Liquid water vaporizes reversibly into steam at exactly () and external pressure: . Which of the fol — Thermodynamics and Thermochemistry Chemistry Question
Question
Liquid water vaporizes reversibly into steam at exactly $100^\circ\text{C}$ ($373 \text{ K}$) and $1 \text{ atm}$ external pressure: $H_2O(l, 373\text{K}, 1\text{atm}) \rightarrow H_2O(g, 373\text{K}, 1\text{atm})$. Which of the following thermodynamic relations universally hold true for this system?
💡 Solution & Explanation
(A) At the normal boiling point, liquid and vapor co-exist in thermodynamic equilibrium, so $\Delta G = 0$. (B) Gas possesses vastly more microstates and disorder than liquid, ensuring $\Delta S_{sys} > 0$. (C) Vaporization requires breaking hydrogen bonds, rendering it endothermic ($\Delta H > 0$). (D) $\Delta U = \Delta H - P\Delta V$. Since $\Delta H = 40.6 \text{ kJ/mol}$ and the expansion work term $P\Delta V \approx RT \approx 3.1 \text{ kJ/mol}$, $\Delta U$ remains strongly positive.