4 replies to this topic

[noookiaaa]

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:

dH=dU+d(pv)

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

dU=Q+W

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.

[Shivshankar]

Noookia,

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

Regards
Shivshankar

[isbarqi]

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

[Shivshankar]

Hello Friends,

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

http://lorien.ncl.ac...s/Main/main.htm

https://ecourses.ou....ok.cgi?topic=th

http://secondlaw.oxy.edu/

Regards
Shivshankar

[kkala]

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.