Talk:Enthalpy

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1 Query
2 Agreed.....
3 Redirect
4 contradiction? - is dQ = dH in isobaric chemical reactions?

[edit] Query

I would really like to know what enthalpy means: help anyone? I myself am just an undergraduate student at the University of Illinois. It appears the word comes from the greek language and means "to warm", or "to heat". A possible meaning of the word could then be expressed as the amount of warmth (which is amount of energy) in a room at a certain temperature and pressure.

For example: In a house each room could be assumed to be at the same pressure (atmospheric pressure). However, the garage would be at a lower temperature than the living room. The amount of enthalpy would be less in the garage (lower temp = less warmth,energy) than in the living room (higher temp = more warmth,energy).

I've believed for many years that enthalpy is an antonym of entropy - to have a tendency towards a more ordered situation (as opposed to entropy), opposite that of moving towards disorder (entropy). Checking with a dictionary, this appears to be a consistent view with the meanings of the words in one particular sense - that of enthalpy being defined as a measure of the ability to perform mechanical work, which will only increase as a function of order. Likewise, the definition of entropy in one context is as a measure denoting the disorder of the system being considered. The mathematical discussion of entropy / enthalpy reflect that in their formulas, but it is left to infer those definitions as a consequence to the understanding of the reader.

Perhaps a short discussion of the meaning
of the words in a more abstract sense would be
beneficial to assist other readers§?

[edit] Agreed.....

I strongly agree. I'm not at all trying to divert this entry from hard science, but I think that if enthalpy is not a tendency towards order, there must be another term for the tendency towards order. I've been wondering about this for quite some time, maybe someone has an answer?

I have this feeling that as hard as entropy is to quantify, so must be enthalpy...... How on earth does a tree grow in the forest when the entropic nature of the universe, as we've been taught to believe, creates an inevitable pull towards a state of unified disorder, the decay and rot of natural matter.

There is a certain duality in forces here, and it just seems to me that enthalpy is the natural opposite..... I just don't know. And I can't seem to find anything defining or discussing enthalpy as a tendency towards order, save for a few sparse references in a few early-mid 20th century physics volumes. And in such cases, there was barely enough to go on.

Perhaps there is a nice Professor of Physic roaming the wikii that might enlighten us.

deolijn: umm just a question. entthalpy is a move towards order but when you mention a tree please understand it is a solid and thus different rules apply

Newbie: I'm no scholar, but i've always used "orthotropy" to be the antonym of entropy. If entropy means "turning in" or "in-turning" (like the way food turns?), then "orthotropy" would mean "right-" or "correct-turning".

You might like to take a look at Second law of thermodynamics for more of a discussion: enthalpy is only one of the factors involved. Physchim62 (talk) 15:09, 9 December 2005 (UTC)

Unregistered Roger: If you're trying to relate entropy to enthalpy then I'm afraid you're going nowhere. Entropy is a statistical measure and indicated what is the most likely state(ie.the one with the highest entropy) and the direction of the change in state. Enthalpy is a measure of energy and is not a fundamental property, but a convenient compound of 'internal energy' and 'pressure x volume'. It turns out that in a system where pressure is constant or where we are looking at the steady flow of a fluid through a system, then the change in enthalpy gives us a measure of the amount of energy that must flow into that system.

Freshman: I'd like to know whose P in the enthalpy equation refer to. The system's or the surrounding's pressure. Some articles listed it as surrounding's. This wikipedia article listed it as the system's. Please clarify the confusion.

[edit] Redirect

Would it be reasonable to redirect heat of reaction here? I don't know enough about the topic, but it's a request article. A redirect here would seem sufficient to me, but can someone more knowledge inform? Would be greatly appreciated! Agentsoo 18:18, 14 August 2005 (UTC)

RE: redirect

redirect from enthalpy to heat of reaction? ... enthalpy has uses and meaning outside chemistry like the thermodynamic one treated here so while a link might be appropriate, it shouldn't redirect automatically

Heat of reaction now redirects to Standard enthalpy chnage of reaction. Physchim62 (talk) 15:09, 9 December 2005 (UTC)

No. This would not be reasonable. The redirect to Standard enthalpy change of reaction is appropriate for now, but ideally there would be a separate Heat of reaction article with the alternate name Enthalpy of reaction to reflect usage under non-standard conditions. I am removing the redirect tag. Flying Jazz 03:08, 13 February 2006 (UTC)

[edit] contradiction? - is dQ = dH in isobaric chemical reactions?

On the one hand it is written here:

It is seen that, not only must the Vdp term be set to zero by requiring the pressures of the initial and final states to be the same, but the

μ i d N i

terms must be zero as well, by requiring that the particle numbers remain unchanged.

But on the other hand, the situation in which the use of dH=dQ is the most important, is in chemical reactions - where the sign of dH decides whether the reaction is exothermic or endothermic. But on chemical reacions $\mu_i dN_i \neq 0$ , and usually (under constant T and P) $\ \sum_i \mu_i dN_i = dG$ , which in spontanous reactions is smaller then 0.

So what do I miss? eman 15:56, 28 June 2006 (UTC)

The last few paragraphs of the "some useful relationships" section may be confusing you. I think the writer has the goal of showing the situations where the first and second law combine to result in the requirement that dH must be less than or equal to zero. Next, the writer includes the posibility of chemical reactions, but only to once again show the situations where the first and second law combine to result in the requirement that dH must be less than or equal to zero. I think that what the writer means by "interpretation of the enthalpy" when writing "any further generalization will add even more terms whose extensive differential term must be set to zero in order for the interpretation of the enthalpy to hold" is an interpretation of enthalpy as a potential whose derivative must be less than or equal to zero.

I think that the writer is trying to say is "The second law means that dH must be less than or equal to zero if entropy, pressure, and the number and types of particles remain unchanged." In other words, you're not missing anything because when there's a chemical reaction, the writer's "interpretation of the enthalpy" no longer applies. By the way, I think you meant "exo-" and "endothermic." I'll try to edit this in a couple days. Flying Jazz 03:29, 29 June 2006 (UTC)

Yes, of course I meant exothermic. I corrected myself above.

My main problem is that I know that chemists use the citeria of the sign of ΔH to determine whether a reaction is exothermic or endothermic, and now I can't see how this can be justified? So could it be that all the chemists, and all the chemistry books are wrong?!

eman 15:21, 29 June 2006 (UTC)

Enthalpy is just heat content, so the chemists are correct by definition. I guess I don't understand why you think they are incorrect. If you take some of the equations you wrote above and combine them with some of the equations in the text, you'll get the equation ΔG = ΔH - TΔS which should be familiar and might explain some of what's going on. Believe the textbooks. The section of the article that you're confused about should be ignored and will be removed soon. Flying Jazz 03:45, 1 July 2006 (UTC)

What bothers me is that for cheical reactions ΔG is usually not zero (for spontanous reactions it should be negative). So $\Delta G \neq 0$ means that $\Delta H - T \Delta S \neq 0$ , thus $\Delta H \neq T \Delta S$ , . eman 10:41, 1 July 2006 (UTC)

Yes. The equations that use the equality only apply for reversible processes (where delta G = 0). This is the case at equilibrium. A spontaneous chemical reaction isn't at equilibrium. After the concentrations change or other reaction conditions change so that equilibrium is achieved, the equalities hold. Flying Jazz 12:25, 1 July 2006 (UTC)

Still it doesn't solve my original problem about the $\ \mu_i dN_i$ in dH. The fact that the Enthalpy is also named "heat content" doesn't help. Nomeclature can be wrong, and has only symbolic importance. eman 11:17, 2 July 2006 (UTC)

OK, I think I understand what you're asking now, and it's similar to the question you asked on Talk:Internal energy with a similar answer, and it's not a simple answer and I might not explain it fully because I don't understand it 100% myself, but I'll try. Just as there are many types of internal energies depending on whether no $\ \mu_i$ , some $\ \mu_i$ or all $\ \mu_i$ are natural variables, there are also many enthalpies for the same reason. For a "black box with some unknown amount of unknown reactions" inside:

$dH = TdS+VdP + \sum_i \mu_i dN_i\,$

and all the natural variables (S, P, and all Ni) are independant of each other. But when you have a "heat of reaction" for a defined chemical reaction, all of the Ni are no longer independant of each other. They are related by stoichiometry, so one of the $\ \mu_i$ must be treated as a natural variable instead of a conjugate variable and subtracted out as discussed here and here. The nomenclature has been ambiguous for a long time, and maybe Alberty's IUPAC paper will change that in the future. Flying Jazz 16:51, 2 July 2006 (UTC)

I want to know the enthalpy

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