Talk:Enthalpy

From Wikipedia, the free encyclopedia

WikiProject Physics This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
B This article has been rated as B-Class on the assessment scale.
Mid This article is on a subject of Mid importance within physics.

Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.
WikiProject Chemistry
Chemistry Portal
This article is within the scope of WikiProject Chemistry, which collaborates on Chemistry and related subjects on Wikipedia. To participate, help improve this article or visit the project page for details on the project.
B This article has been rated as B-Class on the quality scale.
Top This article has been rated as Top-importance on the importance scale.

Article Grading: The article has been rated for quality and/or importance but has no comments yet. If appropriate, please review the article and then leave comments here to identify the strengths and weaknesses of the article and what work it will need.
To-do list for Enthalpy: edit · history · watch · refresh
  • Definately requires all formulae REFERENCING.
  • Section about enthalpy changes needs cleaning up 88.217.26.61 (talk) 18:53, 28 April 2008 (UTC)verification

** Standard enthalpy table -> Standard enthalpy ? Done88.217.26.61 (talk) 18:53, 28 April 2008 (UTC)

Contents

[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".

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

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 μidNi 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

[edit] Article has become messy

The article needs to be re-invigorated by explaining alongside definitions of thermodynamics. I was actually slowed down by downloading an image, and the article looked horrendous when the MathTeX markup wasn't on the screen. There needs to be some definate walkthroughs of the actual processes, rather than just explaining different aspects of enthalpy. This is the wikipedia, not a textbook! J O R D A N [talk ] 16:33, 20 February 2007 (UTC)

[edit] ΔH

Im not sure if this has been mentioned, but in the intro but it describes enthalpy as either H or ΔH, Δ actually denotes the change in a value, so ΔH is actually the change of enthalpy, not the enthalpy itself.

yeah, so change it. —Preceding unsigned comment added by 86.27.184.240 (talk) 12:57, 15 February 2008 (UTC)

[edit] The product of an exothermic reaction will be warm to the touch.

Is this necessarily true? Can an exothermic reaction not occur at -200 degrees, and be very very cold to the touch?

Ariel Hoffman —Preceding unsigned comment added by 87.69.68.65 (talk) 17:36, 31 December 2007 (UTC)

You are right, this is not always true. The statement about "being warm to the touch" is only meaningful if we assume that the reaction started at room temperature and the reaction is fast enough so that the products increase the temperature noticeably before losing the heat to their environment. Of course, if the reaction starts at a high temperature, it can feel warm to the touch whether the reaction is endothermic, exothermic, or not even happening at all! --Itub (talk) 11:06, 7 January 2008 (UTC)

[edit] Molar enthalpy

The requested natural sciences page included a request for "molar enthalpy." I redirected here, as per "standard enthalpy" - should we add a short definition of molar enthalpy to the "definitions" section? Coanda-1910 (talk) 02:27, 6 April 2008 (UTC)

[edit] Useless for the layman

There's an old quote that goes "you're so high-minded that you're of no earthly good." That's what I'm seeing out of a lot of the scientific/technical articles on Wikipedia, including this one. The articles are written on a level that is not appropriate for the layman to quickly grasp the concept. Instead, they drift into lofty and even high-theoretical issues without ever giving a firm grounding.

In this case, I used to know what enthalpy was. It was a term I used a lot back in my nuclear power days. However, I've been out of that industry for a while, and found my knowledge was getting a bit rusty. I took a look at this article, and now I still don't know what it is. That's pretty bad when someone who knows the subject can't figure it out.

I have no problem with the hard-science approach. But that should be only one part of it. We also need an overview that gives the average reader a good understanding of what it is (the article on internal energy does a decent job at that). Then follow it up with the hard science.

The overview in this one borders on the estoeric and really doesn't help much.

I would be happy to re-write it... but first I have to re-discover just what the hell enthalpy is. Izuko (talk) 12:19, 6 May 2008 (UTC)

I agree that the article is more inscrutable than necessary. At least it should have a more legible lead that, instead of bombarding the reader with jargon such as "thermodynamic potential" from the first sentence, tries to put the better-known and simpler aspects of enthalpy first. For example, the definition given in IUPAC's glossary is more readable: "Internal energy of a system plus the product of pressure and volume. Its change in a system is equal to the heat brought to the system at constant pressure."[1] For many people, "heat at constant pressure" is all they need to know about enthalpy, and yet this is hidden in various places throughout the article but not mentioned in the lead. --Itub (talk) 13:19, 6 May 2008 (UTC)

I have to agree with Itub. It is likely that most readers of this article will be students who have come across the term "enthalpy" in a chemistry class. The article is written by physicists for, it seems, physicists. It is important that the chemistry reader comes across the sentence from IUPAC, "Internal energy of a system plus the product of pressure and volume. Its change in a system is equal to the heat brought to the system at constant pressure.". Can we add that to the lead? --Bduke (talk) 23:31, 20 May 2008 (UTC)

Actually despite the jargon, this article contained a lot of mistakes. I corrected most of them. Removing the jargon would be a good idea. I really doubt whether the article was writen by real experts, given the stupid mistakes it contained. I've just rewritten Helmholtz free energy. That article contained so many false statements, it was really abominable. Of course, the article now looks much more technical, but then there is really no shortcut to explaining the things from fundamental concepts. Count Iblis (talk) 23:59, 20 May 2008 (UTC)
I am aware of the errors you fixed and I have no problem with that. We should not make errors. Your comment "there is really no shortcut to explaining the things from fundamental concepts" seems to me to be a typical physicist response. There is a way. It starts from recognising who our readers are and where they are coming from. As I said, I think a lot of our readers are chemistry students. Some might well be people with even less background than that. Just think where people are going to meet the term "Enthalpy" and come to WP to find out about it. It is more than likely to be something chemical, such as heats of reaction. We should start from a simple lead, even if it is skating close to oversimplification, in order to lead people into the content. Perhaps the article should be reorganised so the chemistry student knows when to stop leaving the rest for those who want the physicists detail and absolute rigor. I have spent a career trying to teach physical chemistry to students who every year seemed to have less background in physics and mathematics than those the year before. I am not going to wade in and edit the article unless there is some kind of consensus. I spent a lot of time on Entropy maybe two years ago. It just kept reverting back to being quite unintelligible to people with a poor background in physics and mathematics. This is a major problem in dozens of articles on topics that chemistry students meet in physical chemistry courses. --Bduke (talk) 01:14, 21 May 2008 (UTC)
I do agree with rewriting this article. Thing is that the mathematical formalism and/or jargon is often just a shortcut to a correct explanation. But that correct explanation must be given. If we look at what I corrected, it was the flawed inequalities. But the reason why the mistake was made points to a fundamental misunderstanding about the basic physics. The physics that is behind this can be explained to lay persons as well using only simple examples. Take e.g. the free expansion of a gas. One can explain that as follows:
There the entropy increased despite the fact that no heat is supplied. But the gas does no work either. So, pressure times volume change (assume moving the piston extremely fast by a small amount and then lock it in place so that the gas crashes into the piston at the new locked position). One can then heuristically reason as follows: The P dv term led to the gas being accelerated, increasing the kinetic energy of the gas as it shoots out toward the piston. The gas could have performed work, but all that energy will go to waste as thermal energy in the gas.
For an ideal gas, then end result is that the temperature does not change. If you imagine measuring the temperature during the free expansion, you would actually at first see that the temperature went down as just like in case of adiabatic expansion. But then all that kinetic energy get's dissipated into heat and then the temperature get's back to where it was. That is the cause of the entropy increase you now get that you wouldn't have gotten had the gas performed work.
One can then go a step further and point out that if you make the process more violent the concept of temperature would break down. One can argue that the velocity distribution is not like that of a gas at any temperature so one needs more variables to describe the macroscopic state of the gas as it expands than just pressure, volume and temperature. But you can apply thermodynamics to the initial and final states when things have settled down. From this perspective the equations of thermodynamics becomes much more interesting because it allows you to compute the outcome of complicated processes.
The problem is that the conventions used in wikipedia is to explain some subject comprehensively in a few pages. Then you cannot spend a page to discuss the free expansion experiment in detail to make some point about thermodynamics. But but perhaps we need to change this and do what is necessary to let people really understand the topic. Count Iblis (talk) 02:24, 21 May 2008 (UTC)

[edit] This article containes terrible mistakes  :(

See the discusion here. You can also check my rewrite of the derivations in Helmholtz free energy and of Fundamental thermodynamic relation. I'll rewrite the relevant derivations later in the week if no one else has the time to correct them. It should be understood that keeping these errors in the page does enormous damage to the reputation of wikipedia. Count Iblis (talk) 02:25, 19 May 2008 (UTC)

I have corrected the mistakes. Count Iblis (talk) 19:53, 20 May 2008 (UTC)