Answer the following questions based on the P–T phase diagram of CO2:
(a) CO2 at 1 atm pressure and temperature – 60 °C is compressed isothermally. Does it go through a liquid phase?
(b) What happens when CO2 at 4 atm pressure is cooled from room temperature at constant pressure?
(c) Describe qualitatively the changes in a given mass of solid CO2 at 10 atm pressure and temperature –65 °C as it is heated up to room temperature at constant pressure.
(d) CO2 is heated to a temperature 70 °C and compressed isothermally. What changes in its properties do you expect to observe?
Answers
(a) No, it is in vapour phase .
if it is compressed isothermally. I.e, when pressure is increased without changing the temperature, it will go to solid phase directly without going through the liquid phase .
(b) CO2 at 4atm pressure and at room temperature ( 298 K ) is vapour if it is cooled at constant pressure , it will again condense to solid without going through liquid phase.
(c) CO2 at 10atm pressure and -65°C temperature is solid . As CO2 is heated up to room temperature at constant pressure , it will go in liquid phase and then go vapour phase .because the initial line through the initial point intersect both the fusion and vaporisation curves .
(d) it won't exhibit any clear phase transition to the liquid phase . However CO2 gas will depart more and more from the ideal gas behavior as its pressure increases .
Answer:The P-T phase diagram for CO
2
is shown in the following figure:
(a) Since the temperature −60
o
C lies to the left of 56.6
o
C on the curve i.e. lies in the region vapour and solid phase, so carbon dioxide will condense directly into the solid without becoming liquid.
(b) Since the pressure 4 atm is less than 5.11 atm the carbon dioxide will condense directly into solid without becoming liquid.
(c) When a solid CO
2
at 10 atm pressure and −65
o
C temperature is heated, it is first converted into liquid. A further increase in temperature brings it into the vapour phase. At P = 10 atm, if a horizontal line is drawn parallel to the T-axis, then the points of intersection of this line with the fusion and vaporization curve will give the fusion and boiling points of CO
2
at 10 atm.
(d) Since 70
o
C is higher than the critical temperature of CO
2
, so the CO
2
gas can not be converted into liquid state on being compressed isothermally at 70
o
C. It will remain in the vapour state. However, the gas will depart more and more from its perfect gas behavior with the increase in pressure.
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