Two samples—one of naphthalene and one of lauric acid—are heated from to complete…

PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Thermodynamics · Thermodynamics

Problem

Two

25.0\,\text{g}

samples—one of naphthalene and one of lauric acid—are heated from

20\,^{\circ}\text{C}

to complete melting under identical conditions. The following information is obtained from separate heating-curve experiments: Naphthalene heating-curve description: • Temperature rises linearly from

20\,^{\circ}\text{C}

to a plateau at

80\,^{\circ}\text{C}

. The plateau lasts

3.0\,\text{min}

. Lauric acid heating-curve description: • Temperature rises linearly from

20\,^{\circ}\text{C}

to a plateau at

44\,^{\circ}\text{C}

. The plateau lasts

8.0\,\text{min}

. The rate of energy input from the hot plate is constant at

120\,\text{J s}^{-1}

for each experiment. Assume no heat loss to the surroundings and negligible heat capacity of the container. (a) Using the time lengths of the plateaus, calculate the quantity of heat absorbed during melting for each substance. (b) Determine the molar enthalpy of fusion

\Delta H_{\text{fus}}

for each substance. (Molar masses: naphthalene

=128.2\,\text{g mol}^{-1}

; lauric acid

=200.3\,\text{g mol}^{-1}

.) (c) Which substance exhibits stronger intermolecular forces in the liquid phase relative to the solid phase? Support your answer with quantitative evidence from parts (a) and (b).

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