What are the pressure and volume components of enthalpy?

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SUMMARY

Enthalpy (H) is defined as the sum of internal energy (U) and the pressure-volume product (pV), expressed as H = U + pV. The pressure and volume components of enthalpy are particularly relevant in the context of isobaric processes, such as the expansion of gases in a cylinder. While the pV term is crucial for gases, its relevance diminishes in solid systems, where pressure may be negligible. The introduction of enthalpy simplifies the analysis of open systems, especially when fluids enter or exit these systems.

PREREQUISITES
  • Understanding of thermodynamic concepts, specifically enthalpy and internal energy.
  • Familiarity with the ideal gas law and isobaric processes.
  • Basic knowledge of open and closed thermodynamic systems.
  • Awareness of the significance of pressure-volume work in thermodynamics.
NEXT STEPS
  • Study the derivation and applications of the first law of thermodynamics.
  • Explore the concept of isothermal and isobaric processes in detail.
  • Learn about the implications of enthalpy in chemical reactions and phase changes.
  • Investigate the role of enthalpy in real-world applications, such as refrigeration cycles.
USEFUL FOR

This discussion is beneficial for students and professionals in thermodynamics, chemical engineering, and mechanical engineering, particularly those focusing on energy systems and fluid dynamics.

syang9
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okay, i get that enthaply is a measure of the thermodynamic potential of a system. but what exactly are the 'pressure' and 'volume' components of this? (I'm referring to the definition H = U + pV). I've read that it's "motivated" by the isobaric expansion of a gas inside a cylinder, but how does that describe the thermodynamic potential of any arbitrary system (let's say for example, a slab of metal that has been heated to some temperature)-- is the 'pressure' simply zero? is the pV term only there to describe gases?
 
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I will summarize some comments that I found in an old Thermodynamics book I had laying around: "Enthalpy is always defined as a property equal to the sum of internal energy and the pressure-volume product (H = U +PV). Only in the case of a fluid entering or leaving an open system, however, does the PV term represent energy..." According to the author, the main reason for introducing the property enthalpy is to effect simplifications that might arise in problems involving open systems.

This doesn't directly answer your question, but maybe it will help.
 

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