Explain free expansion.

(N/A) The expansion of a gas into a vacuum $(p_{ex} = 0)$ is known as free expansion.
During the free expansion of an ideal gas,no work is performed,regardless of whether the process is reversible or irreversible.
We can express the first law of thermodynamics,$\Delta U = q + w$,in various ways depending on the process.
Substituting $w = -p_{ex} \Delta V$ into the equation,we get:
$\Delta U = q - p_{ex} \Delta V$.
If a process occurs at constant volume $(\Delta V = 0)$,then $\Delta U = q_{V}$,where the subscript $V$ indicates that heat is exchanged at constant volume.