Answer the following questions based on the P-T phase diagram of carbon dioxide:
Answer the following questions based on the P-T phase diagram of carbon dioxide:
(a) At what temperature and pressure can the solid, liquid and vapour phases of CO2 co-exist in equilibrium?
(b) What is the effect of decrease of pressure on the fusion and boiling point of CO2?
(c) What are the critical temperature and pressure for CO2? What is their significance?
(d) Is CO2 solid, liquid or gas at (a) –70 °C under 1 atm, (b) –60 °C under 10 atm, (c) 15 °C under 56 atm?
Solution 1
The P-T phase diagram for CO2 is shown in the following figure.
C is the triple point of the CO2 phase diagram. This means that at the temperature and pressure corresponding to this point (i.e., at –56.6°C and 5.11 atm), the solid, liquid, and vaporous phases of CO2co-exist in equilibrium.
b) The fusion and boiling points of CO2 decrease with a decrease in pressure.
(c) The critical temperature and critical pressure of CO2 are 31.1°C and 73 atm respectively. Even if it is compressed to a pressure greater than 73 atm, CO2 will not liquefy above the critical temperature.
(d) It can be concluded from the P-T phase diagram of CO2 that:
1)CO2 is gaseous at –70°C, under 1 atm pressure
2)CO2 is solid at –60°C, under 10 atm pressure
3)CO2 is liquid at 15°C, under 56 atm pressure
Solution 2
(a) At the triple point, temperature = – 56.6 °C and pressure = 5.11 atm.
(b) Both the boiling point and freezing point of CO2 decrease if pressure decreases.
(c) The critical temperature and pressure of CO2 are 31.1°C and 73.0 atm respectively. Above this temperature, CO2 will not liquefy/even if compressed to high pressures.
(d) (i) The point (- 70 °C, 1.0 atm) lies in the vapour region. Hence, CO2 is vapour at this point.
(ii) The point (- 60 °C, 10 atm) lies in the solid region. Hence, CO2 is solid at this point.
(iii) The point (15 °C, 56 atm) lies in the liquid region. Hence, CO2 is liquid at this point
