Free Expansion - Ideal Gas vs Real Gas
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.