Free Expansion - Ideal Gas vs Real Gas By free expansion, I am referring to gas kept in a piston-cylinder arrangement freely allowed to expand against vacuum.
It is clear to me that free expansion is an irreversible process because if it were not then it could get compressed as well at equilibrium, but we know it doesn't as it would be violating the 2nd law of thermodynamics.
I know that all the parameters work 'W', Change in Internal Energy '∆U', and Heat transfer '∆Q' would all be zero in the case of an ideal gas.
But, what I want to know is how will all the parameters vary if gas is real?
Also, I wanted to ask whether the answer will change if the surface between the piston and cylinder is not frictionless?
For a real gas, the internal energy varies with both temperature and molar volume.  The variation is described by the equation:
$$\mathrm dU=C_V\,\mathrm dT-\left[P-T\left(\frac{\partial P}{\partial T}\right)_V\right]\mathrm dV,$$ where $U$ is the internal energy per mole and $V$ is the molar volume.  This means that, unless the equation of state is such that pressure is directly proportional to temperature at constant molar volume (like an ideal gas), the internal energy will depend on molar volume.