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Question

What is the relation between H and U?


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Solution

Internal energy:

  • The heat absorbed at constant volume is equal to the change in the internal energy."
  • It is expressed as:
  • U=qV
  • This equation can also be written as U=qpPV …………….(i) at constant pressure,
  • Where qp is heat absorbed by the system and PV represent expansion work done by the system.

Enthalpy:

  • “The another thermodynamic function, the enthalpy H [Greek word enthalpien, to warm or heat content]”
  • It is expressed as:
  • H=U+PV……………….(ii)

Relation between H and U:

  • From equation (i), it can be written as:
  • U2U1=qpp(V2V1)
  • On rearranging, qp=(U2+PV2)(U1+PV1)
  • Thus, this equation can become, qp=H2H1=H
  • Finally, the change in enthalpy can be written as: H=U+PV (since p is constant)

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