isenthalpic
Definition and meaning of isenthalpic in chemistry.
Isenthalpic describes a thermodynamic process during which the enthalpy of a system remains constant. No net change in enthalpy occurs, even as other properties like pressure, temperature, or volume may change.
In more detail
In an isenthalpic process, the total enthalpy H is conserved (ΔH = 0). This differs from adiabatic processes, where no heat transfers but enthalpy may still change. It also differs from isothermal processes in real, non-ideal fluids: temperature stays constant, but enthalpy can vary if pressure or phase changes occur (for an ideal gas, by contrast, enthalpy depends on temperature alone, so an isothermal process is also isenthalpic). Throttling (expansion of a gas through a valve or restriction) is the most common real-world example of an isenthalpic process. During throttling, no shaft work is done by the system, no heat enters or leaves, and enthalpy is conserved. Isenthalpic processes are critical in refrigeration cycles and industrial gas expansion applications.
Key facts
| Constraint | Enthalpy H remains constant (ΔH = 0) |
|---|---|
| Field | Physical Chemistry |
| Key Process | Throttling and valve expansion |
| Related Coefficient | Joule-Thomson coefficient (determines temperature change) |
When a high-pressure gas expands through a valve into a lower-pressure region (Joule-Thomson expansion), the process is isenthalpic. Although pressure and temperature may both change, the total enthalpy remains constant throughout the expansion.
Frequently asked questions
How does an isenthalpic process differ from an adiabatic process?
Adiabatic means no heat transfer (Q = 0), but enthalpy can still change. Isenthalpic means enthalpy is constant (ΔH = 0), but heat may transfer. Throttling happens to be both adiabatic and isenthalpic, which is why the two terms are sometimes conflated.
What happens to temperature during an isenthalpic process?
Temperature may increase or decrease depending on the gas and its Joule-Thomson coefficient. Real gases exhibit temperature changes proportional to pressure drops, with the direction of change (heating or cooling) determined by whether the gas is below or above its Joule-Thomson inversion temperature.