Notices
Results 1 to 7 of 7

Thread: internal energy and enthalpy

  1. #1 internal energy and enthalpy 
    Forum Freshman
    Join Date
    May 2009
    Location
    Belgium
    Posts
    11
    Hello,
    could anyone help me understand the difference between internal energy U and enthalpy H.

    enthalpy: ?this is the energy in the form of heat? that a gas (liquid,...) posesses. So we must eliminate the 'work' energy. And so we do H=U+PV

    U: this is the total internal energy of a system.

    Is the underlined text correct?

    Greets


    Reply With Quote  
     

  2.  
     

  3. #2  
    Reptile Dysfunction drowsy turtle's Avatar
    Join Date
    Mar 2009
    Location
    UK
    Posts
    2,255
    Enthalpy is the energy stored inside bonds. An increase in enthalpy means a decrease in temperature, and vice versa.


    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
    Reply With Quote  
     

  4. #3 RE: internal energy and enthalpy 
    Forum Freshman
    Join Date
    May 2009
    Location
    Belgium
    Posts
    11
    thank you, with so little you explain so much, ... hard to understand why they make it so difficult in books etc.
    One question still:
    "An increase in enthalpy means a decrease in temperature, and vice versa"
    Is this applied in an isolated system? And if so, when energy from outside the system comes into the system, and the enthalpy doesn't change, does the temperature rises then?

    Greets
    Reply With Quote  
     

  5. #4  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    Quote Originally Posted by drowsy turtle
    Enthalpy is the energy stored inside bonds. An increase in enthalpy means a decrease in temperature, and vice versa.
    No, this is wrong. Enthalpy includes the energy stored in bonds, but it also includes the kinetic energy of atoms and molecules, and the PV energy. Increasing the temperature usually increases the enthalpy, except in the critical region, as can be seen in this enthalpy temperature diagram for water.

    http://www.tpub.com/content/doe/h101...h1012v1_71.htm
    Reply With Quote  
     

  6. #5 RE: internal energy and enthalpy 
    Forum Freshman
    Join Date
    May 2009
    Location
    Belgium
    Posts
    11
    So enthalpy isn't just the energy stored in bonds

    What do you mean with 'PV' energy though? Isn't this equal to -w (expansion work). You have dU= w+q, so dH=w+q+PV=q=heat .
    Greets,
    Reply With Quote  
     

  7. #6  
    Reptile Dysfunction drowsy turtle's Avatar
    Join Date
    Mar 2009
    Location
    UK
    Posts
    2,255
    Quote Originally Posted by Bunbury
    Quote Originally Posted by drowsy turtle
    Enthalpy is the energy stored inside bonds. An increase in enthalpy means a decrease in temperature, and vice versa.
    No, this is wrong. Enthalpy includes the energy stored in bonds, but it also includes the kinetic energy of atoms and molecules, and the PV energy. Increasing the temperature usually increases the enthalpy, except in the critical region, as can be seen in this enthalpy temperature diagram for water.

    http://www.tpub.com/content/doe/h101...h1012v1_71.htm
    OK, my apologies.
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
    Reply With Quote  
     

  8. #7 RE: internal energy and enthalpy 
    Forum Freshman
    Join Date
    May 2009
    Location
    Belgium
    Posts
    11
    I forgive you :wink:

    Greetz
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •