Expansion into a vacuum, also called free expansion or Joule expansion, is not the same as an adiabatic process, there is no piston and no work is done. For an

Expansion into a vacuum, also called free expansion or Joule expansion, is not the same as an adiabatic process, there is no piston and no work is done. For an ideal gas the internal energy does not depend on the volume, so conversely such an expansion does not change the temperature. And of course it's not isentropic either, since the same amount of gas with same amount of kinetic energy now can spread itself over a larger volume.