Joule–Thomson effect: During throttling at approximately ambient conditions, all real gases cool except which of the following?

Introduction / Context:
The Joule–Thomson (JT) effect describes temperature change during throttling (adiabatic, no shaft work). Whether a gas cools or warms depends on its inversion temperature relative to the operating temperature. This principle is crucial in gas liquefaction and cryogenics.

Given Data / Assumptions:

• Throttling at roughly atmospheric pressure and room temperature.
• Real-gas behavior; no external heat exchange (adiabatic throttling).
• We compare common gases CO2, H2, O2, N2.

Concept / Approach:

If the operating temperature is below the JT inversion temperature of the gas, throttling causes cooling; if above, it causes warming. Most common gases (CO2, O2, N2) have inversion temperatures well above room temperature, so they cool upon throttling. Hydrogen (and helium) have inversion temperatures below room temperature; thus, at ambient conditions, they warm during throttling instead of cooling.

Step-by-Step Solution:

Verification / Alternative check:

Cryogenic process design uses pre-cooling or expansion engines for hydrogen/helium before JT valves, reflecting their warming behavior near ambient conditions.

Why Other Options Are Wrong:

CO2, O2, N2 commonly cool upon throttling at ambient temperatures.

Common Pitfalls:

Generalizing JT cooling to all gases at all conditions; ignoring the role of inversion temperature.

Final Answer:

H2