A decorative "ice" sculpture is carved from dry ice (solid CO2) and held at its sublimation point of –78.5°C. Consider the process in which a CO2 sculpture, weighing 389 g, sublimes on a granite tabletop. The temperature of the granite is 12.0°C and the process occurs reversibly. Assume a final temperature for the CO2 vapor of –78.5°C. The enthalpy of sublimation of CO2 is 26.1 kJ/mol.a. Calculate the entropy of sublimation for carbon dioxide (the system) ___ J/Kmol b. Calculate the entropy of the universe for this reversible process. Use three significant figures in the answer __J/K

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Answer:

The answers to the questions are;

a. The entropy of sublimation for carbon dioxide (the system) is  

134.07 J/Kmol.

b. The entropy of the universe for this reversible process is 376 J/K.

Explanation:

Entropy of sublimation is the entropy change experienced following the transformation of a mole of solid to vapor at  the temperature where the sublimation is taking place

a. We note that the mass of the solid CO₂ = 389 g

Molar mass of CO₂ = 44.01 g/mol

Number of moles of CO₂ in the sculpture = Mass/(Molar mass)

= (389 g)/(44.01 g/mol) = 8.84 Moles

Entropy of sublimation is given by

ΔS[tex]_{sublimation}[/tex] = [tex]S_{vapor}[/tex] - S[tex]_{solid}[/tex] = [tex]\frac{\Delta H_{sublimation}}{T}[/tex]

Where:

ΔH[tex]_{sublimation}[/tex]  = 26.1 KJ/mol

T = Temperature = –78.5°C = ‪194.65‬ K

Therefore the amount of heat required to cause the 389 g of dry ice to sublime =    26.1 KJ/mol  × 8.84 Moles = 230.695 KJ

Therefore the entropy of sublimation = ΔS[tex]_{sublimation}[/tex] = [tex]\frac{230.695 KJ}{194.65 K}[/tex]

= 1.185 KJ/K

= 1185 J/K = 1185/8.84 J/Kmol = 134.07 J/Kmol

b. The entropy of the universe is given by;

ΔS[tex]_{universe}[/tex] = [tex]\Delta S_{system}[/tex] + ΔS[tex]_{surrounding}[/tex]  

If the heat absorbed by the system is the same as the heat given off by the surrounding, then we have;

ΔS[tex]_{universe}[/tex] = [tex]\frac{Q}{ T_{system}}[/tex]  [tex]-\frac{Q}{T_{surrounding}}[/tex]  

                =1.185 KJ/K - [tex]-\frac{230.695 KJ}{285.15K}[/tex] = 1.185 KJ/K - 0.809 KJ/K = 0.376 KJ/K

= 376 J/K.

The entropy sublimation will be "134.1 J/mol.K".

Given:

  • Mass = 389 g
  • Sublimation point = –78.5°C
  • Granite temperature = 12.0°C
  • Enthalpy of sublimation = 26.1

Now,

The Entropy of sublimation of CO₂ will be:

= [tex]\frac{\Delta H_{sublimation}}{T_{sublimation}}[/tex]

By substituting the values, we get

= [tex]\frac{26100 }{273.15-78.5 }[/tex]

= [tex]134.1 \ J/mol.k[/tex]

Thus the answer above is right.

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