A 25.0g sample of solid naphthalene (molar mass 128g mol−1) is initially at its melting point of 80.2∘C. In a closed, well-insulated vessel open to the atmosphere, the following sequence of events takes place:
1. The solid melts completely to liquid at 80.2∘C.
2. The liquid is then vaporized completely at its boiling point of 218∘C.
3. The hot vapor cools and condenses completely back to liquid at 218∘C.
No temperature changes other than those listed occur, and pressure remains at 1 atm throughout.
Molar enthalpies (at 1 atm):
• ΔHfus=19.1kJ mol−1 • ΔHvap=41.4kJ mol−1
Answer the following questions about the overall energetic effect of the sequence.
(a) Calculate the quantity of heat absorbed or released (include sign) during step 1.
(b) Calculate the quantity of heat absorbed or released (include sign) during step 2.
(c) Calculate the quantity of heat absorbed or released (include sign) during step 3.
(d) Determine the net heat absorbed or released by the naphthalene sample for the entire sequence, and state whether the overall process is endothermic or exothermic.
(e) Justify your answer in part (d) by discussing how the direction of energy flow in each individual step combines to produce the overall result.