Thermodynamics: Spontaneity and Chemical Equilibrium

Thermodynamics provides the framework for predicting whether a chemical reaction will occur spontaneously and where the equilibrium will lie.

The Three Laws of Thermodynamics

First Law (Energy Conservation) Energy cannot be created or destroyed, only transferred. For a chemical system: ΔU = q + w

Second Law (Entropy) The total entropy of an isolated system always increases for spontaneous processes: ΔS_universe = ΔS_system + ΔS_surroundings > 0

Third Law The entropy of a perfect crystal at absolute zero is zero.

Gibbs Free Energy

The Gibbs free energy combines enthalpy and entropy into a single criterion for spontaneity at constant T and P:

ΔG = ΔH - TΔS

• ΔG < 0: Spontaneous (exergonic)
• ΔG = 0: At equilibrium
• ΔG > 0: Non-spontaneous (endergonic)

Temperature Dependence

The effect of temperature on spontaneity depends on the signs of ΔH and ΔS:

| ΔH | ΔS | Spontaneous at | |----|----|----| | - | + | All temperatures | | - | - | Low temperatures | | + | + | High temperatures | | + | - | No temperature |

Relationship to Equilibrium

The standard free energy change relates to the equilibrium constant: ΔG° = -RT ln(K)

This powerful equation connects thermodynamics to equilibrium: - K > 1: ΔG° < 0, products favored - K < 1: ΔG° > 0, reactants favored - K = 1: ΔG° = 0

Practical Applications

Understanding these principles is essential for: - Predicting reaction feasibility - Optimizing industrial processes (e.g., Haber process) - Understanding biochemical pathways - Designing electrochemical cells