# Equilibrium Overview

> AP Chemistry · Unit 7: Equilibrium
> Source: https://www.owlsprep.com/study/ap-chemistry-u7-overview/
> Weight: 7-9% of overall AP Chemistry exam score

This unit explores dynamic chemical equilibrium for all types of reversible reactions, from general homogeneous equilibria to solubility equilibria. You will learn to quantify equilibrium, predict shifts, and connect equilibrium to thermodynamics, core concepts tested on the AP Chemistry exam.

**Prerequisites:** Unit 6: Thermodynamics; Reaction stoichiometry and kinetics basics

## Learning objectives

- Distinguish between dynamic chemical equilibrium and static equilibrium/completion reactions
- Calculate and interpret the reaction quotient Q and equilibrium constant K for reversible reactions
- Predict equilibrium shifts in response to stress using Le Châtelier’s principle
- Solve quantitative problems involving solubility equilibria, common ion effect, and pH-dependent solubility
- Relate standard Gibbs free energy change to the equilibrium constant for a reaction

## 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](https://www.owlsprep.com/study/ap-chemistry-u7-introduction-to-equilibrium/) — Learn the definition of dynamic equilibrium and how it differs from static equilibrium and completion reactions.
- [AP Chemistry Magnitude of K](https://www.owlsprep.com/study/ap-chemistry-u7-magnitude-of-k/) — Interpret what the size of K tells you about the relative amounts of reactants and products at equilibrium.
- [AP Chemistry Reaction quotient Q](https://www.owlsprep.com/study/ap-chemistry-u7-reaction-quotient-q/) — Understand how Q is calculated and how it compares to the equilibrium constant K.
- [AP Chemistry Direction of reversible reactions](https://www.owlsprep.com/study/ap-chemistry-u7-direction-of-reversible-reactions/) — Use Q and K to predict which direction a reaction will shift to reach equilibrium.
- [AP Chemistry Calculating the equilibrium constant K](https://www.owlsprep.com/study/ap-chemistry-u7-calculating-the-equilibrium-constant-k/) — Calculate K from known initial and equilibrium concentrations of reactants and products.
- [AP Chemistry Calculating equilibrium concentrations](https://www.owlsprep.com/study/ap-chemistry-u7-calculating-equilibrium-concentrations/) — Use ICE tables to find unknown equilibrium concentrations when you know the value of K.
- [AP Chemistry Le Châtelier’s principle](https://www.owlsprep.com/study/ap-chemistry-u7-le-ch-telier-s-principle/) — Predict how changes in concentration, temperature, and pressure shift the equilibrium position.
- [AP Chemistry Solubility equilibria](https://www.owlsprep.com/study/ap-chemistry-u7-solubility-equilibria/) — Apply equilibrium rules to sparingly soluble ionic compounds and calculate the solubility product Ksp.
- [AP Chemistry Common ion effect](https://www.owlsprep.com/study/ap-chemistry-u7-common-ion-effect/) — Calculate the solubility of ionic compounds in solutions that already contain one of their ions.
- [AP Chemistry pH and solubility](https://www.owlsprep.com/study/ap-chemistry-u7-ph-and-solubility/) — Explain how changing pH impacts the solubility of ionic compounds with basic anions.
- [AP Chemistry Free energy and equilibrium](https://www.owlsprep.com/study/ap-chemistry-u7-free-energy-and-equilibrium/) — Relate the standard free energy change of a reaction to its equilibrium constant K.
- [AP Chemistry Free energy of dissolution](https://www.owlsprep.com/study/ap-chemistry-u7-free-energy-of-dissolution/) — Calculate the standard free energy of dissolution from Ksp and interpret solubility trends.

## Common pitfalls

- **Wrong:** Confusing Q and K, and using K to predict reaction direction before equilibrium is reached
  - Why it fails: K only describes the system at equilibrium, while Q describes the system at any point during the reaction
  - Correct: Always use Q (not K) to predict which direction a reaction will shift to reach equilibrium
- **Wrong:** Assuming a large K means a reaction will proceed quickly
  - Why it fails: Equilibrium describes the extent of a reaction (thermodynamics), not how fast it proceeds (kinetics)
  - Correct: Always separate equilibrium extent from reaction rate when interpreting the value of K
- **Wrong:** Forgetting to raise concentration terms to the power of their stoichiometric coefficients in Q/K expressions
  - Why it fails: Exponents in Q and K directly match the coefficients from the balanced reaction equation
  - Correct: Double-check that all concentration terms have the correct exponent when calculating Q or K

## Cheatsheet

| Concept/Formula | Key Description |
| --- | --- |
| Dynamic Equilibrium | Forward reaction rate = Reverse reaction rate; concentrations are constant |
| $Q = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ | Reaction quotient, calculated from current reactant/product concentrations |
| Q vs K Rule: $Q<K$ shift right; $Q>K$ shift left | Predict direction of reaction to reach equilibrium |
| $\Delta G^\circ = -RT \ln K$ | Relationship between standard free energy change and equilibrium constant |
| $K_{sp} = [M^{n+}]^a[X^{m-}]^b$ | 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.

- [Introduction to Equilibrium (First sub-topic of Unit 7)](https://www.owlsprep.com/study/ap-chemistry-u7-introduction-to-equilibrium/)
- [Unit 8: Introduction to Acids and Bases (Next Unit)](https://www.owlsprep.com/study/ap-chemistry-u8-introduction-to-acids-and-bases/)
- [Direction of reversible reactions](https://www.owlsprep.com/study/ap-chemistry-u7-direction-of-reversible-reactions/)

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