Substance Q melts at and boils at at

PHYSICS · Multi-part · A Level · Physics · Further mechanics and thermal physics · Further mechanics and thermal physics

Problem

Substance Q melts at 18 ∘C18\,^\circ\text{C} and boils at 96 ∘C96\,^\circ\text{C} at 1 atm1\,\text{atm}. Relevant thermodynamic data are provided below. ΔHfus=7.0 kJ mol−1\Delta H_{\text{fus}} = 7.0\,\text{kJ}\,\text{mol}^{-1}  ΔHvap=31.0 kJ mol−1\Delta H_{\text{vap}} = 31.0\,\text{kJ}\,\text{mol}^{-1} The molar heat capacity of liquid Q is 112 J mol−1 K−1112\,\text{J}\,\text{mol}^{-1}\,\text{K}^{-1}. A sample containing 3.50 mol3.50\,\text{mol} of solid Q is initially at its melting point (18 ∘C18\,^\circ\text{C}). The external pressure is held constant at 1 atm1\,\text{atm}. (a) Calculate the heat, in kJ\text{kJ}, required to convert the sample from solid to liquid at 18 ∘C18\,^\circ\text{C}. (b) Determine the additional heat required to raise the temperature of the liquid from 18 ∘C18\,^\circ\text{C} to its normal boiling point. (c) Calculate the heat needed to vaporize the sample completely at 96 ∘C96\,^\circ\text{C}. (d) Using your results from parts (a)–(c), justify which phase transition absorbs more energy per mole. In your explanation include a discussion of both changes in particle motion and intermolecular forces.

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