use ICE tables to find unknown equilibrium concentrations when you know the value of K.
★★★⏱ 6 min
AP Chemistry Le Châtelier’s principle
Predict how changes in concentration, temperature, and pressure shift the equilibrium position.
★★★⏱ 5 min
AP Chemistry Solubility equilibria
Apply equilibrium rules to sparingly soluble ionic compounds and calculate the solubility product Ksp.
★★★⏱ 5 min
AP Chemistry Common ion effect
Calculate the solubility of ionic compounds in solutions that already contain one of their ions.
★★★★⏱ 5 min
AP Chemistry pH and solubility
Explain how changing pH impacts the solubility of ionic compounds with basic anions.
★★★★⏱ 4 min
AP Chemistry Free energy and equilibrium
Relate the standard free energy change of a reaction to its equilibrium constant K.
★★★★⏱ 5 min
AP Chemistry Free energy of dissolution
Calculate the standard free energy of dissolution from Ksp and interpret solubility trends.
★★★★★⏱ 4 min
2. Common Pitfalls
Wrong move:
Confusing Q and K, and using K to predict reaction direction before equilibrium is reached
Why:
K only describes the system at equilibrium, while Q describes the system at any point during the reaction
Correct move:
Always use Q (not K) to predict which direction a reaction will shift to reach equilibrium
Wrong move:
Assuming a large K means a reaction will proceed quickly
Why:
Equilibrium describes the extent of a reaction (thermodynamics), not how fast it proceeds (kinetics)
Correct move:
Always separate equilibrium extent from reaction rate when interpreting the value of K
Wrong move:
Forgetting to raise concentration terms to the power of their stoichiometric coefficients in Q/K expressions
Why:
Exponents in Q and K directly match the coefficients from the balanced reaction equation
Correct move:
Double-check that all concentration terms have the correct exponent when calculating Q or K
3. Quick Reference Cheatsheet
Concept/Formula | Key Description |
|---|---|
Dynamic Equilibrium | Forward reaction rate = Reverse reaction rate; concentrations are constant |
Reaction quotient, calculated from current reactant/product concentrations | |
Q vs K Rule: shift right; shift left | Predict direction of reaction to reach equilibrium |
Relationship between standard free energy change and equilibrium constant | |
Solubility product constant expression for sparingly soluble ionic compounds | |
Le Châtelier’s Principle | Equilibrium systems shift to counteract applied stress to re-establish equilibrium |
ICE Table | Tracks initial, change, and equilibrium concentrations for quantitative calculations |
What's Next
Start your study of equilibrium with the first sub-topic below, which lays the conceptual foundation for all quantitative work in this unit. Once you complete all sub-topics in Unit 7, you will move on to Unit 8: Acids and Bases, which builds heavily on the equilibrium concepts you learn here.