AP Chemistry Thermodynamics Lecture Notes

Introduction

• Presenter: Cara
• Topic: Unit 6: Thermodynamics
• Upcoming AP Exam: Thursday
• Focus: Thermodynamics, with reference to Units 5 & 7

Laws of Thermodynamics

Zeroth Law

• If object A is in thermal equilibrium with object B, and B is with C, then A is with C.
• Defines temperature based on thermal equilibrium.

First Law: Conservation of Energy

• Equation: ΔU = Q + W
  • ΔU: Change in internal energy
  • Q: Heat added to the system
  • W: Work done on the system
• Sign Convention:
  • Q is positive when heat is added, negative when removed.
  • W is positive when work is done on the system, negative when done by the system.
• Work Equation: W = PΔV (not common in chemistry but useful)
• Alternate Equation: ΔU = (3/2)nRΔT

Enthalpy (H)

• Analogous to internal energy (U).
• Equation: H = U + PV
• Calorimetry: Measures heat of chemical reactions
  • Example: Dissolving NaCl in water
• Exothermic vs. Endothermic:
  • Exothermic: System releases energy, ΔH < 0
  • Endothermic: System absorbs energy, ΔH > 0
• Hess's Law:
  • Allows calculation of ΔH for a reaction by using enthalpy changes from other reactions.
• Enthalpy of Formation:
  • ΔH when a compound forms from its elements.
  • Elements in standard state have ΔH_f = 0.
• Bond Enthalpies:
  • Energy required to break bonds.
  • ΔH = Bonds broken - Bonds formed

Entropy (S)

• Measure of disorder in a system.
• Second Law of Thermodynamics: Entropy of the universe is always increasing.
• Phase Changes:
  • Solid to liquid (melting) increases entropy.
  • Dissolving solids increases entropy.
  • More gas molecules increase entropy.
• Entropy Change (ΔS) in Reactions:
  • Positive ΔS: System becomes more disordered.
  • Negative ΔS: System becomes more ordered.

Gibbs Free Energy (G)

• Equation: ΔG = ΔH - TΔS
• Spontaneity:
  • ΔG < 0: Spontaneous
  • ΔG > 0: Non-spontaneous
• Relation to Equilibrium:
  • ΔG = 0 at equilibrium
  • Q (reaction quotient) relates to product/reactant favorability
  • ΔG = ΔG° + RT ln(Q)
  • ΔG° = -RT ln(K) at equilibrium

Conclusion

• Covered key thermodynamics concepts critical for the AP exam.
• Encouragement to review material and focus on understanding equations and concepts.
• Mention of upcoming kinetics section (Part 2).