Overview

Thermodynamics tells us if a reaction will happen (spontaneity) and how much energy it involves. It deals with macroscopic properties like temperature, pressure, and volume.

Core Idea

First Law: Energy cannot be created or destroyed (Conservation of Energy). Second Law: The entropy of the universe always increases (Disorder tends to increase).

Formal Definition (if applicable)

Gibbs Free Energy ($\Delta G$): A measure of the maximum reversible work that may be performed by a thermodynamic system. $$ \Delta G = \Delta H - T\Delta S $$ If $\Delta G < 0$, the reaction is spontaneous.

Intuition

  • Enthalpy ($\Delta H$): Heat content. Is the reaction releasing heat (burning wood) or absorbing it (melting ice)?
  • Entropy ($\Delta S$): Disorder. Is the room getting messier (gas expanding) or cleaner (crystal forming)?
  • Nature prefers low energy (exothermic) and high disorder (high entropy).

Examples

  • Combustion: Highly exothermic (releases heat) and increases entropy (solid wood $\rightarrow$ gas). Very spontaneous.
  • Ice Melting: Endothermic (absorbs heat) but increases entropy. Spontaneous above 0°C.
  • Photosynthesis: Non-spontaneous (requires energy input from the sun).

Common Misconceptions

  • “Spontaneous means fast.” (No, spontaneous means it can happen without external energy. Rusting is spontaneous but slow. Speed is Kinetics, not Thermodynamics.)
  • “Entropy is just messiness.” (It’s statistically defined as the number of microstates.)
  • Equilibrium: When the forward and reverse reaction rates are equal ($\Delta G = 0$).
  • Heat Capacity: How much energy is needed to raise temperature.
  • Calorimetry: Measuring heat changes.

Applications

  • Engines: Maximizing efficiency of heat-to-work conversion.
  • Batteries: Storing chemical energy.
  • Refrigeration: Moving heat against the gradient.

Criticism / Limitations

Thermodynamics predicts the final state but says nothing about how long it takes to get there. Diamond turning into graphite is thermodynamically favorable but kinetically impossible at room temperature.

Further Reading

  • Atkins, Physical Chemistry
  • Fermi, Thermodynamics