• AMA with Certified Student Loan Professional

    Join SDN on December 7th at 6:00 PM Eastern as we host Andrew Paulson of StudentLoanAdvice.com for an AMA webinar. He'll be answering your questions about how to best manage your student loans. Register now!

deltaH and q


Full Member
2+ Year Member
Jul 21, 2015
    Hi everyone,

    I'm having trouble wrapping my head around the deltaH = deltaU + PdeltaV equation
    My main issue is this: doesn't deltaU already include PV work in its equation deltaU = q + w = heat + PV work? So why is it considered twice in the enthalpy equation, and why is deltaH = q when there is no change in volume valid?

    This concept has been driving me crazy. Thanks!


    Full Member
    2+ Year Member
    Jul 27, 2015
    1. Medical Student (Accepted)
      1. The equation for enthalpy you give is already assuming constant pressure. Enthalpy is actually defined as dH=dU+d(pV), the change of pressure and volume, not dH=dU+P*dV which is change only in volume (http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpy)
      This does not include a redundant term since dU=dq+dW and dW=-p*dV so dU=dq-p*dV

      2. dH=q when pressure is constant
      with the 2 definitions above -> dH=dU+d(pV) dU=dq-p*dV
      plug dU into dH -> dH=(dq-p*dV)+d(pV)
      product rule (https://en.wikipedia.org/wiki/Product_rule) -> dH=(dq-p*dV)+(p*dV+V*dp)
      cancel terms -> dH=dq+V*dp
      so when there is no change in pressure, dp=0 so V*dp=0 -> dH=q
      This thread is more than 6 years old.

      Your message may be considered spam for the following reasons:

      1. Your new thread title is very short, and likely is unhelpful.
      2. Your reply is very short and likely does not add anything to the thread.
      3. Your reply is very long and likely does not add anything to the thread.
      4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
      5. Your message is mostly quotes or spoilers.
      6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
      7. This thread is locked.