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### Thermodynamics Question

06/16/2012 12:15 PM

can any1 define enthalpy and entropy with real examples......

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#1

### Re: thermodynamics question

06/16/2012 12:31 PM

Enthalpy = ice melting... Entropy = molecules in ice compared to molecules in water....

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#2

### Re: Thermodynamics Question

06/16/2012 11:53 PM

The Second Law, by P.W. Atkins, might be helpful.

Enthalpy, the easier of the two, is simply the total heat in a substance (both sensible and latent) above a chosen base temperature.

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#3

### Re: Thermodynamics Question

06/17/2012 1:28 AM

Enthalpy is a term given to total energy due to both the pressure and temperature of the fluid (steam/water). Take it as at 620 psi & 880F Vs 620 psi & 650F, the 620 psi & 850F will do more work than the 620 psi & 650F.

Entropy is unavailable energy. It measures how much heat is rejected and not converted to work.

Summarization: enthalpy is energy available for useful work and entropy is dissipated in form of waste heat (like ice melting in a room).

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#4

### Re: Thermodynamics Question

06/18/2012 2:03 AM

Great question ujjawal-bajpai !

Enthalpy is the one that is used frequently in practical Process Engineering/ Chemical Engineering - e.g. on the process plant. The other one, Entropy is rarely used in practical applications on process plants (my experience any way).

It is often difficult to understand and to answer e.g. in exams. Sometimes it is given vague definitions and explanations in text-books. I would imagine in practice few process engineers use entropy in their regular work on plants. If someone has examples to the contrary, please do entertain us with examples.

Enthalpy is used frequently in Energy Balances. It may be seen as a bread-and-butter variable in Chem. Eng. Note also the complexities (wrt enthalpy) that come into play when there are chemical reactions. The word Energy is used more commonly in plant practice.

Thanks U-B for reminding us of bread-and-butter stuff. Good idea is to keep the reference books close by on a shelf. In this case I use Smith and van Ness, a standard text, "Thermodynamics for chemical engineers"

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#6

### Re: Thermodynamics Question

06/18/2012 6:35 AM

Hello Ducon

Is it possible form you to put some numbers and values to a particular simplified worked example to demonstrate a 'before and after' situation. You mentioned water and steam, but what if you started with an enclosed cylinder with a piston pushing against a spring in it's neutral position, with the space under the piston filled entirely with ice.

Heat the vessel until the ice melts and turns to steam, then allow the vessel to cool to the starting temperature. The piston will move to do work against the spring that then returns the work as the vessel cools.

What is the change in enthalpy and entropy and what happens to the difference?

It would help understand the problem.

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#5

### Re: Thermodynamics Question

06/18/2012 4:21 AM

You can find definition of these terns in nay good thermodynamics books and few posts have already discussed on this. Practice will make you to realize the terms like inertia, emf entropy when you deal with real time engineering problems.

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#7

### Re: Thermodynamics Question

06/19/2012 12:00 PM

enthaply is total internal energy of system.ie H=U+pV.ENERGY NEEDED TO CREATE a system.whereas entropy is measure of disorder in a system.It mainly depends on temperature.example -minimum entropy when there is an ice cube as it melts to water max. entropy .this shows energy require to do work

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