Dry ice (solid CO2) can either melt to liquid CO2 or sublime directly to gaseous CO2, depending on the pressure conditions. Consider the following phase changes at 194K, the triple-point temperature of CO2.
Process I: CO2(s)→CO2(ℓ)
Process II: CO2(s)→CO2(g)
(a) Predict the sign of ΔS∘ for each process and justify your answer based on the relative degrees of freedom available to molecules in each phase.
(b) Compare the magnitudes of ΔS∘ (Process I) and ΔS∘ (Process II). Provide a molecular-level explanation for any difference you predict.
(c) Carbon dioxide is frequently used as a refrigerant by exploiting the sublimation rather than the melting process. Discuss how the entropy considerations from parts (a) and (b) help explain this practical choice.
(a) Predict and justify the sign of ΔS∘ for Process I.
(a) Predict and justify the sign of ΔS∘ for Process II.
(b) Compare ∣ΔS∘∣ for the two processes and explain the difference.
(c) Connect your entropy analysis to the practical use of CO2 sublimation in refrigeration.