Lecture Notes: Laws of Thermodynamics

Introduction

• Thermodynamics helps understand energy flow.
• Concepts may seem like common sense but there's underlying math.
• We'll discuss conceptual descriptions of the laws.

First Law of Thermodynamics: Conservation of Energy

• Energy is not created or destroyed, only changes forms (e.g., potential to kinetic to heat).
• Applies well for chemists despite exceptions at the quantum level.
• Energy has a preferred direction of flow.

Second Law of Thermodynamics: Entropy

• Entropy: Measure of disorder.
• Entropy of a system and surroundings always increases.
• Classic analogy: Rooms become messy over time.
• High entropy: Energy is dispersed.
• Example: Solid ionic vs. liquid state (more disorder in liquid).
• Entropic influence examples: Heat flows from hot to cold naturally.
• Entropy is measured in joules per kelvin.
• Entropy is about energy distribution, not energy itself.

Third Law of Thermodynamics

• Perfect crystalline solid at absolute zero has zero entropy.

Enthalpy and Gibbs Free Energy

• Enthalpy: Describes energy of a system.
• Gibbs Free Energy (G): Determines spontaneity of a process.
• Equation: ΔG = ΔH - TΔS
  • Negative ΔG: Spontaneous process.
  • Positive ΔG: Non-spontaneous process.
• Favorability:
  • Exothermic (negative ΔH) and increased entropy (positive ΔS) is spontaneous.
  • Endothermic (positive ΔH) can be spontaneous if entropy increase is large enough.
  • Temperature affects spontaneity:
    • Higher temps favor entropically favorable processes.
    • Lower temps favor energetically favorable processes.

Misconceptions and Examples

• Incorrect use of entropy to suggest order can't happen spontaneously.
• Entropically unfavorable processes can be spontaneous if energetically favorable.
• Example: Soap and micelles formation:
  • Soap molecules have polar heads and nonpolar tails.
  • Form micelles trapping dirt, making them water-soluble.
  • Demonstrates order formation on a small scale, but overall entropy increase.

Conclusion

• Entropy of the universe is always increasing despite local order.
• Understanding these laws helps explain spontaneous processes.

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