Overview
This lecture discusses how entropy and enthalpy influence whether a process, like water freezing, is spontaneous, and examines the roles of the system and surroundings.
Entropy Changes in Freezing Water
- Freezing water is a phase change from liquid to solid.
- During freezing, entropy (ΔS) of the water (system) decreases (ΔS < 0).
- According to the second law of thermodynamics, total entropy of the universe must increase for a process to be spontaneous.
- The system (water) is only a part of the universe; the surroundings must also be considered.
System and Surroundings
- The universe is divided into the system (the reaction or process) and the surroundings (everything else).
- Change in entropy for the universe: ΔS_univ = ΔS_sys + ΔS_surr.
- For freezing to be spontaneous, the increase in entropy of the surroundings must outweigh the decrease in entropy of the system.
Role of Enthalpy (ΔH)
- Freezing water has a negative enthalpy change (ΔH_sys < 0); this is an exothermic process.
- Exothermic means heat is released from the system to the surroundings.
- Released heat increases the temperature of the surroundings (ΔT_surr > 0).
- As temperature of the surroundings increases, their entropy (ΔS_surr) also increases.
Entropy of the Surroundings
- The increase in entropy of the surroundings (ΔS_surr > 0) is due to heat transfer from the exothermic process.
- If ΔS_surr is large enough, it makes ΔS_univ positive, allowing the process to be spontaneous.
- If the process were endothermic (ΔH_sys > 0), heat would be absorbed from the surroundings, decreasing ΔS_surr.
Key Terms & Definitions
- Entropy (ΔS) — A measure of disorder or randomness in a system.
- Enthalpy (ΔH) — Heat content of a system at constant pressure.
- Exothermic — A process that releases heat to the surroundings (ΔH < 0).
- Endothermic — A process that absorbs heat from the surroundings (ΔH > 0).
- System — The part of the universe being studied (e.g., freezing water).
- Surroundings — Everything outside the system.
Action Items / Next Steps
- Review examples of spontaneous and non-spontaneous processes using entropy and enthalpy.
- Practice calculating ΔS_univ for given reactions.