Free expansion (Joule expansion) – reversibility classification A free expansion of a gas (unrestrained expansion into vacuum; non-flow) is best classified as which type of process?

Introduction / Context:
Free expansion (Joule expansion) is a classic thought experiment where a gas expands into an evacuated space without doing external work. It highlights the difference between energy conservation (First law) and process directionality (Second law), and is a standard example of an irreversible process.

Given Data / Assumptions:

• Gas initially confined; a partition is removed allowing expansion into vacuum.
• No external pressure opposing the expansion; surroundings do no work.
• Insulated container → no heat transfer (adiabatic).

Concept / Approach:

From the First law for a closed, adiabatic system: ΔU = Q − W = 0 − 0 = 0 for an ideal gas (since external work W = 0 and Q = 0). However, the process is highly non-equilibrium and cannot be retraced without external agency, violating reversibility criteria. Entropy increases, so the Second law classifies it as irreversible. For an ideal gas, temperature remains constant (ΔT = 0) because ΔU = 0 and u = u(T); real gases may show slight temperature changes due to Joule effect, but irreversibility remains.

Step-by-Step Solution:

Verification / Alternative check:

Reversing free expansion would require compressing the gas, doing work on it and removing entropy—impossible without external intervention and compensation, confirming irreversibility.

Why Other Options Are Wrong:

Reversible / quasi-static: A huge pressure gradient prevents a sequence of equilibrium states.Isentropic, isobaric, polytropic with n = 1: Do not describe free expansion; pressure is undefined during the unrestrained expansion.

Common Pitfalls:

Assuming ΔU = 0 implies reversibility; energy conservation alone does not guarantee reversibility.

Final Answer:

irreversible