AP Chemistry Unit 9: Thermodynamics and Electrochemistry

Introduction

• Presenter: Jeremy Krug
• Focus: Thermodynamics and Electrochemistry in AP Chemistry

Entropy

• Definition: Measure of disorder or dispersion in a system.
  • Solids: Lowest entropy (compact).
  • Liquids: Higher entropy.
  • Aqueous solutions: Even higher entropy.
  • Gases: Highest entropy.
• Effects:
  • Temperature: Higher temperature increases entropy (molecules move faster).
  • Volume: Greater volume increases entropy (molecules have more space).
• Chemical Reaction:
  • Going from solid to gas increases entropy.
  • More gas molecules = more entropy.
• Calculation: Sum of products' entropy minus sum of reactants' entropy.

Thermodynamic Favorability and Gibbs Free Energy

• Thermodynamically Favored: Process occurs naturally.
• Gibbs Free Energy (ΔG):
  • Negative ΔG: Process is favored.
  • Positive ΔG: Process is not favored.
• Standard Conditions: 25°C, 1 atm, 1 mol/L.
• Calculating ΔG:
  • Products' Gibbs Free Energies minus reactants' Gibbs Free Energies.
  • ΔG = ΔH - TΔS.
  • Units: Ensure entropy (Joules) aligns with others (kilojoules).

Reaction Conditions

• Exothermic & Entropy Increase: Favored at all temperatures.
• Endothermic & Entropy Decrease: Not favored at any temperature.
• Positive ΔH & Positive ΔS: Favored at high temperatures.
• Negative ΔH & Negative ΔS: Favored at low temperatures.
• Kinetic Control: Thermodynamically favored but very slow rate (e.g. rusting).

Relation to Equilibrium Constant

• Equation: ΔG = -RT ln(K)
  • R = 8.314 J/mol·K
• Favored Reaction:
  • Negative ΔG, large equilibrium constant (lots of product).
  • Positive ΔG, small equilibrium constant (few products).

Solubility and Driving Forces

• Example: KCl dissociation in water.
  • ΔG = Negative, process favored.
  • ΔH = Positive, not favored.
  • ΔS = Positive, favored and drives the process.

Unfavorable Reactions

• Energy Addition: External energy (e.g., electricity, light) can drive reactions.
• Coupling Reactions: Pairing unfavorable with favorable can make a process favorable (e.g., copper from copper(II) sulfide).

Galvanic Cells (Batteries)

• Function: Use electron flow from redox reactions to power loads.
  • Anode: Oxidation, electron source.
  • Cathode: Reduction, electron destination.
  • Electrons: Flow from anode to cathode.
• Salt Bridge: Maintains charge balance.
• Cell Potential: Voltage determined by cathode minus anode potential.
  • Positive voltage = favored.
  • Negative voltage = unfavored.

Non-Standard Conditions and Nernst Equation

• Nernst Equation: Used when conditions deviate from standard.
  • Reaction Quotient (Q): Affects voltage.
  • Increased product concentration or decreased reactant concentration lowers voltage.
  • Decreased product concentration or increased reactant concentration raises voltage.

Electrolytic Cells

• Function: Non-favored processes driven by external power.
  • Used for metal plating.
• Calculation:
  • Current (amps) = Charge (coulombs) / Time (seconds).
  • Convert charge to grams using stoichiometry and Faraday’s constant.

Conclusion

• Emphasis on reviewing laboratory techniques in the next session.
• Encouragement to use additional resources for AP Chemistry preparation.