Liquefaction of Gases

Thomas Andrews plotted isotherms of carbon dioxide at various temperatures.

From these he surmised that,

® Real gases behaved like carbon dioxide.

® Isotherms of carbon dioxide at high temperatures looked like an ideal gas.

® At lesser temperature the shape of the curve changes.

® The gas could not be liquefied even at high temperatures.

® Liquid carbon dioxide first appears at 31 ͦ C, which is called critical temperature T_c.

® Volume of one mole of gas at this temperature is called critical volume V_c.

® Pressure at this point is called critical pressure p_c.

® On more pressure being applied at 21 ͦ C the gas changes to liquid at point 1.

® Thus we see that a point like 4 represents gaseous state. A point like 3 represents liquid state and a point 2 or 1 under the dome shaped area represents existence of liquid and gaseous carbon dioxide in equilibrium.

® All the gases upon compression at constant temperature (isothermal compression) show the same behaviour as shown by carbon dioxide.

® Compression brings the molecules in close vicinity and cooling slows down the movement of molecules therefore, intermolecular interactions may hold the closely and slowly moving molecules together and the gas liquefies.

® It is possible to change a gas into liquid or a liquid into gas by a process in which always a single phase is present.

Thus there is continuity between the gaseous and liquid state. The term fluid is used for either a liquid or a gas to recognise this continuity. Thus a liquid can be viewed as a very dense gas. Liquid and gas can be distinguished only when the fluid is below its critical temperature and its pressure and volume lie under the dome, since in that situation liquid and gas are in equilibrium and a surface separating the two phases is visible. A gas below the critical temperature can be liquefied by applying pressure, and is called vapour of the substance.

Problem:

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.