What is the name of the process where a gas expands into a vacuum without doing work?

The process in which a gas expands into a vacuum without doing work is called Joule expansion. In the classic experiment, gas occupies one side of a thermally insulated container while the other side is evacuated; opening a partition lets the gas fill the combined volume.

Because the gas expands against essentially no external pressure, the system does no boundary work. Insulation also prevents heat transfer, so the first law gives a constant internal energy, ΔU = 0. The process is irreversible, and its entropy increases even though the internal energy does not change.

For an ideal gas, internal energy depends only on temperature, so a Joule expansion leaves the temperature unchanged. Real gases can cool or warm because intermolecular forces alter the balance between kinetic and potential energy. James Prescott Joule used this type of experiment in 1845 while investigating the mechanical equivalent of heat.

Joule expansion is often confused with Joule–Thomson expansion. The latter is a throttling process through a valve or porous plug and is approximately constant-enthalpy, whereas Joule expansion is free expansion with constant internal energy.