7. Critical phenomenon & Liquefaction of gases:
The phenomena of converting a gas into liquid is known as liquefaction. The liquefaction of gas is achieved by controlling P and T as follows:
1. Increasing pressure: An increase in pressure results in an increase in attraction among molecules.
2. Decreasing temperature: A decrease in temperature results in decrease in kinetic energy of molecules.
Critical temperature (Tc) : It is defined as the characteristic temperature for a given gas below which a continuous increase in pressure will bring liquefaction of gas and above which no liquefaction is noticed although pressure may be increased e.g. Tc for CO2 is 31.2°C.
Critical pressure (Pc) : It is defined as the minimum pressure applied on 1 mole of gas placed at critical temperature, to just liquefy the gas
Critical Volume (Vc) : The volume occupied by 1 mole of gas placed at critical conditions.
VC = 3b (i.e. P = Pc and T = Tc)
8. Collision parameters
- Mean free path
= ( = coefficient of viscosity)
k = Boltzman constant ; = collision diameter.
- Collision frequency (z) : no. of collision taking place per second per unit volume.
- Collision diameter : Closest distance between the centre of two molecule which are participating in collision.
- Relative Humidity (RH) :
At a given temperature
- Loschmidth Number
The number of molecules present in 1 cc of gas (or) vapour at STP.
value : 2.617 × 1019 1 cc