Advanced Topic 251 of 1102 States of Matter & Solutions
Joule-Thomson Effect
Temperature change of a real gas when expanded through a valve at constant enthalpy.
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In-depth explanation of Joule-Thomson Effect
The Joule-Thomson effect is the change in temperature of a real gas when it expands through a throttling valve or porous plug while its enthalpy stays constant. For most gases below their inversion temperature, the gas cools on expansion because work is done to overcome the attractive forces between molecules as they separate. Above the inversion temperature the gas instead warms up. The size of the change is expressed by the Joule-Thomson coefficient, and the effect is an important property of real gases. It is the basis of refrigeration cycles and of the Linde process for liquefying air and other gases.
Going deeper: Joule-Thomson Effect
Key ideas: The coefficient mu_JT = (dT/dP) at constant H is positive for cooling on expansion. The inversion temperature is about 2a/(Rb) for a van der Waals gas. At room temperature, N2 and O2 cool on expansion, with about 0.25 K per atm for N2, while H2 and He warm because their inversion temperatures are far below room temperature.
Common mistakes and connections: A typical error is confusing it with free expansion of an ideal gas, which has no temperature change. Another is thinking that the process is adiabatic reversible; throttling is irreversible and isenthalpic. Hydrogen must be pre-cooled below about 200 K before cooling on expansion. Linked to liquefaction, enthalpy and real gas attractions.
Visualization of Joule-Thomson Effect
When a real gas expands through a throttle, attractions are overcome so most gases cool (Joule-Thomson), unless above the inversion temperature. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Expanding compressed carbon dioxide from a fire extinguisher cools it so much that dry-ice snow forms at the nozzle.
Frequently asked questions about Joule-Thomson Effect
What is Joule-Thomson Effect?
Temperature change of a real gas when expanded through a valve at constant enthalpy.
Is Joule-Thomson Effect a beginner, intermediate or advanced topic?
Joule-Thomson Effect is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Joule-Thomson Effect?
Study these earlier topics in States of Matter & Solutions first: Van der Waals Equation, Compressibility Factor, Critical Temperature.
What comes after Joule-Thomson Effect?
The next topics in States of Matter & Solutions are Liquefaction of Gases and Liquid State.
Can I practise Joule-Thomson Effect with quizzes?
Yes. The interactive lesson includes a self-check question on Joule-Thomson Effect, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Joule-Thomson Effect fits in States of Matter & Solutions
Joule-Thomson Effect is topic 32 of 109 in States of Matter & Solutions.