The vapor pressure of solid CO2 (dry ice) is 280 torr at -90. oC and 105 torr at

Question

The vapor pressure of solid CO2 (dry ice) is 280 torr at -90. oC and 105 torr at -100. oC. a) What is the value of Go for the reaction CO2(s) CO2(g) at each temperature? -90oC -100oC Go = kJ/mol 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. So = kJ/mol-K Ho = kJ/mol 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

Explanation / Answer

Solution:

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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