Question

At 0 oC and pressures greater than 0 but less than 30 atm, Z of H2 gas is > 1, while the Zs of N2 and CO2 gases are < 1. Beyond 200 atm and still at 0 oC, Zs of N2 and H2 are > 1 while Z of CO2 is still < 1. Explain in terms of attractive and repulsive forces.

Answer 1

The ratio PV/RT is called compressibility factor or Z

For ideal gases it is 1. At a given temperature the curve Z v.s P is affected by attractions repulsion and kinetic energy of molecules.

For low temperature and low pressure, gases have low kinetic energy and thus attractive forces dominates making actual volume of gas smaller than ideal. This deviation depends on molecular diameter , H₂  is much smaller in size and thus has no such attractive forces showing positive deviation.

But at higher Pressure , Kinetic energy of gases increases. Here attractive interactions are less important and repulsive more. This makes actual volume greater than ideal and Z>1 for H₂ and N₂ . But CO₂ needs further more energy to overcome these attractive interactions and thus, Z is still negative.