AP Chemistry - Unit 9: Thermodynamics and Electrochemistry

Introduction

• This is the last unit in the AP Chemistry curriculum.
• Focus on thermodynamics and electrochemistry.
• Will revisit concepts from Unit 6 (thermochemistry).
• New material on electrochemistry.

Key Concepts

Thermodynamics

• Study of energy exchanges in chemical reactions and physical processes.
• Helps predict whether a process is spontaneous (thermodynamically favored).
• Does not tell how long a process will take (unlike kinetics).
• Spontaneous Process: Occurs without external intervention (e.g., a ball rolling down a hill).

Entropy (ΔS)

• Measure of randomness or disorder.
• Key term: Degrees of Freedom (AP preferred term).
• Solid < Liquid < Gas in terms of entropy.
• Increase in entropy generally leads to a thermodynamically favored process.

Entropy Changes in Processes

• Positive ΔS: Process becomes more disordered, increasing degrees of freedom.
• Negative ΔS: Process becomes more organized, decreasing degrees of freedom.

Examples of Entropy Changes

1. Sugar dissolving in hot tea: Positive ΔS (solid to aqueous, increased freedom).
2. Lake freezing: Negative ΔS (liquid to solid, decreased freedom).
3. Evaporation of rainwater: Positive ΔS (liquid to gas, increased freedom).
4. Chemical reactions:
   • Solid/elastic to solid: Negative ΔS (less freedom).
   • Gaseous reactants to fewer gaseous products: Negative ΔS.
   • Solid dissolving into aqueous ions: Positive ΔS (increased freedom).

Practice Problem

• Heating Curve Analysis:
  • Segment X: Liquid to gas phase change (vaporization).
  • ΔH (Enthalpy): Positive (endothermic process).
  • ΔS (Entropy): Positive (increase in degrees of freedom).

General Observations

• Increase in Entropy: Usually indicates a thermodynamically favored process.
• Decrease in Entropy: Process may not be favored in given conditions.
• Nature tends towards chaos and disorder, supporting increased entropy.

Conclusion

• Understanding of entropy is crucial for predicting thermodynamically favored processes.
• More detailed discussion in upcoming subunits.