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Liquefaction of Gases: Critical Temperature, Volume, Pressure






Liquefaction of Gases

Low temperature and high pressure are the fact condition for Liquefaction of gases. The reason is that at high pressure and low temperature intermolecular forces start operating strongly between the molecules of gases because they come close to each other.
 Under suitable temperature and pressure conditions gases can be liquefied.
Critical Temperature: -
It is defined as the temperature above which a gas cannot be liquefied. However, high pressure may be applied on the gas.
         $ T_c=\frac {8a}{27Br}$
Critical Pressure: -
It is the pressure required to liquefy the gas at critical temperature.
          $P_c=\frac {a}{27B^2}$
Critical Volume: -
The volume occupied by one mole of gas at critical temperature and critical pressure
E.g.: - For CO2;
                           TC = 30.98o C; PC = 73.9
                           VC = 95 ml mole-1.
These together are called Critical constants

Example

Question 1
Critical temperature for CO2 and CH4 are 31.1o C & -81.9o C respectively. Which of these have stronger intermolecular forces & why?
Answer
Higher the critical temperature easily the gas can be liquefied and more the force of attraction
hence CO2 is having more intermolecular forces than CH4.
Question 2
Name the temperature above which a gas cannot be liquefied by any amount of pressure
Answer
Critical Temperature
Question 3
Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the gas particles. Critical
temperatures of ammonia and carbon dioxide are 405.5 K and 304.10 K respectively. Which of these gases will liquefy first when you start cooling from
500 K to their critical temperature ?
Solution
Ammonia will liquefy first because its critical temperature will be reached first. Liquefaction of CO2 will require more cooling.

Liquid State

Liquids may be considered as continuation of gas phase into a region of small volume and very strong molecular attractions. Molecules of liquid do not separate from each other but Molecule of liquids can move past one another freely, therefore, liquids can flow, can be poured and can assume the shape of the container in which these are stored
 
Vapour pressure
The equilibrium pressure by vapour of liquid in a container at given temperature (T)
Boiling Point
When the liquid is heated in the open container, its vapour pressure start increasing. When Vapour pressure reaches atmospheric pressure, then vaporization occurs in bulk of the liquid and vapours freely moves in the surrounding. This temperature is called Boiling point.
At 1 atm pressure boiling temperature is called normal boiling point. If pressure is 1 bar then the boiling point is called standard boiling point of the liquid
At higher altitude, the boiling point of water decreases because the atmospheric pressure is less than one atmosphere.
Surface Tension (V)
 It is force acting per unit length perpendicular to the line drawn on the surface: (N/m): It decreases with increases in T, it increases with increase in external pressure, because of it falling drops of liquid are spherical, liquid in capillary tube rises.
It has dimensions of kg s-2 and in SI unit it is expressed as N m-1
 
 Viscosity (η)
It is resistance offered to the flow of liquid due to friction between layer of fluids.
F= η.A.(du/dv)
Viscosity decreases with increase in T, and increases with increase in P.
 



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