Solid ethanol can slowly disappear from a laboratory freezer over time because it sublimes. Selected thermodynamic data for ethanol are provided below.
• Molar enthalpy of fusion at its melting point (–114 °C): ΔHfus=5.02 kJ mol−1
• Molar enthalpy of vaporization at its normal boiling point (78.3 °C): ΔHvap=38.6 kJ mol−1
Assume that these molar enthalpies remain approximately constant over the temperature range considered and that ethanol behaves ideally.
(a) Write separate, balanced thermochemical equations (including physical states and enthalpy changes) for the fusion of ethanol and for the vaporization of liquid ethanol.
(b) Construct a balanced thermochemical equation for the sublimation of solid ethanol at 1 atm and use the data provided to calculate the molar enthalpy of sublimation, ΔHsub.
(c) Determine the value (with sign) of q required to sublime 2.00 mol of solid ethanol at 1 atm under the same temperature assumptions.
(d) State the sign and magnitude of q for the deposition of 2.00 mol of gaseous ethanol to solid under identical conditions, and explain how your answer demonstrates the thermodynamic relationship between forward and reverse phase changes.