#AP Chemistry: Unit 9 - Thermodynamics - The Final Frontier! 🚀

Hey there, future AP Chem master! You've made it to the last unit – Thermodynamics! This is where we tie together a lot of what you've learned, focusing on energy, spontaneity, and how reactions actually happen. Let's make sure you're ready to ace this!

#Introduction to Thermodynamics

Thermodynamics is all about energy and whether a reaction will occur on its own (spontaneity). Forget just temperature; we're diving into the heart of why things happen!

• System: Where the reaction/process is happening.
• Surroundings: Everything outside the system.
• Universe: System + Surroundings.

We'll be exploring:

• Entropy (S°): A measure of disorder.
• Gibbs Free Energy (ΔG°): Determines spontaneity.
• Enthalpy (ΔH): Remember this from Unit 6? It's back!

#Major Topics in Unit 9

#9.1-9.2: Entropy (S°)

Entropy (S°) is all about disorder. The more chaotic a system, the higher its entropy.

• High Entropy: More disorder (e.g., gas > liquid > solid).
• Low Entropy: More order.

Image: Entropy increases as a substance changes from solid to liquid to gas.

Entropy can be calculated using standard entropy values, similar to how we calculated enthalpy changes.

#9.3-9.6: Gibbs Free Energy (ΔG°), Spontaneity, and Equilibrium

Gibbs Free Energy (ΔG°) combines enthalpy (ΔH) and entropy (ΔS) to determine spontaneity.

• ΔG° < 0: Spontaneous (thermodynamically favorable)
• ΔG° > 0: Nonspontaneous (thermodynamically unfavorable)

#Kinetic Control

Reactions can be spontaneous (ΔG° < 0) but slow due to high activation energy. This is kinetic control.

Image: Activation energy is the barrier to reaction progress.

#Spontaneity and Equilibrium

• ΔG < 0: Reaction favors product formation.
• ΔG > 0: Reaction favors reactant formation.
• ΔG = 0: Reaction is at equilibrium.

#ΔG° and the Equilibrium Constant (K)

• ΔG° < 0: K > 1 (product-favored)
• ΔG° > 0: K < 1 (reactant-favored)

#Coupled Reactions

Nonspontaneous reactions can be made spontaneous by coupling them with spontaneous reactions.

#9.7-9.10: Electrochemistry

Electrochemistry studies redox reactions (electron transfer) and their connection to thermodynamics.

• Galvanic Cells: Spontaneous reactions generate electricity.
• Electrolytic Cells: Nonspontaneous reactions require electricity.

Image: A basic galvanic cell setup.

We'll connect cell potential to ΔG°, Q, and K.

#Final Exam Focus 🎯

• High-Priority Topics:
  • Entropy and its calculations.
  • Gibbs Free Energy and spontaneity.
  • Relationship between ΔG°, K, and spontaneity.
  • Basic electrochemistry.
• Common Question Types:
  • MCQs involving entropy changes in reactions.
  • FRQs calculating ΔG° and interpreting spontaneity.
  • Questions linking thermodynamics to equilibrium.
  • Problems involving galvanic and electrolytic cells.

#Practice Questions

Practice Question

Multiple Choice Questions

1. Which of the following processes is accompanied by an increase in entropy? (A) Freezing of water (B) Condensation of steam (C) Dissolving sugar in water (D) Formation of a crystal from a supersaturated solution

2. A reaction has a positive enthalpy change (ΔH > 0) and a positive entropy change (ΔS > 0). Under what conditions will this reaction be spontaneous? (A) At all temperatures (B) At high temperatures (C) At low temperatures (D) Never

3. For the reaction A + B ⇌ 2C, the equilibrium constant K is 10. Which of the following statements is true? (A) ΔG° > 0 and the reaction is nonspontaneous (B) ΔG° < 0 and the reaction is spontaneous (C) ΔG° = 0 and the reaction is at equilibrium (D) ΔG° > 0 and the reaction favors reactants

Free Response Question

Consider the following reaction:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The standard enthalpy change (ΔH°) for this reaction is -92.2 kJ/mol, and the standard entropy change (ΔS°) is -198.7 J/mol·K at 298 K.

(a) Calculate the standard Gibbs free energy change (ΔG°) for this reaction at 298 K. (b) Is this reaction spontaneous under standard conditions at 298 K? Explain. (c) Calculate the equilibrium constant (K) for this reaction at 298 K. (R = 8.314 J/mol·K) (d) How does increasing the temperature affect the spontaneity of this reaction? Explain.

Scoring Breakdown:

(a) (3 points) * 1 point for correct conversion of ΔS° to kJ/mol·K (-0.1987 kJ/mol·K) * 1 point for using the correct formula ΔG° = ΔH° - TΔS° * 1 point for correct calculation: ΔG° = -92.2 kJ/mol - (298 K)(-0.1987 kJ/mol·K) = -32.9 kJ/mol

(b) (2 points) * 1 point for stating that the reaction is spontaneous * 1 point for the explanation that ΔG° is negative

(c) (3 points) * 1 point for using the correct formula ΔG° = -RTlnK * 1 point for correct substitution: -32900 J/mol = -(8.314 J/mol·K)(298 K)lnK * 1 point for correct calculation: K = 8.4 x 10⁵

(d) (2 points) * 1 point for stating that increasing temperature makes the reaction less spontaneous * 1 point for the explanation that the negative ΔS° term becomes more significant at higher temperatures, making ΔG° less negative

You've got this! Go get that 5! 💪