Questions 39-41 refer to the following graph, which shows the heating curve for methane, CH4. [VISUA — Thermodynamics Chemistry Question

STEPS:

1. Identify the physical processes from the heating curve: By examining the heating curve for methane, the process occurring at the horizontal plateau at $90\text{ K}$ (plateau Q) is melting (fusion), and the process occurring at the horizontal plateau at $110\text{ K}$ (plateau S) is vaporization (boiling).
2. Understand the molecular behavior during melting (at $90\text{ K}$): During melting, solid methane transitions to liquid methane. In this transition, the rigid crystal lattice is disrupted, allowing the molecules to flow past one another. However, because the molecules in a liquid remain in close contact, their intermolecular attractions are only partially disrupted or weakened, not completely broken.
3. Understand the molecular behavior during vaporization (at $110\text{ K}$): During vaporization, liquid methane transitions to gaseous methane. In the gas phase, molecules are extremely far apart and move independently of one another. To achieve this state of complete separation, intermolecular attractions must be completely overcome.
4. Relate the extent of IMF disruption to energy requirements: Because completely overcoming intermolecular attractions requires a massive input of energy compared to simply weakening them, vaporization requires significantly more energy than melting. This is reflected in the fact that the enthalpy of vaporization ($\Delta H_{\text{vap}} = 8.2\text{ kJ/mol}$) is nearly nine times larger than the enthalpy of fusion ($\Delta H_{\text{fus}} = 0.94\text{ kJ/mol}$). This confirms Option A is the correct statement.

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WHY_OTHERS_WRONG:

• Option B is incorrect: Intermolecular attractions are strongest in the solid phase (where thermal energy is low enough that attractions hold particles in fixed, rigid positions) and become progressively weaker in the liquid phase. Stating that attractions in the solid phase are weaker than in the liquid phase is factually incorrect.
• Option C is incorrect: Polarizability is a measure of how easily an electron cloud can be distorted to form a dipole, which depends primarily on the size and number of electrons in the molecule. It does not change significantly with small temperature changes, nor is it the explanation for why vaporization requires more energy than melting.
• Option D is incorrect: Phase changes are isothermal processes, meaning they occur at a constant temperature. While vaporization occurs at a higher temperature than melting, the process of vaporization itself occurs at a constant temperature of $110\text{ K}$ and does not involve any increase in temperature.