Overview
This lecture explains how the signs of enthalpy change (ΔH) and entropy change (ΔS) affect the spontaneity of a process using the Gibbs free energy equation, ΔG = ΔH – TΔS.
Spontaneity and Gibbs Free Energy
- Spontaneity is determined by the sign of ΔG: negative ΔG indicates a spontaneous reaction, positive ΔG indicates non-spontaneous.
- The equation ΔG = ΔH – TΔS is used to predict spontaneity, considering both enthalpy and entropy changes and temperature.
Four Scenarios for ΔH and ΔS
- Both ΔH negative, ΔS positive: ΔG always negative, reaction is spontaneous at all temperatures.
- Both ΔH positive, ΔS negative: ΔG always positive, reaction is non-spontaneous at all temperatures.
- Both ΔH and ΔS positive: ΔG becomes negative (spontaneous) only at high temperatures.
- Both ΔH and ΔS negative: ΔG becomes negative (spontaneous) only at low temperatures.
Detailed Examples
- Endothermic (ΔH > 0) & entropy-increasing (ΔS > 0): Becomes spontaneous at high temperatures (e.g., boiling water).
- Exothermic (ΔH < 0) & entropy-decreasing (ΔS < 0): Becomes spontaneous at low temperatures (e.g., freezing water).
Temperature Dependence
- When both ΔH and ΔS are positive or both are negative, the temperature determines whether the process is spontaneous.
Key Terms & Definitions
- ΔG (Gibbs Free Energy) — Determines if a reaction is spontaneous; calculated as ΔG = ΔH – TΔS.
- ΔH (Enthalpy Change) — Heat absorbed or released in a reaction; negative for exothermic, positive for endothermic.
- ΔS (Entropy Change) — Change in disorder; positive means more disorder, negative means more order.
- Spontaneous Process — Occurs without external work (ΔG < 0).
- Non-spontaneous Process — Requires energy input (ΔG > 0).
Action Items / Next Steps
- Practice calculating ΔG for different combinations of ΔH and ΔS at various temperatures.
- Review textbook examples of phase changes (boiling, freezing) and their relation to spontaneity.