# internal energy and enthalpy

• May 23rd, 2009, 10:46 AM
scienceboy
internal energy and enthalpy
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 :D
• May 23rd, 2009, 12:12 PM
drowsy turtle
Enthalpy is the energy stored inside bonds. An increase in enthalpy means a decrease in temperature, and vice versa.
• May 23rd, 2009, 01:29 PM
scienceboy
RE: internal energy and enthalpy
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 :D
• May 23rd, 2009, 01:47 PM
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
• May 23rd, 2009, 02:29 PM
scienceboy
RE: internal energy and enthalpy
So enthalpy isn't just the energy stored in bonds :roll:

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, :lol:
• May 23rd, 2009, 03:31 PM
drowsy turtle
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.
• May 23rd, 2009, 03:49 PM
scienceboy
RE: internal energy and enthalpy
I forgive you :wink:

Greetz