Which statement defines the Joule-Thomson coefficient?

Which statement defines the Joule-Thomson coefficient?

• A. The rate of change of temperature with respect to pressure during throttling at constant enthalpy
• B. The rate of change of volume with pressure at constant temperature
• C. The rate of change of enthalpy with volume at constant pressure
• D. The rate of change of entropy with temperature at constant pressure

Answer: (A)

Joule–Thomson coefficient measures how temperature changes when a real gas is throttled—that is, expanded with essentially no heat transfer and no external work, so enthalpy stays the same. It is defined as μ_JT = (∂T/∂P)_H. This is the rate of change of temperature with pressure during a throttling process at constant enthalpy. For real gases, intermolecular forces cause a temperature change during throttling: if μ_JT is positive, a drop in pressure cools the gas; if negative, it warms. In the ideal gas limit, enthalpy depends only on temperature, so μ_JT = 0 and no temperature change occurs during throttling.

Joule–Thomson coefficient measures how temperature changes when a real gas is throttled—that is, expanded with essentially no heat transfer and no external work, so enthalpy stays the same. It is defined as μ_JT = (∂T/∂P)_H. This is the rate of change of temperature with pressure during a throttling process at constant enthalpy. For real gases, intermolecular forces cause a temperature change during throttling: if μ_JT is positive, a drop in pressure cools the gas; if negative, it warms. In the ideal gas limit, enthalpy depends only on temperature, so μ_JT = 0 and no temperature change occurs during throttling.