Solid carbon dioxide (dry ice) at −78.5∘C is placed in a sealed, rigid, evacuated chamber whose walls are maintained at a constant temperature of 25.0∘C by an external thermostat. Over several hours the following sequence occurs:
1. The dry ice sublimates completely to gaseous CO2 at −78.5∘C.
2. The gaseous CO2 subsequently deposits as a thin layer of solid on the cooler inner wall of a connected side arm that is maintained at −90.0∘C.
(Assume the bulk gas temperature remains at −78.5∘C throughout.)
Molar enthalpy values at 1 atm (assume temperature independence over the range considered):
• ΔHsub=25.2kJ mol−1
• ΔHdep=−25.2kJ mol−1
Answer the following questions about the combined process.
(a) Using appropriate signs, state the heat exchanged between the CO2 system and its surroundings during step 1 and during step 2.
(b) Determine the net heat exchanged by the CO2 system for the overall two-step process.
(c) Explain, with reference to the first law of thermodynamics, why the temperature of the chamber walls must remain constant even though energy is exchanged in each step.
(d) At the particulate level, compare the molecular arrangements in the original dry ice and in the solid CO2 that forms on the side arm wall, and discuss why the enthalpy change for deposition has the sign you listed in part (a).