Talk:Enthalpy

message: I would really like to know what enthalpy means:  help anyone? 
—Preceding unsigned comment added by 62.252.0.5 (talk) 00:42, January 9, 2005 (UTC)
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).
—Preceding unsigned comment added by 12.203.115.91 (talk) 16:52, January 31, 2005 (UTC)
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?
—Preceding unsigned comment added by 24.255.211.121 (talk) 08:02, July 5, 2005 (UTC)
 The following was moved here from a section called "Agreed....." by Riick (talk) 05:01, 2 August 2010 (UTC)
thread_title: Query

message: 1. )  "Standard enthalpy change of denaturation" should link to a page titled "Standard enthalpy change of denaturation," not, as it now does, to the page on "Denaturation."  This does not define ∆H(denat).  Or, if "denaturation" is the preferred link, only that word should be highlighted.  (12sep09)
—Preceding unsigned comment added by Schmin (talk • contribs) 20:56, September 12, 2009 (UTC)
2.)  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)
replies: text: 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)
thread_title: Redirect

message: 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)
thread_title:  Article has become messy 

message: 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.
—Preceding unsigned comment added by 81.178.106.177 (talk) 22:34, April 22, 2007 (UTC)
yeah, so change it.  —Preceding unsigned comment added by 86.27.184.240 (talk) 12:57, 15 February 2008 (UTC)
thread_title:  ΔH 

message: 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)
replies: text: 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)
thread_title:  The product of an exothermic reaction will be warm to the touch. 

message: 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)
thread_title: Molar enthalpy

message: 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)
replies: text: While I neither bother, nor presume, to check your remediation, please accept this (however belated) verbal and virtual barnstar.--JerzyA (talk) 16:21, 20 June 2019 (UTC)
thread_title:  This article containes terrible mistakes  :( 

message: The "original definition" section has a question "?? Except that the pressure in a combustion cylinder is far from constant! Maybe the example is referring to a steam engine with 100% duty cycle?"
That is true, the pressure in a practical cylinder is far from constant if the internal reaction is a typical explosive combustion.  If one wished to determine the work done by the gas during combustion by considering the pressure on the interior face of the piston, one would certainly have to know the detailed history of pressure vs. distance; a constant pressure could not usually be assumed.  However, it is simpler to observe that a long time after the combustion is complete, when the system has returned to a quiescent state, the work done by the gas on the interior face of the piston must equal the work received by the environment at the exterior face.  If the exterior face is in contact with a very large reservoir, e.g. the Earth's atmosphere, and the combustion isn't too fast, the pressure there is so close to constant that constant may be safely assumed.  
Of course, a practical cylinder has a massy piston, and if the combustion is explosive, the piston may move quickly enough for the gas on its exterior face to be out of equilibrium with its surroundings. There may be sudden or oscillatory motion, producing either sound or shock waves in the environment subsequent to combustion.  This real occurrence is inconsistent with assuming a constant exterior pressure.  Nonetheless, for the purposes of a definition, the custom is to assume an ideal cylinder with a massless piston, and ideal combustion in which heat is produced slowly enough to maintain equilibrium of the exterior gas.  The difference between real and ideal situations must be taken into account for experimental work, but is conventionally neglected for theoretical work.
The difference in sign between work done and work received was neglected in the text.  This resulted from neglecting: a complete definition of work dW = F dot-product dX; whether the force under discussion is that exerted on the piston by the exterior gas or on the exterior gas by the piston; and a definition of the direction of increasing x.  The positive signum shown in the text applies if one chooses these definitions: the force is that exerted by the piston on the exterior gas, and x increases on the exterior side away from the piston.  
128.149.22.239 (talk) 18:55, 8 September 2009 (UTC)
thread_title:  Constant Pressure in the Definition 

message: This entry's basic definition uses the word "Fluid" but i believe enthalpy is not specific to fluids;just a system.  Fluids are simply commonly used when measuring enthalpy.  Could someone weigh in on this? - tnx JScribner (talk) 22:40, 14 September 2009 (UTC)
replies: text: I agree.  Internal energy is defined in terms of a thermodynamic system.  Enthalpy should be defined the same way, and thermodynamic system is more accurate than fluids.  I will make the change. Dolphin51 (talk) 01:45, 15 September 2009 (UTC)
thread_title:  Fluid 

message: I think the link Standard conditions redirects to Standard conditions for temperature and pressure. From what I understand enthalpy is calculated at a constant pressure not pressure and temperature. Shouldn't it redirect to standard state? --kupirijo (talk) 10:36, 25 October 2009 (UTC)
thread_title:  Standard condidtions 

message: There was a sentence that said "the change in enthalpy at constant pressure is equal to the heat added plus the work done". That is false. I changed it to "the change in enthalpy at constant pressure is equal to the heat added".
After all, that is why the concept of enthalpy even exists, it was because for the scientists in previous centuries with bad equipment, it let them run their experiments with pistons at constant pressure. Enthalpy eliminated a variable "work". This made life easier for them, so they created the concept of enthalpy.  —Preceding unsigned comment added by Apc3161 (talk • contribs) 17:10, 5 January 2010 (UTC)
thread_title:  Changed a sentence 

message: in the 'Difference between H and U: the additional term pV' the writer suddenly introduces a variable uppercase P to represent pressure [as far as i can tell]. I'm almost certain that lowercase 'p' is intended, but the switch is confusing. Perhaps someone with a more complete knowledge of the subject would have the confidence to make the appropiate modifications.
Hai2410 (talk)  —Preceding undated comment added 01:00, 24 January 2010 (UTC)
replies: text: It's always been P in my experience. JIMp talk·cont 22:55, 6 June 2011 (UTC)
thread_title:  Sudden change of notation

message: In the beginning of this wiki, the term "non-mechanical" is used 6 times. So, I assume this means heat energy or something kind of like the type of energy when a photon of energy enters an atom system? Does this mean heat is going into the system, or out of? Clarification would be helpful.  —Preceding unsigned comment added by 71.193.147.120 (talk) 20:40, 7 June 2010 (UTC)
replies: text: The statement "ΔH of a system is equal to the sum of non-mechanical work done on it and the heat supplied to it." is a bit baffling. It is not clear what the writer meant by "non-mechanical work".  From the definition of H = U + PV, it follows that dH = dU + PdV + VdP. The heat flow is δQ = dU + δW where δW is the work done BY the system.  For reversible processes, δW = PdV so dH = (dU + PdV) + VdP = δQrev + VdP. This means that ΔH = Qrev + ∫VdP. So, ΔH is equal to the sum of the integral of VdP and the reversible heat flow.  So it seems to me that the statement as it stands is wrong, as well as being unclear. AMSask (talk) 00:50, 4 June 2014 (UTC)
thread_title:  Non-mechanical = heat?? 

message: This version of the article says, "In the absence of an external field, the enthalpy may be defined, as it is generally known, by: H = U + pV".  I have removed the bit about the absence of the external field because it seems the definition should still hold even in the presence of an external field.  Either the external field affects the internal energy, pressure, and/or volume; or it does not.  In either case I cannot envision a situation where the presence of an external field would make this definition incorrect. Anyone else want to weigh in on this?  Riick (talk) 19:34, 16 August 2010 (UTC)
thread_title:  External fields 

message: There's a contradiction here, and I don't have the grounding in this subject to be able to fix it.
    Adding or removing energy through heat is the only way to change the enthalpy, and
    The amount of change in enthalpy is equal to the amount of energy added through heat.
...
However, heat is not the only way to change enthalpy.
Maybe I'm missing something? As I'm re-reading, I'm getting that my interpretation of the section is incorrect, but I can't figure out why. Could use clarification.  — Preceding unsigned comment added by 108.57.35.49 (talk) 15:27, 31 May 2011 (UTC)
replies: text: While I shan't waste time critiquing the pedagogy head-on, failing to link path function, enthalpy (and state function) is at least supercilious, even if they're already linked in the article (tho I also won't further decorate this talk page, beyond the arguably redundant remark that the connecting algebra raises this question: was the deeper purpose of that wording to either mock or preen? ... oh, well: for that matter, "Look at the big vocab on Jerzy".)~~----  — Preceding unsigned comment added by JerzyA (talk • contribs) 15:59, 20 June 2019 (UTC)
thread_title: Enthalpy is Not Heat

message: In the Section “Formal definition” it is stated that p is the pressure at the boundary of the system and its environment. This is wrong/unclear for the following reasons:
#The pressure need not be the same all over the boundary of the system. E.g. in the case of throttling usually an open system is chosen that contains the plug. In this case the pressures at the inlet and outlet differ. Which pressure should be taken in this case?
# This formulation leaves unclear which pressure (and temperature?) must be chosen in calculating U. Is this also the value at the boundary?
# In an inhomogeneous system the value of the enthalpy of the total system depends on the pressure and temperature distribution inside the system and not only on the pressure at the boundary.
The Article refers to Zumdahl, but Zumdahl talks about “the” pressure of the system (suggesting a homogeneous value) and not about the pressure at the boundary of the system and its environment.
The solution is simple: first define the enthalpy for homogeneous systems. This has also the advantage that δQ=TdS, since no irreversible processes can take place in a homogeneous system. Also the pressure is well-defined so δW=pdV. Next use the property that enthalpy is an extensive parameter. So the enthalpy of an inhomogeneous system is the sum of the enthalpies of the subsystems. In case of continuous variation of p and T composition etc. the summation becomes an integration.
The Subsection “Difference between enthalpy and internal energy” is not part of the formal definition and should be moved to a separate Section.
I have modified the text accordingly.
I like to add that there is a lot of overlap between the various Sections that needs to be cleaned up.
(Adwaele (talk) 21:05, 21 July 2012 (UTC)
thread_title: Changes by Adwaele on June 21, 2012

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thread_title:  External links modified (January 2018) 

message: My first glimmer of understanding was when I saw "BTU."  I moved the units to the first paragraph.  
I came away with a still-tenuous grasp of the concept.  My learning mind needs a concrete example to then be able to grasp a generalization  A single concrete example at the beginning could go a long way to giving a basic grasp of the concept to non-technical people who don't already have it.  
After a quick peruse, the below candidate explanatory example is my best guess of what an example intro could be, but I defer to someone who understands the concept better than me to provide a useful example.  
:For example, a canister of compressed butane under pressure at room temperature can be thought of having two kinds of energy.  One is the potential energy from release of the pressure of the gas inside the canister.  This can do work such as propelling gas out of the canister until pressure is equalized with the room.  The second is the latent chemical energy of the butane, which is released when the butane is burned.  A rough measure of the enthalpy of the system can be calculated by measuring the units of energy represented by the pressure and chemical energy in the butane canister and adding them together.  
This example reflects that after a quick read, I wasn't able to exclude chemical energy as part of the measurement of the enthalpy of the system.  If it's not, an example that says enthalpy does not include the chemical energy from burning the butane would be very helpful, since I think of calories and BTUs in terms of how much a given quantity of water can be heated by burning a specific quantity of gas.  
Including a very basic correct(ed) example like a charged butane cannister may seem like stating the obvious, but that is exactly what is needed by readers who come to the article without any clue of what enthalpy is, and absent an example leave in the same state.  Which may be a rather large percentage of the visitor to the page.   
I suggest a concise very concrete example, very early in the article. - Preceding edit added by User talk:Ocdcntx on 18 February 2018.
:Enthalpy is not energy so an explanation based on adding different kinds of energy won’t be the example you are seeking. (It might be helpful to observe that momentum is also not energy, but momentum is a very useful property nonetheless.)
:I can provide a sound explanatory example although I would hesitate before inserting it in Wikipedia because Wikipedia isn’t a text book. If J joules of heat flow into a solid or liquid, the internal energy of the solid or liquid increases by J joules. Conversely, if the internal energy of a solid or liquid increases by J joules we can say with confidence that J joules of heat have flowed into it.
:Similarly, if J joules of heat flow into a body of gas constrained at constant volume, the internal energy will increase by J joules.
:Now imagine that the constraint is removed from the body of gas, allowing it to expand so that its pressure returns to its original value. As it expands it does N newton.metres of positive work on the surroundings. Also its temperature is reduced, but not to its original temperature. In accordance with the first law of thermodynamics the overall change in the internal energy of the gas is J minus N (the heat added, minus the work done.)
:The internal energy, plus an allowance for the work that can be done, is called enthalpy. So if the enthalpy of a body of gas increases by J joules as it is heated at constant pressure, we can say with confidence that J joules of heat flowed into the gas. Conversely, if J joules of heat flow into a gas at constant pressure, the increase in enthalpy will be J joules even though the increase in internal energy will be less than J joules.
:The “allowance for the work that can be done” is quantified by PV, the product of the pressure and the volume of the body of gas. Enthalpy H is equal to internal energy U plus PV:
::H = U + PV
:Dolphin (t) 23:27, 18 February 2018 (UTC)
thread_title:  Suggestion on how to make concept accessible to non-technical readers 

message: I have seen that the article is littered with the nonstandard term matter transfer/flow claimed by some editor (only in an edit description!) to be different than ordinary/usual mass transfer. The claimed difference (if any) is not clearly explained in article if it is claimed to be an important distinction. I have removed some occurrences of the nonstandard term which should not be used in a wikiarticle. I may have missed other cases of use of this nonstandard language. I'll remove them when spotted.--37.251.221.101 (talk) 23:26, 17 August 2019 (UTC)
replies: text: A small survey (alphabetically stopping at 'G': Adkins 3rd edition, Buchdahl, Callen 2nd edition, Denbigh 4th edition, Guggenheim 5th edition; for now I will not burden this page with the extensive quotes to make this a full account) of standard reliable sources on thermodynamics as a branch of physics finds no instances of the term 'mass transfer'. Terms such as 'transfer of matter' and 'transfer of material' are found instead. There are abundant examples of the shift from 'transfer of xxx' to 'xxx transfer', where 'xxx' is used as a noun in the former and as an adjective in the latter. Callen is a reliable source for such questions; he uses the term 'matter transfer' twice. True, the term 'mass transfer' is the title of a Wikipedia article on that topic, but that does not make the term standard in thermodynamics. It is not our task to invent standardization of terms. A Wikipedia article is not a reliable source.Chjoaygame (talk) 19:55, 18 August 2019 (UTC)
replies: text: I have seen the mentioned survey of expressions from sources containing matter transfer/transfer of matter (from an editor sandbox). The mentioned wordings in sources are an example of sloppy use (in ordinary language) of the word matter to mean substance or material. Such sloppy/careless uses of words in ordinary language are not acceptable when scientific terminology is involved, ordinary language is very frequently affected by ambiguities and use of words having the meaning of other words. Scientific language is a rather formal(ized) language or controlled language because the inconsistencies and ambiguities of natural ordinary language need to be purged. Similarly, some editor's fascination (or obsession?) with ordinary language and its weak features is also not acceptable for scientific articles on Wikipedia.--37.251.220.113 (talk) 13:26, 20 August 2019 (UTC)

text: Another aspect to be analyzed involves the claimed lack of standard status of the term mass transfer in thermodynamics, despite its use in transport phenomena. It is claimed that the term mass transfer in the usual sense re transport phenomena is not accepted in thermodynamics as the sloppy (and undefined) expression transfer of matter is considered somehow distinct in sense from mass transfer. I don't understand the supposed distinction in terms between the transport phenomena and thermodynamics, as if the non-equilibrium thermodynamics which includes transport phenomena were non-existent.--37.251.220.113 (talk) 13:58, 20 August 2019 (UTC)
thread_title:  Nonstandard term matter transfer/flow 

message: There are several ways of considering enthalpy.
It is said to be a function of state, but also to be a state variable.
If the word 'function' here is to have its usual mathematical meaning, then enthalpy as a function of state would have arguments, with enthalpy evaluated for a specific set of values of the arguments. This way of talking is not much in evidence in the article.
In more detail, the article writes;
:H = T\,dS + V\,dp + \sum_i \mu_i\,dN_i,\,\,\,\,\,\,\,(1)
This naturally leads to the form of expression of H as a mathematical function
:H(S,p,\{N_i\}),\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(2)
where S,p, and \{{N_i}\} denote the obvious variables of state, called the natural arguments of the enthalpy, the arguments of that function of state.
That form of expression is a Legendre transform of
:U(S,V,\{N_i\}),\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(3)
which exhibits the internal energy as another function of state, with different but related arguments, also state variables.
The two arguments denoted by S in the two formulas (2) and (3) are measured under different conditions, and more properly would have different symbols, but I will pass over that for brevity here.
For reasons that need not be stated here, knowing the form (2), one can also write
:S(H,p,\{N_i\}),\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(4)
which exhibits S as a function of state variables H,p, and \{{N_i}\}.Chjoaygame (talk) 07:42, 5 February 2020 (UTC)
thread_title:  variable of state 

message: I undid this edit. Its edit note said "Deleted the false statement that 'a system enclosed so as to prevent mass transfer' is a requirement for the heat transfer at constant pressure to be equal to the change in enthalpy. A reaction occurring in an open container at atmospheric pressure would also undergo heat transfer equal to the change in enthalpy."
The sentence in the article, that the undone edit note says was false, did not say that a closed system is a requirement. It said that when a system is closed then such and such holds, but did not say that closure is a necessary condition.
For identification of heat transfer in a vessel that has a path that allows matter transfer, the heat transfer needs to be by separate path that does not allow transfer of matter.Chjoaygame (talk) 00:45, 1 May 2020 (UTC)
thread_title:  undone edit 

message: When every introductory chemistry textbook gives the example of deltaH = q at constant pressure as a reaction occurring in an open container, would it not be prudent to state why that it should be enclosed?. Whilst it may be a tacit assumption for those with wider knowledge in the area, its nitpicky and confusing from a pedagogical point of view.   — Preceding unsigned comment added by 2A00:23C6:B106:4600:8906:64C8:7E10:2A16 (talk) 15:51, 4 May 2020 (UTC)
replies: text: I made a change that I hope may be a reasonable compromise, from "enclosed" to 'contained'.Chjoaygame (talk) 16:54, 4 May 2020 (UTC)
thread_title: Clarification on the opening definition

message: I have made some efforts at finding Clausius' mind, but I haven't quite enough to nail it against all dispute. Though Gillispie offers 'transformation' as an English translation of Clausius, I think Liddell & Scott's 'turning' is a better rendering of the Greek word. They don't mention 'transformation' in their medium-length entry for the word. Wikipedia is not obliged to be uniform between articles. With external quoting, there is a danger of Wikipedia generating circular errors, that may be hard to undo. If you concur, I would like to follow Liddell & Scott on this.Chjoaygame (talk) 00:31, 26 November 2021 (UTC)
replies: text: I think that any detailed discussion of the translation of τροπή belongs not here but in the Entropy article, where I notice that the History section (though not the Etymology section) mentions both 'transformation' and 'turning' and cites the appropriate authors. For this article on enthalpy, we could just insert the alternate translation at the end of the sentence. Perhaps we could say Entropy uses the Greek word  (tropē) meaning transformation or turning. The reader who wants to know the authors of both translations can consult the Entropy article. Dirac66 (talk) 18:49, 1 December 2021 (UTC)
replies: text: Good thinking.Chjoaygame (talk) 02:38, 2 December 2021 (UTC)
replies: text: Done. Dirac66 (talk) 00:50, 12 December 2021 (UTC)
thread_title:  etymology