Why does hydrogen gas warm instead of cool upon sudden expansion?

Answer 1

This happens in certain circumstances due to what is called the Joule Thomson Effect

For most fluids, flow through a valve results in a dropping of both pressure and temperature. Some fluids such as helium,hydrogen have a "reverse Joule-Thompson" effect, which simply means that instead of the temperature dropping along with the pressure, the temperature actually increases.

If the gas molecules were so close together that moving apart would give

them a lower potential energy, then the molecules repel each other.

Expansion lowers the intermolecular potential energy. This means that

expansion releases energy from the intermolecular potential, and that energy

is picked up by the molecules' kinetic energy. In other words, the gas

molecules speed up and the temperature increases.