Page "Adiabatic process" Paragraph 42

For an adiabatic free expansion of an ideal gas, the gas is contained in an insulated container and then allowed to expand in a vacuum.

Because there is no external pressure for the gas to expand against, the work done by or on the system is zero.

Since this process does not involve any heat transfer or work, the First Law of Thermodynamics then implies that the net internal energy change of the system is zero.

For an ideal gas, the temperature remains constant because the internal energy only depends on temperature in that case.

Since at constant temperature, the entropy is proportional to the volume, the entropy increases in this case, therefore this process is irreversible.