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Why does carbon dioxide show larger abnormal deviation from ideal gas behaviour than other gases. Why does carbon dioxide first show negative deviation and then positive?

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  • $\begingroup$ What does showing negative deviation of positive deviation mean (pressure, temperature, volume, z factor??)? $\endgroup$ Dec 24, 2015 at 14:50

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When the compressibility factor z for CO2 is expressed in terms of reduced pressure and reduced temperature (i.e., using the law of corresponding states), the PVT behavior of CO2 is not very different from that of other gases. The reduced temperature and reduced pressure are defined as the actual pressure and temperature normalized in terms of the critical pressure and critical temperature, respectively. The real question is, "what causes the critical temperature and critical pressure of CO2 to be what they are?"

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An ideal gas is the assumption that:

  1. you have a large number of particles in the gas with a Boltzmann distribution
  2. the size of the particles is negligible, and
  3. there are no forces acting between the particles except for elastic collisions

In a gas with very weak forces, such as hydrogen which has only van der Waals forces, these assumptions are quite reasonable. In a gas with strong intermolecular forces, such as ammonia which can form hydrogen-bonds as well as reglar dipole-dipole interactions, these assumptions no longer make as much sense and the gas deviates from 'ideal' behaviour.

Molecules of CO$_{2}$ do not have an overall dipole so it only has van der Waals forces acting on it, but it is much larger and has more electrons than hydrogen molecules so the van der Waals forces are stronger. When you begin to compress the gas the molecules are compressed within range where these forces attract the molecules closer together and the volume is smaller than expected (your negative deviation from ideal gas).

Eventually there comes a point where this extra attraction is overcome by repulsive forces, because the particles of all gases do have size and can't be compressed infinitely. At the point the gas occupies a larger volume than you would expect for the pressure (the positive deviation from the ideal gas shown by all gases at high pressure).

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