The vapor pressure of solid CO 2 (dry ice) is 280 torr at -90. o C and 105 torr
ID: 892848 • Letter: T
Question
The vapor pressure of solid CO2 (dry ice) is 280 torr at -90. oC and 105 torr at -100. oC.
b) Use the above values for the standard free energies to determine the standard enthalpies and entropies for the sublimation. Assume both quantities are constant over this temperature range.
A dry ice bath is used routinely in the laboratory to keep things cold. It is made by mixing dry ice powder with a solvent to make a slurry. Estimate the temperature of the bath to the nearest degree Celcius by determining the temperature at which the vapor pressure of CO2 is 1 atm.
t = oC
-90oC -100oC Go = kJ/molExplanation / Answer
delta Go = - RT lnK
For T = - 90 degreeC or ( - 90 +273) K = 183 K
delta Go = 1.52 kJ/mol
For T = - 100 degreeC or ( 273 - 100) = 173 K
deltaGo = 2.85 kJ/mol
deltaGo = deltaHo - T*deltaSo
1.62 kJ/mol = deltaHo - 183 * deltaSo
2.86 kJ/mol = deltaHo - 173 * deltaSo
1.33 = 10 * deltaSo
deltaSo = 0.133 kJ/K-mol
deltaHo = - 8.573 + ( 183 * 0.188)
deltaHo = 25.9 kJ/mol
Vapor pressure of CO2 = 1 atm
From the phase diagram the temperature at which vapor pressure of CO2 is 1 atm = - 78.5 degreeC or 197.5 K
Temperature of the bath = - 78.5 degreeC
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