A chemistry student investigates the energy changes that occur when a pure…
PHYSICS · Multi-part · A Level · Physics · Further mechanics and thermal physics · Further mechanics and thermal physics
Problem
A chemistry student investigates the energy changes that occur when a pure crystalline solid sample of naphthalene (melting point 80.2 °C) is warmed gently in an insulated beaker on a hot plate that maintains a constant temperature of 85 °C.
The student observes that once the solid begins to melt, the thermometer reading remains essentially constant until all of the solid has turned to liquid, even though the hot plate continues to supply energy.
(a) Describe, in terms of molecular-level interactions, why the temperature of the naphthalene remains constant during the interval in which the solid and liquid phases coexist.
(b) Naphthalene releases 19.0 kJ mol⁻¹ of energy when it solidifies at 80.2 °C. Explain how this value is related to the energy absorbed during the melting process described above.
(c) Suppose the student starts with 0.080 mol of solid naphthalene at 80.2 °C. Without performing a numerical calculation, predict whether the magnitude of the energy released on freezing this sample back to solid at 80.2 °C would be greater than, less than, or equal to the magnitude of the energy absorbed during melting. Justify your prediction.