Question

Below the triple point (-56.6°C) the vapor pressure of solid CO2 (also known as “dry ice”) is given as:

ln p = -3116/T + 16.01

The molar heat of melting of CO2 is 8330 J. Use just the data given here about CO2. State any assumptions you need to make to answer the following questions.

(a) Calculate the vapor pressure exerted by the liquid CO2 at 25°C.

(b) Explain why solid CO2 sitting on a laboratory bench evaporates rather than melts.

Answer 1

the vapour pressure exerted by liquid CO2 at 25^oC is 72.97 atm.

b) Sublimation is the process of direct conversion of a solid to gaseous state. Solid CO2 sublimes at room temperature because at room temperature (25^oC) and atmspheric pressure (1 atm) carbon dioxide exist in gaseous form.It will be clear from the phase diagram of carbon dioxide. Liquid CO2 only forms at pressure above 5.11 atm

In the phase diagram, the y-axis is pressure (in atm) and the x-axis is temperature (in ^oC). If we draw a horizontal line across the graph at a pressure of 1 atmosphere, we will see that carbon dioxide goes directly from a solid to a gas at a very low temperature of -78.5^oC.  The surface of the solid CO2 is in contact with the room temperature air (at ~25^oC) and so the molecules on the surface of the solid CO2 immediately go from the solid state to the gaseous state.

In terms of intermolecular forces, CO₂ is a non-polar molecule, and the molecules are attracted to each other only by a very weak force called the "van der Waals" force. Since the molecules are very weakly attracted to each other, CO₂ molecules very readily go from the solid state to the gas state at low temperatures.