Substance Z exhibits the following molar enthalpies at 1 atm: and
CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics
Problem
Substance Z exhibits the following molar enthalpies at 1 atm: ΔHfus=8.0kJ mol−1 and ΔHvap=32.0kJ mol−1. At 0.20 atm, Z sublimates at a temperature below its normal melting point. Assume these enthalpy values are approximately pressure-independent. A student claims that the molar enthalpy of sublimation of Z at 0.20 atm must equal 32.0kJ mol−1 because the substance goes directly to the gas phase. Which statement best evaluates the student’s claim?
(a) The claim is incorrect because the enthalpy of sublimation equals the sum of the enthalpies of fusion and vaporization, so it should be about 40kJ mol−1.
(b) The claim is correct because sublimation bypasses melting, so only the vaporization enthalpy is needed.
(c) The claim is incorrect because the enthalpy of sublimation equals the enthalpy of vaporization minus the enthalpy of fusion, so it should be about 24kJ mol−1.
(d) The claim is incorrect because sublimation releases energy at low pressure, so the enthalpy of sublimation should be about −40kJ mol−1.