#AP Chemistry Unit 7: Equilibrium - The Night Before 🚀

Hey there, future AP Chem master! Let's make sure you're feeling confident and ready to ace this exam. We're going to break down Unit 7, focusing on what's really important, and make sure you've got the tools you need to succeed. Let's dive in!

#Unit Overview: Dynamic Equilibrium

Unit 7 is all about equilibrium, a dynamic state where forward and reverse reactions occur at the same rate. It's a concept that ties together kinetics (Unit 5) and thermodynamics (Unit 8), so it's super important! Expect to see this topic frequently on both MCQs and FRQs.

#Big Ideas 💡

• Reversible Reactions: Most reactions can go both ways (reactants ⇌ products).
• Dynamic Equilibrium: Reactions don't stop; they reach a balance where rates are equal.
• Le Châtelier's Principle: Systems shift to relieve stress (changes in concentration, temperature, etc.).
• Equilibrium Constant (K): A numerical way to describe the extent of a reaction.

#Key Questions to Ponder 🤔

• Why is a waterfall considered a spontaneous reaction?
• How can reactions occur in more than one direction?
• How is caffeine removed from coffee? ☕
• Why is food stored in a refrigerator? ❄️

# 7.1 Introduction to Equilibrium

• Rate of Reaction: R = Δ[P]/ΔT = -Δ[R]/ΔT (change in concentration over time)
• Equilibrium is a dynamic process, not a static one. Reactions are always occurring, even when at equilibrium.

# 7.2 Direction of Reversible Reactions

• At equilibrium, the rates of the forward and reverse reactions are equal. This does not mean the reaction stops!
• Concentrations of reactants and products remain constant at equilibrium.

• Image Explanation: The graph shows how the rates of the forward and reverse reactions become equal over time, reaching equilibrium.

# 7.3 Reaction Quotient (Q) and Equilibrium Constant (K)

• Equilibrium Constant (K): Describes the ratio of products to reactants at equilibrium. It's a constant value for a given reaction at a specific temperature.
• Reaction Quotient (Q): Describes the ratio of products to reactants at any point in time. It is used to predict the direction a reaction will shift.

# 7.4 Calculating the Equilibrium Constant (K)

• Formula: For a reaction aA + bB ⇌ cC + dD, $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ (products over reactants, each raised to the power of their stoichiometric coefficient).
• Key Points:
  • K is unitless.
  • Only aqueous solutions (aq) and gases (g) are included in the equilibrium expression. Solids (s) and liquids (l) are not included because their concentrations are constant.

# 7.5 Magnitude of the Equilibrium Constant

• K > 1: Product-favored (more products at equilibrium).
• K = 1: Neither product- nor reactant-favored (equilibrium).
• K < 1: Reactant-favored (more reactants at equilibrium).

# 7.6 Properties of the Equilibrium Constant

• Reversed Reaction: Invert K (1/K).
• Multiplied by a Constant (n): Raise K to the power of n ($K^n$).
• Added Reactions: Multiply the K values of each reaction ($K_1$ x $K_2$ x $K_3$...)

# 7.7 Calculating Equilibrium Concentrations

• ICE Tables: Use ICE (Initial, Change, Equilibrium) tables to find equilibrium concentrations. This is a MUST-KNOW skill!
  1. Write out the balanced equation.
  2. Set up the ICE table with initial concentrations.
  3. Use the change in concentration based on reaction stoichiometry.
  4. Use the equilibrium constant expression to solve for x, and calculate equilibrium concentrations.

# 7.8 Representations of Equilibrium

• Equilibrium can be represented mathematically (K, ICE tables) and conceptually (particle diagrams).
• Be ready to explain equilibrium using particle diagrams on FRQs.

# 7.9 Introduction to Le Châtelier’s Principle

• Le Châtelier's Principle: If a system at equilibrium is stressed, it will shift to relieve that stress. Stresses include changes in concentration, temperature, and pressure.

# 7.10 Reaction Quotient (Q) and Le Châtelier’s Principle

• Q vs. K:
  • If Q < K, the reaction shifts right (towards products).
  • If Q > K, the reaction shifts left (towards reactants).
  • If Q = K, the system is at equilibrium.
• Temperature: K is temperature-dependent. Changes in temperature change the value of K.

# 7.11 Introduction to Solubility Equilibria

• Solubility Product (Ksp): The equilibrium constant for the dissolution of a solid ionic compound. It represents the extent to which a solid dissolves in a solution.
• Molar Solubility: The concentration of the metal cation in a saturated solution. It is the amount of a solid that dissolves in a liter of solution.

# 7.12 Factors Affecting Solubility

• Common Ion Effect: The reduction in solubility of a sparingly soluble salt when a soluble salt containing a common ion is added to the solution.
• pH: The solubility of some salts depends on pH. For example, the solubility of hydroxide salts increases in acidic solutions.
• Temperature: The solubility of most solids increases with increasing temperature.

# 7.13 Gibbs Free Energy and Equilibrium

• Gibbs Free Energy (ΔG): The amount of energy available to do work. A negative ΔG indicates a spontaneous reaction.
• Relationship to Equilibrium: ΔG = -RTlnK. This equation shows how the spontaneity of a reaction is related to the equilibrium constant.

#Practice Questions

Practice Question

Multiple Choice Questions

1. Which of the following statements is true regarding a system at equilibrium? (A) The forward reaction has stopped. (B) The reverse reaction has stopped. (C) The rate of the forward reaction equals the rate of the reverse reaction. (D) The concentrations of reactants and products are equal.

2. For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), what is the correct equilibrium constant expression? (A) K = [NH3]/[N2][H2] (B) K = [NH3]2/[N2][H2]3 (C) K = [N2][H2]/[NH3] (D) K = [N2][H2]3/[NH3]2

3. If the equilibrium constant K for a reaction is 0.01, which of the following is true? (A) The reaction is product-favored. (B) The reaction is reactant-favored. (C) The reaction is at equilibrium. (D) The reaction is spontaneous.

4. According to Le Châtelier’s Principle, what happens to the equilibrium of an exothermic reaction when the temperature is increased? (A) It shifts towards the products. (B) It shifts towards the reactants. (C) It does not shift. (D) The equilibrium constant increases.

5. Which of the following factors does NOT affect the value of the equilibrium constant? (A) Temperature (B) Pressure (C) Concentration (D) Catalyst

Short Answer Questions

1. Explain the difference between the reaction quotient (Q) and the equilibrium constant (K).

2. Describe how to use an ICE table to calculate equilibrium concentrations.