Unit 7: Equilibrium

AP Chemistry — 24 practice questions with detailed explanations.

Unit Study Guide

Executive Summary

Unit 7 stands as one of the most heavily tested and conceptually foundational units in AP Chemistry, bridging kinetics, thermodynamics, and solution chemistry. Mastery of equilibrium means you can write correct equilibrium constant expressions, calculate unknown concentrations using ICE tables, predict the direction a reaction will shift using the reaction quotient Q, and apply Le Chatelier's Principle to both qualitative and quantitative scenarios. On the AP exam, equilibrium appears across both multiple-choice and free-response sections, often interwoven with acid-base chemistry (Unit 8) and thermodynamics (Unit 9). Students who command this unit recognize that equilibrium is dynamic, not static, and that only a temperature change alters the value of the equilibrium constant K itself. Achieving fluency here unlocks partial credit on complex FRQ items that require multi-step reasoning, mathematical setup, and clear explanation of molecular-level events.

Deep-Dive

Dynamic Equilibrium

A reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of all reactants and products remain constant over time, but they are not necessarily equal to one another. Both microscopically and macroscopically, the system appears unchanging, yet individual molecules continue to react. This distinction between constant and equal concentrations is a cornerstone conceptual point.

The Equilibrium Constant K

The equilibrium constant, K, is a temperature-dependent ratio that relates the equilibrium concentrations (or partial pressures) of products to reactants, each raised to the power of their stoichiometric coefficients. For a general reaction aA + bB ⇌ cC + dD, the concentration-based constant is Kc = [C]^c[D]^d / ([A]^a[B]^b). Importantly, solids and pure liquids (including water as solvent in dilute solutions) are excluded from the expression because their activities are essentially constant. The magnitude of K reveals the position of equilibrium: a large K (>>1) indicates products are favored at equilibrium, while a small K (<<1) indicates reactants are favored.

For gas-phase equilibria, Kp uses partial pressures instead of molar concentrations. The relationship Kp = Kc(RT)^Δn connects the two, where Δn is the change in moles of gas (moles gaseous products minus moles gaseous reactants) and R = 0.08206 L·atm·mol⁻¹·K⁻¹, with T in Kelvin. When Δn = 0, Kp equals Kc.

Reaction Quotient Q

The reaction quotient Q has the same mathematical form as K but uses instantaneous concentrations or pressures that may not be at equilibrium. Comparing Q to K predicts the direction of shift:

Q < K: reaction proceeds forward (toward products)

Q > K: reaction proceeds in reverse (toward reactants)

Q = K: system is at equilibrium

This comparison is the quantitative backbone of Le Chatelier predictions.

Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will shift in a direction that partially counteracts the disturbance. Disturbances include changes in concentration, pressure (or volume) for gaseous systems, and temperature. Only a temperature change changes the value of K itself. Adding a reactant shifts the equilibrium toward products; increasing pressure by decreasing volume shifts the equilibrium toward the side with fewer moles of gas. A catalyst increases the rate at which equilibrium is reached but does not change the position of equilibrium or the value of K.

ICE Tables and Equilibrium Calculations

ICE (Initial, Change, Equilibrium) tables organize quantitative equilibrium problems. You write initial concentrations, express changes in terms of a variable (often x), and sum to obtain equilibrium expressions. These equilibrium concentrations are then substituted into the K expression to solve for x. The 5% approximation (or small-x approximation) can be used when K is small and the initial concentration of the limiting species is large, simplifying the algebra by neglecting x in the denominator.

Introduction to Solubility Equilibria

Solubility equilibria involve the dissolution of sparingly soluble ionic compounds. The solubility product constant, Ksp, is written using only the dissolved ions, since the solid is excluded from the expression. Molar solubility is the number of moles of solute that dissolve per liter of saturated solution. The common ion effect—a manifestation of Le Chatelier's Principle—decreases solubility when a solution already contains one of the ions in the dissolution equation.

AP Exam Trap (FRQ)

Including solids or pure liquids in K expressions. Students often write K expressions that include the concentration of a solid or pure liquid. Correction: omit all solids and pure liquids from the equilibrium expression because their activities are constant and incorporated into K. Model exam sentence: "The solid CaCO₃ is excluded from the K expression because pure solids have constant activity."

Confusing "shift" with "new equilibrium concentrations are identical to the old ones." A shift changes concentrations, but they do not return to original values (unless the disturbance is removed). Correction: Le Chatelier's Principle describes how the system reaches a new equilibrium with new concentrations. Model exam sentence: "After adding NO₂, the system shifts toward reactants and establishes a new equilibrium with a higher [N₂O₄] than before the disturbance."

Assuming any disturbance changes K. Only a change in temperature changes K. Changes in concentration, pressure, or adding a catalyst shift the equilibrium position but do not alter K. Correction: distinguish between changes in the position of equilibrium and changes in the value of K. Model exam sentence: "Increasing the volume decreases the partial pressures of all gases, causing a shift toward the side with more moles of gas, but Kp remains unchanged because temperature is constant."

Ignoring units in Kp vs Kc conversions or using the wrong value of R. Students forget that Kp and Kc are related by Kp = Kc(RT)^Δn and may use R = 8.314 instead of R = 0.08206. Correction: use R = 0.08206 L·atm·mol⁻¹·K⁻¹ for Kp/Kc conversions involving pressure in atm. Model exam sentence: "Using R = 0.08206 L·atm·mol⁻¹·K⁻¹ and T = 298 K, Kp = Kc(0.08206 × 298)^Δn."

Counting the same ion twice when calculating molar solubility in the presence of a common ion. Students sometimes double-count the contribution of the common ion. Correction: the common ion's initial concentration is given; only the additional dissolution contributes a small increment. *Model exam sentence:* "In a solution already 0.10 M in Cl⁻, the molar solubility of AgCl is calculated by setting [Ag⁺] = s and [Cl⁻] = 0.10 + s ≈ 0.10, because s is negligible compared to 0.10."

Interactive Glossary

Term	Definition
------	------------
Dynamic Equilibrium	A state in which the forward and reverse reaction rates are equal, so concentrations remain constant over time. Individual molecules continue to react, so the system is not static.
Equilibrium Constant (K)	A temperature-dependent ratio of product concentrations to reactant concentrations at equilibrium, each raised to their stoichiometric coefficients. It indicates the extent to which a reaction proceeds.
Reaction Quotient (Q)	A ratio calculated using the same expression as K but with instantaneous, non-equilibrium concentrations. Comparing Q to K predicts the direction the reaction will shift to reach equilibrium.
Le Chatelier's Principle	A principle stating that a system at equilibrium will respond to a disturbance by shifting to partially counteract the change. This shift re-establishes a new equilibrium position.
ICE Table	A tabular method organizing Initial concentrations, the Change in concentrations as the system shifts, and the Equilibrium concentrations reached. It is the standard tool for solving quantitative equilibrium problems.
Kc	The equilibrium constant expressed in terms of molar concentrations (mol/L). It is used for reactions in aqueous solution or when concentration data are provided.
Kp	The equilibrium constant expressed in terms of partial pressures of gases (atm). It is related to Kc by the equation Kp = Kc(RT)^Δn.
Ksp (Solubility Product Constant)	The equilibrium constant for the dissolution of a sparingly soluble ionic compound. It is written only in terms of the dissolved ions, excluding the solid.
Molar Solubility	The number of moles of solute that dissolve per liter of saturated solution. It can be calculated from Ksp and is reduced by the common ion effect.
Common Ion Effect	The decrease in solubility of an ionic compound when a solution already contains one of the ions in the compound's dissolution equation. It is a specific application of Le Chatelier's Principle.
5% Approximation	A simplification assuming that x is small enough to be neglected in the denominator of the K expression. It is valid when the calculated x is less than 5% of the initial concentration.
Heterogeneous Equilibrium	An equilibrium involving reactants and products in more than one phase, such as a solid and a gas. Pure solids and pure liquids are omitted from the K expression.
Δn (Delta n)	The difference between the total moles of gaseous products and the total moles of gaseous reactants in a balanced equation. It determines the relationship between Kp and Kc.
Catalyst	A substance that increases the rate of both the forward and reverse reactions equally, allowing equilibrium to be reached faster. A catalyst does not change the value of K or the position of equilibrium.
Shift Direction	The predicted movement of a reaction toward products (forward) or reactants (reverse) in response to a disturbance. It is determined by comparing Q to K.

Skill-Set

ICE Tables for Equilibrium Calculations: Write balanced equilibrium equations, set up ICE tables with appropriate initial concentrations and a variable for change, substitute equilibrium expressions into K, and solve for the unknown. Verify the 5% approximation if used.

K Expression Writing (Solids/Liquids Excluded): Construct correct Kc, Kp, and Ksp expressions. Omit solids, pure liquids, and solvent water from the expression. Include only gaseous and aqueous species.

Q Calculation and Comparison to K: Calculate Q from given instantaneous concentrations, compare Q to K, and state whether the reaction proceeds forward, in reverse, or is already at equilibrium.

Le Chatelier Predictions for Concentration/Pressure/Temperature Changes: Predict the direction of shift when a reactant or product is added or removed, when volume or pressure changes for gas-phase equilibria, and when temperature changes. State whether K changes (only temperature alters K).

Study Moves

[ ] Write the K expression for five different reaction types (gas-phase, aqueous, heterogeneous, solubility, and a reaction with a pure liquid) and verify that solids and pure liquids are excluded.

[ ] Set up and solve three full ICE table problems: one with a straightforward K calculation, one requiring the 5% approximation, and one involving Kp and Kc conversion.

[ ] Create a quick-reference card listing the effects of concentration, pressure, volume, temperature, and catalyst additions on both the direction of shift and the value of K.

[ ] Practice five Q-vs-K comparison problems: calculate Q, compare to the given K, and write a one-sentence prediction of the shift direction.

[ ] Solve two solubility equilibria problems, including one with a common ion, to reinforce Ksp expression writing and molar solubility calculations.

[ ] Review past AP FRQ scoring guidelines for equilibrium problems and note the specific language that earns the explanation point.

Exam Linkage

AP Chemistry free-response questions on equilibrium frequently use the task verbs "justify," "explain," "calculate," and "predict." When asked to justify or explain, you must provide both a correct claim and the reasoning connecting it to a chemical principle (e.g., Le Chatelier's Principle or the definition of K). A calculation alone does not earn the explanation point. When asked to calculate, show your work: write the K expression, set up the ICE table, substitute into the expression, and solve algebraically. Units and significant figures matter. When asked to predict, state the direction of shift or the qualitative change in concentration clearly. Graders look for precise language: say "shifts toward products" rather than "moves right," and specify that "K does not change" when the disturbance is not a temperature change. Linking your answer to the reaction quotient Q ("Q < K, so the reaction proceeds forward") demonstrates the quantitative reasoning that earns full credit.

Top 5 Concepts to Master

1Dynamic equilibrium occurs when forward and reverse rates are equal, resulting in constant but unequal concentrations.
2The equilibrium constant K is a temperature-dependent ratio; only temperature changes alter K itself.
3The reaction quotient Q compared to K predicts the direction of shift: Q < K shifts forward, Q > K shifts reverse.
4Le Chatelier's Principle predicts how concentration, pressure, and temperature changes shift equilibrium position.
5ICE tables organize quantitative equilibrium calculations, and the 5% approximation simplifies algebra when K is small.

Key Terms & Definitions

Practice with Flashcards

Dynamic Equilibrium

A state in which the forward and reverse reaction rates are equal, so concentrations remain constant over time. Individual molecules continue to react, so the system is not static.

Equilibrium Constant (K)

A temperature-dependent ratio of product concentrations to reactant concentrations at equilibrium, each raised to their stoichiometric coefficients. It indicates the extent to which a reaction proceeds.

Reaction Quotient (Q)

A ratio calculated using the same expression as K but with instantaneous, non-equilibrium concentrations. Comparing Q to K predicts the direction the reaction will shift to reach equilibrium.

Le Chatelier's Principle

A principle stating that a system at equilibrium will respond to a disturbance by shifting to partially counteract the change. This shift re-establishes a new equilibrium position.

ICE Table

A tabular method organizing Initial concentrations, the Change in concentrations as the system shifts, and the Equilibrium concentrations reached. It is the standard tool for solving quantitative equilibrium problems.

The equilibrium constant expressed in terms of molar concentrations (mol/L). It is used for reactions in aqueous solution or when concentration data are provided.

The equilibrium constant expressed in terms of partial pressures of gases (atm). It is related to Kc by the equation Kp = Kc(RT)^Δn.

Ksp (Solubility Product Constant)

The equilibrium constant for the dissolution of a sparingly soluble ionic compound. It is written only in terms of the dissolved ions, excluding the solid.

Molar Solubility

The number of moles of solute that dissolve per liter of saturated solution. It can be calculated from Ksp and is reduced by the common ion effect.

Common Ion Effect

The decrease in solubility of an ionic compound when a solution already contains one of the ions in the compound's dissolution equation. It is a specific application of Le Chatelier's Principle.

5% Approximation

A simplification assuming that x is small enough to be neglected in the denominator of the K expression. It is valid when the calculated x is less than 5% of the initial concentration.

Heterogeneous Equilibrium

An equilibrium involving reactants and products in more than one phase, such as a solid and a gas. Pure solids and pure liquids are omitted from the K expression.

Δn (Delta n)

The difference between the total moles of gaseous products and the total moles of gaseous reactants in a balanced equation. It determines the relationship between Kp and Kc.

Catalyst

A substance that increases the rate of both the forward and reverse reactions equally, allowing equilibrium to be reached faster. A catalyst does not change the value of K or the position of equilibrium.

Shift Direction

The predicted movement of a reaction toward products (forward) or reactants (reverse) in response to a disturbance. It is determined by comparing Q to K.

⚠️ Common Misconceptions — Exam Traps

❌ Including solids or pure liquids in K expressions.

✅ Correct: Omit all solids and pure liquids from the equilibrium expression because their activities are constant and incorporated into K.

❌ Confusing 'shift' with 'new equilibrium concentrations are identical to the old ones.'

✅ Correct: A shift changes concentrations, and the system reaches a new equilibrium with new concentrations that do not return to their original values unless the disturbance is removed.

❌ Assuming any disturbance changes K.

✅ Correct: Only a change in temperature changes K. Changes in concentration, pressure, or adding a catalyst shift the equilibrium position but do not alter K.

❌ Ignoring units in Kp vs Kc conversions or using the wrong value of R.

✅ Correct: Use R = 0.08206 L·atm·mol⁻¹·K⁻¹ for Kp/Kc conversions when pressure is in atm, and always account for Δn correctly.

❌ Counting the same ion twice when calculating molar solubility in the presence of a common ion.

✅ Correct: The common ion's initial concentration is given separately; only the dissolution contributes a small additional increment, which is often negligible relative to the initial concentration.

❌ Believing equilibrium means the reaction has stopped.

✅ Correct: Equilibrium is dynamic; both forward and reverse reactions continue at equal rates, so microscopic activity persists even though macroscopic concentrations are constant.

❌ Thinking a catalyst shifts equilibrium.

✅ Correct: A catalyst speeds up both forward and reverse reactions equally, so equilibrium is reached faster but the position of equilibrium and K remain unchanged.