1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Question regarding change in enthalpy

  1. Jan 18, 2014 #1
    This isn't a formal homework question so much as a conceptual question for my own edification.

    I'm reading my textbook's section on enthalpy and energy, and given the expression:


    It states that, "we can use this expression to represent the change in enthalpy when n moles of an ideal gas are heated, regardless of any conditions on pressure or volume."

    I know that the ideal gas law stats that PV = nRT, and thus T is proportional to PV.

    How can it be, then, that enthalpy change is only affected by temperature change and not affected by changes in pressure and/or volume? :confused:
  2. jcsd
  3. Jan 18, 2014 #2


    User Avatar

    Staff: Mentor

    First, that equation for enthalpy holds for constant pressure. At const. P, the volume will change when T changes of course. But it is taken care of by considering only T because P is fixed.

    What the book mentions are "conditions" on P and V. This means that you do not need to know what P is, or what the initial and final volumes are. So long as you know ΔT, you can calculate ΔH.
  4. Jan 18, 2014 #3
    What they are saying it that the enthalpy of an ideal gas is independent of pressure. If we regard the enthalpy (per unit mass) of a pure substance to be a function of pressure and temperature, the we can write:

    H = H(T,P)

    From this it follows that:

    [tex]dH=\frac{\partial H}{\partial T}dT+\frac{\partial H}{\partial P}dP[/tex]
    But, by definition,
    [tex]C_p=\frac{\partial H}{\partial T}[/tex]
    [tex]dH=C_pdT+\frac{\partial H}{\partial P}dP[/tex]
    For real gases in the limit of low pressures, it has been found experimentally that:
    [tex]\frac{\partial H}{\partial P}→0[/tex]
    But real gases approach ideal gas behavior in the limit of low pressures. So, for ideal gases, the enthalpy is independent of pressure.
    Last edited: Jan 18, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted