Question

calculate the temperature of ammonia and carbon dioxide are 405.5k and 304.10 k respectively. which of these gases will liquefery first when you start cooloing from 500k to their critical temoeratyre?

Answer 1

If the temperature is being lowered monotonically from an initial value of 500K, it seems reasonable that you would reach a value of 405.5 K before reaching 304.1 K.

HOWEVER! This does NOT mean that the ammonia will liquefy before the CO2 does. Room Temperature is about 300K, so why don’t we see liquid CO2 or ammonia?

You are ignoring a very important variable - Pressure.

The Critical PRESSURE of CO2 is 72.9 atm, 7.39 MPa, 1,071 psi. This explains why you can have liquid CO2 in a steel fire extinguisher but not in any wimpy container that can’t withstand 1000 psi.

The Critical Pressure of ammonia is 111.3 atm, 11.28 MPa, 1636 psi, considerably higher than that of CO2.

So it might well be easier to liquefy CO2 because of the lower pressure needed.

Nomenclature: For substances that are above both the Critical Temperature and Pressure, we call them fluids, neither liquid not gas. They only become liquid or gas once they are below the Critical Point on the Phase Diagram.

Answer 2

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• Ammonia will liquify first because its critical temperature will be reached first.
• Liquefication of ${CO2}$will require more cooling.