Jump to content

Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

Internal Energy Vs Enthalpy?

This topic has been archived. This means that you cannot reply to this topic.
4 replies to this topic
Share this topic:
| More

#1 noookiaaa


    Junior Member

  • Members
  • 11 posts

Posted 09 April 2012 - 04:42 AM

Hello everybody

I have tried to understand why the enthalpy property was invented while there is another property (internal energy) that does the the same function. (as I understand)

This is really confusing:


the work term in the above equation is already included in the internal energy term


What is the difference between both of these properties?

So, why should work be taken into account two times in enthalpy? - and what is the physical meaning of enthalpy?

Edited by Art Montemayor, 09 April 2012 - 03:25 PM.

#2 Shivshankar


    Gold Member

  • Members
  • 257 posts

Posted 09 April 2012 - 05:36 AM


In an system molecules or micro state particles have Kinetic Energy, Potential Energy, Electrical energy, Bond energy i.e have all types of energy particles inclusive is called as internal energy.

Internal Energy= Heat added to the system -Work done by the system

dU=Q-W ( Work done by the system)

Internal Energy= Heat added to the system +Work done on the system

dU=Q+W (Work done on the system)

The Physical meaning of Enthalpy (Heat Content) is When system temperature changes there will be change in Enthalpy of the system.

dH=dU+d(PV), Now put dU=Q-W (Work done by the system)

dH=Q-W-d(PV), Now W=PdV put in the equation

dH=Q-PdV+d(PV), Now Remember at constant pressure the term (-PdV+d(PV)) will cancel out

And you will get

dH=Q i.e Enthalpy= Heat Content (Under Constant Pressure)

Go to following websites videos and listen carefully.

http://www.khanacademy.org/#chemistry ( Go First law of thermodynamics/Internal energy More on internal energy Enthalpy and Enthalpy intuition Videos)

I hope this will help you lot. Just start lessons and you will get right


#3 isbarqi


    Junior Member

  • Members
  • 26 posts

Posted 12 April 2012 - 11:13 PM

Thanks to Shivshankar,,,
your uploaded videos has broadened and clarified my vision.....

#4 Shivshankar


    Gold Member

  • Members
  • 257 posts

Posted 13 April 2012 - 12:55 AM

Hello Friends,

More Information on Thermodynamics can be found here. Its very good information on webpages.





#5 kkala


    Gold Member

  • Banned
  • PipPipPipPipPip
  • 1,939 posts

Posted 13 April 2012 - 08:34 AM

1. Suppose an horizontal adiabatic fluid flow (without shaft work). At any point the energy per unit mass of the fluid can be considered as the sum of internal energy (U), kinetic energy, and pressure energy (p*V), that is U + v2/2 + p*V. It is convenient to combine U+p*V into a single quantity H (enthalpy).
Note: "Principles of Unit Operations" by A Foust, L Wenzel, C Clump (Wiley, 1960), chapter 20 - The energy balance and its applications, offer a clear presentation.
2. As Shivshankar has pointed out, heat absorbed or given by a system under constant pressure actually represents its enthalpy change. Constant pressure (isobaric) process is quite common in Chemical Engineering. Hence enthalpy is sometimes called heat content, but this is valid under isobaric process. This is its physical meaning, ΔH=Cp*ΔT (Cp=constant pressure heat capacity), or better ΔH=integral of Cp*dT.
3. Consequently convenience may be the reason of using the enthalpy function H, instead of U+p*V. It is used more frequently than U, entropy, free energy.
4. A good reference for those interested in Thermodynamics "philosophy" is "Heat and Thermodynamics" by M Zemansky, McGraw-Hill, several editions from 1937 on. Practical applications or exercises also improve understanding, since only theory may not be adequate. But I have weak points, not having studied theory systematically.
5. Videos by P McCord enjoyed, intensive and extensive (e.g. volume) properties have been clear now.

Similar Topics