Why we needed to define enthelpy?

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SUMMARY

The introduction of the enthalpy term is essential for thermodynamic calculations, particularly when dealing with processes at constant pressure. Enthalpy (H) is defined as H = U + p·V, where U is internal energy, p is pressure, and V is volume. This formulation simplifies the relationship between heat transfer and work done, allowing for easier calorimetric measurements under constant pressure conditions. The differential form dH = dq at constant pressure directly relates heat absorbed to enthalpy change, eliminating the complexities associated with measuring internal energy and volumetric work.

PREREQUISITES
  • Understanding of thermodynamic principles, specifically the first law of thermodynamics.
  • Familiarity with the concepts of internal energy (U) and pressure (p).
  • Knowledge of calorimetry and its measurement techniques.
  • Basic grasp of differential calculus as it applies to thermodynamic equations.
NEXT STEPS
  • Study the derivation and applications of the first law of thermodynamics.
  • Learn about calorimetric techniques for measuring heat transfer at constant pressure.
  • Explore the implications of enthalpy in chemical reactions and phase changes.
  • Investigate the relationship between enthalpy and Gibbs free energy in thermodynamic processes.
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Students and professionals in thermodynamics, chemical engineering, and physical chemistry who seek to deepen their understanding of enthalpy and its practical applications in heat transfer and energy calculations.

Frigus
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If we know that at constant pressure heat absorbed or gained is path independent then what was the need for introducing this new enthalpy term.
If its answer is that it is not only for a special case in which pressure is constant then how can we even use it because we cannot measure the internal energy.
Thanks
 
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I don't understand your question. Please provide a specific problem (with actual temperatures, pressures, amounts of material, etc) that better illustrates what your issue is.
 
Hemant said:
If we know that at constant pressure heat absorbed or gained is path independent then what was the need for introducing this new enthalpy term.

The change of internal energy is dU = dq + dw. It is easy to eliminate non-volumetric work. But than you still have dU = dq - p·dV. Eliminating the volumetric work would require keeping the volume constant which is difficult in practice. That means that an accurate calorimetric measurement would always need to be accompanied by a measurement of the change in volume.

This problem is solved by the definition of enthalpy H = U + p·V. That results in dH = dU + p·dV + V·dp, without non-volumentric work in dH = dq + V·dp and under constant pressure in dH = dq. If you manage to keep the pressure constant (which is quite easy in practice) the calorimetric measurement directly gives you the change of enthalpy.
 

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