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AP Chemistry
6.5 Energy of Phase Changes
IntermediateMCQGraphicalConceptual14.4k
A 2D Cartesian coordinate system with bare axes in grayscale. The horizontal axis is labeled 'Heat Added (kJ)' and the vertical axis is labeled 'Temperature (°C)'. A single solid black curve traces the heating curve of a pure substance from bottom-left to top-right through five connected linear segments: segment 1 rises with positive slope from (0, -20) to (2, 40); segment 2 is a flat horizontal plateau labeled 'Melting' extending from (2, 40) to (5, 40), spanning 3 kJ; segment 3 rises with positive slope from (5, 40) to (9, 120); segment 4 is a flat horizontal plateau labeled 'Vaporization' extending from (9, 120) to (18, 120), spanning 9 kJ; segment 5 rises with positive slope from (18, 120) to (20, 160). Dashed vertical projection lines connect the plateau endpoints to the horizontal axis. No other curves, labels, or annotations appear.
Heating curve for a pure molecular substance heated at a constant rate under \(1.0 \text{ atm}\).
A pure sample of a molecular substance is heated at a constant rate under a constant pressure of \(1.0 \text{ atm}\). The heating curve for the sample is shown below.

Which of the following correctly compares the molar enthalpy of fusion, \(\Delta H_{\text{fus}}\), to the molar enthalpy of vaporization, \(\Delta H_{\text{vap}}\), of the substance and provides the correct particulate-level justification?

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