Free expansion (real vs ideal gas)

Free expansions are considered to be adiabatic. There are two reasons for this:

1. In a theoretical free expansion, we just want to study the effect of free expansion which will get affected if heat entered / existed. So, system is considered insulated

2. In an actual free expansion, the process takes place very rapidly. The expansion is very rapid and any rapid process is considered to be adiabatic.

Rapid processes are assumed to be adiabatic and extremely slow processes are assumed to be isothermal. You can use this concept in solving numericals of tire bursting as well

Free expansion of a real gas will cause cooling since real gases have intermolecular forces. Atoms have to spend energy in order to move farther by overcoming intermolecular forces. Thus expansion of real gas occurs at the expense of internal energy which reduces temperature. So, there are two takeaways for real gas:

1. Expansion causes cooling
2. Work done in free expansion of real gases is not zero as there is a force of attraction between molecules of real gas

On the other hand, ideal gases are not assumed to have any interatomic forces so free expansion neither include any work nor there is any cooling (since no internal energy was spent in overcoming interatomic forces). Hence, for free expansion of ideal gases not only δQ = 0 but also δW = 0, hence ∆U = 0.

For ideal gases, as we know, internal energy and enthalpy are a function of temperature only, so if internal energy U remains constant, temperature T also remains constant which means enthalpy also remains constant.

From these concepts, we can conclude that:
1. During free expansion of an ideal gas, both internal energy and enthalpy remain constant
2. During free expansion of a real gas, none of internal energy and enthalpy remains constant.