Chapter 15: Thermochemistry

Entropy (S)

A measure of the disorder or randomness in a system, with units of J/(mol·K).

Second Law of Thermodynamics

The total entropy of an isolated system always increases over time or remains constant for a reversible process.

Gibbs Free Energy (G)

The energy available to do work in a system at constant temperature and pressure.

Gibbs Free Energy Change (ΔG)

Determines whether a reaction is spontaneous. Negative ΔG indicates a spontaneous process.

Factors Affecting Entropy

• Phase Changes: Entropy increases when a substance changes from solid to liquid to gas.
• Number of Particles: Entropy increases as the number of particles increases in a system.
• Temperature: Higher temperatures lead to greater entropy due to increased molecular motion.

Gibbs Free Energy Equation

The equation for Gibbs Free Energy: $\mathrm{\Delta G}=\mathrm{\Delta H}-T\times \mathrm{\Delta S}$

• ΔG < 0: Reaction is spontaneous.
• ΔG = 0: Reaction is at equilibrium.
• ΔG > 0: Reaction is nonspontaneous.

Spontaneity and Reaction Favorability

• Exothermic Reactions (ΔH < 0): Tend to be spontaneous if ΔS is positive.
• Endothermic Reactions (ΔH > 0): Can be spontaneous if ΔS is sufficiently positive and temperature is high.
• Reactions that increase disorder (ΔS > 0) are more likely to be spontaneous.

Applications of Gibbs Free Energy

• Chemical Reactions: Predicting whether a reaction will proceed without external input.
• Biological Systems: Understanding energy transfer in metabolic pathways.
• Industrial Processes: Designing efficient and spontaneous chemical reactions.

Questions for Students

1. Define entropy and explain how it relates to the Second Law of Thermodynamics.
2. What factors affect entropy? Provide examples.
3. Write and explain the Gibbs Free Energy equation. What does a negative ΔG mean?
4. How do phase changes affect entropy?
5. Give an example of how Gibbs Free Energy is used in industrial or biological applications.