Question

1) Explain the reason for heating and cooling when a non-ideal gas experiences a decease in pressure ?

2) The Joule-Thompson coefficient of argon is 0.32 K/arm. Estimate the final temperature of Ar at 30 atm and 50C that is forced through a small orifice to final pressure of 1atm?

Answer 1

1). When the pressure of a non - ideal gas is decreased, average distance between the gas molecules decrease.  Because of the attractive part of the intermolecular force, contraction causes an increase in the kinetic energy of the gas. Thus causes the heating of the gas.

During gas molecule collisions, kinetic energy is temporarily converted into potential energy (corresponding to the repulsive part of the intermolecular force). As the average intermolecular distance increases, there is a drop in the number of collisions per unit time, which causes a decrease in average kinetic energy causing the cooling of the gas.

2). Joule-Thompson coefficient of argon, _JT  = 0.32 K/atm

Initial pressure, P1 = 30 atm

Initial temperature, T1 = 50 oC = 323 K

Final pressure, P2 = 1 atm

Final temperature, T2 = ?

Using Joule-Thompson effect -

_JT = T / P

_JT = T1 - T2 / P1 - P2

0.32 = 323 - T2 / 30 - 1

0.32 = 323 - T2 / 29

9.28 = 323 - T2

T2 = 323 - 9.28

T2 = 313.72 K

T2 = 40.72 oC

Finat temperature = 40.72 oC