## Weird and Deep question about enthalpy...

## Weird and Deep question about enthalpy...

(OP)

Supposedly the enthalpy would only depend on the temperature and pressure "h (T, P)" or at least this is how commonly enthalpy is expressed in partial derivatives...

So why in an isothermal and isobaric transformation is it different from 0?

*We are taliking of real subtances here, nor ideal gases.

*We know that is non zero!, beacuse we see it all the time on state changes and some chemical reactions.

That equation must be incomplete... or just misleading.

Maybe i'm failing to grasp a mathematical concept that I do not remember.

Please help or guidance.

So why in an isothermal and isobaric transformation is it different from 0?

*We are taliking of real subtances here, nor ideal gases.

*We know that is non zero!, beacuse we see it all the time on state changes and some chemical reactions.

That equation must be incomplete... or just misleading.

Maybe i'm failing to grasp a mathematical concept that I do not remember.

Please help or guidance.

## RE: Weird and Deep question about enthalpy...

David Simpson, PEMuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual.Galileo Galilei, Italian Physicist## RE: Weird and Deep question about enthalpy...

There is the catch...

In many phase changes, and in many Chemical Reactions we consider P and T constant. But energy change is obviously non-zero and you really have some enthalpy difference. Work is done by the volume change of the substance.

So that means the h(P;T) derivative notation does not hold all the information? Because dH=0 if I use that partial derivative equation... but there is work done by the change of volume, that do not appears on that equation.

So... V is a totally independent variable of T and P. For a real substance.

## RE: Weird and Deep question about enthalpy...

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

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## RE: Weird and Deep question about enthalpy...

## RE: Weird and Deep question about enthalpy...

Assume a constant P calorimeter where liquid water is at 100 ºC and is brought to water vapor also to 100ºc with the mixture of a hot fluid. Calculate Q needed. (enthalpy)

*T and P are constant on this process.

*We know also that Hv-Hl=L where "L" is the Latent heat, that indeed is different from 0.

BUT

The problem remains on:

It certainly does not work here, but ... why???. If there is only a cuastiestatic work (Volume expansion) that equation should work.

## RE: Weird and Deep question about enthalpy...

Is your question what is the mechanistic underpinning of a change in enthalpy due to a change of phase? That's really easy to understand actually, if you understand the forces which act between molecules.

The way you've stated your last question puzzles me. If you want to take saturated liquid water to saturated water vapour at constant temperature and pressure, the thing you're adding is energy (i.e. heat), not "mixture with a hot fluid". The difference in enthalpy between the liquid and the vapour state in this case is precisely the heat energy you need to add to make this transition, which we call the enthalpy of evaporation. Where does the heat go? It goes into overcoming the attractions between the molecules in the liquid- in the case of water it's both van der Waals attraction and hydrogen bonding.

## RE: Weird and Deep question about enthalpy...

Are you talking an open or closed system? It's kind of important.

You will have an easier time thinking about this with a more fundamental equation with no assumptions (open or closed system) attached to it, such as:

dH = TdS + VdP

Good luck,

Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

## RE: Weird and Deep question about enthalpy...

The thing comes to dependence between P and T. When there is a phase change or a chemical reaction they become dependent and they do not follow the State Postulate that allows one to write h(T;P).

If I define h(T;v) then I can use that equation on phase changes and chemical reactions without drama.the

## RE: Weird and Deep question about enthalpy...

## RE: Weird and Deep question about enthalpy...

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

## RE: Weird and Deep question about enthalpy...

So that seems to say to me that even if I have a mulphase/multicomponent system enthalpy h of the whole system can always be calculated has h(T,v)? Is that OK?

*I know that h "natural variables" are h(s,P) but it seems that h(T,v) should also work.

**There is no reference to chemical nor phase changes on the book I'm using.

## RE: Weird and Deep question about enthalpy...

No. In a multi-phase system you have another degree of freedom (x, i.e. mol fraction liquid or vapour) and hence you need another state variable. In fact, you will have many more such fractions in multicomponent systems, and each such fraction is another degree of freedom.

## RE: Weird and Deep question about enthalpy...

Isn't that precisely what your citation says?

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

## RE: Weird and Deep question about enthalpy...

If you do not consider adding energy to a closed volume of water to create a phase change in water, you appear to be proposing an energy free boiler. Changing volume against a real opposing force requires work, sort of how your car engine needs gasoline, air and a spark before the piston moves.

To the mechanical engineer, it doesn't matter if the glass is half full or half empty, it only matters if the bottom half is mine.

## RE: Weird and Deep question about enthalpy...