Q17 of 26 Page 294

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?

The P-T phase diagram of CO2 is as follows:


(a) Since the temperature -60° C lies to the left of 56.7° C on the curve i.e., it lies in the region vapour and solid phase, carbon dioxide will condense directly into the solid without becoming liquid.


(b) Since the pressure 4 atm is less than 5.1 atm the carbon dioxide will condense directly into solid without becoming liquid.


(c) When a solid CO2 at 10 atm pressure and -65° C temprature 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 Temperature-axis, then the points of intersection of this line with the fusion and vaporization curve will give the fusion and boiling points of CO2 at 10 atm.


(d) Since 70° C is higher than the critical temperature of CO2, the CO2 gas cannot be converted into liquid state on being compressed isothermally at 70° C. It will remain in the vapour state. However, the gas will deviate from its perfect gas behaviour with increasing pressure.


More from this chapter

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15

Given below are observations on molar specific heats at room temperature of some common gases.































Gas



Molar specific heat (Cv) (cal mo1–1 K–1)



Hydrogen



4.87



Nitrogen



4.97



Oxygen



5.02



Nitric oxide



4.99



Carbon monoxide



5.01



Chlorine



6.17



The measured molar specific heats of these gases are markedly different from those for monatomic gases. Typically, molar specific heat of a monatomic gas is 2.92 cal/mol K. Explain this difference. What can you infer from the somewhat larger (than the rest) value for chlorine?

 16

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?

 18

A child running a temperature of 101°F is given an antipyrin (i.e. a medicine that lowers fever) which causes an increase in the rate of evaporation of sweat from his body. If the fever is brought down to 98 °F in 20 min, what is the average rate of extra evaporation caused, by the drug. Assume the evaporation mechanism to be the only way by which heat is lost. The mass of the child is 30 kg. The specific heat of human body is approximately the same as that of water, and latent heat of evaporation of water at that temperature is about 580 cal g–1.

 19

A ‘thermacole’ icebox is a cheap and efficient method for storing small quantities of cooked food in summer in particular. A cubical icebox of side 30 cm has a thickness of 5.0 cm. If 4.0 kg of ice is put in the box, estimate the amount of ice remaining after 6 h. The outside temperature is 45 °C, and co-efficient of thermal conductivity of thermacole is 0.01 J s–1 m–1 K–1. [Heat of fusion of water = 335 × 103 J kg–1]