Talk:Isenthalpic process

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False statements[edit]

The old version said:

In an isenthalpic process there is no transfer of heat to (or from) the surroundings, and no work done on (or by) the surroundings.

Which is as wrong as it could be. Because if you keep a system insulated at constant volume (which is what you need to do if you wan the sysem not to do work and not absorb heat), then the internal energy will stay constant. The enthalpy, E + P V will, in general, not stay constant. When not? precisely not when the pressure increases. The next sentence said that during such a process the temperature and pressure could increase. That last statement made the paragraph "as wrong as it could be" :( Count Iblis (talk) 15:13, 16 June 2008 (UTC)


Count Iblis, you are saying that if you want the system not to do work you must keep it at constant volume. This is the error in your thinking that is causing you to post erroneous statements in this article. We are talking about the system not doing work on the surroundings (and not allowing the surroundings to do work on the system.) It is only necessary to consider work not crossing the boundary of the system. If one element in the system does work on another element in the system that has not violated the consideration that work must not cross the boundary of the system. As an example, consider the throttling process. Gas at high pressure flows across a restriction to a region of lower pressure. The volume of the escaping gas increases significantly and yet the process is isenthalpic. An important consideration that explains why the process is isenthalpic is that the system has not not performed worked on the surroundings – the escaping gas has not driven a turbine, raised a piston or generated an electric current.
You have written that a process in which the pressure and volume remain constant is an isenthalpic process. If the pressure and volume remain constant with no exchange of heat or work the temperature will also remain constant. If these things all remain constant all the properties of the system also remain constant – enthalpy, entropy, and internal energy. This is a trivial case and not worth mentioning.
You summarised your most recent revert to Isenthalpic process by writing “Surely a version with so many elementary mistakes is unacceptable?” You have made the claim that the article contains many elementary mistakes, but you haven’t given much information about all the alleged elementary mistakes. The only mistakes I am aware of are ones you have introduced. If you only make changes that you can support with a citation pointing to some authoritative source your work on Wikipedia will be much more valuable. Dolphin51 (talk) 03:46, 17 June 2008 (UTC)


Dolphin, you have written this article with some very specific gas dynamics seting in mind, even though to a random reader this is not clear at all. The so-called trivial case of constant volume that is allegedly "not worh mentioning "is not trivial at all, because a system may evolve from one equilibrtium state to another, (e.g. if a chemical reaction happens). This is explained in the main enthalpy article (in which I had to do a lot of rewriting).

If this article is about gas dynamics, then one should demand that it contain a detailed introduction of gas dynamics, Navier Stokes equations and a derivation of the relevant conservation equations involving the enthalpy like 1/2 v^2 + h = const. along flowlines etc. etc.

But if that context is missing and someone with my background reads it, the statements do not make sense.

B.t.w. when discussing technical subjects, demanding sources to back up something (other than terminology) is not a good way to proceed, as I explained here yesterday Count Iblis (talk) 13:33, 17 June 2008 (UTC)

Flawed statement[edit]

"Process is isenthalpic if there is no transfer of heat to (or from) the surroundings and no work done on (or by) the surroundings."

Which is trivially false.

So, I reverted to the previous version. I understand that you can have isenthalpic processes under certain circumstances. But then those circumstances should be mentioned first, and then you can say that in such and such a setting the process is isenthalpic.

Count Iblis (talk) 13:51, 17 June 2008 (UTC)

Count Iblis's additions[edit]

Count Iblis has repeatedly deleted text and replaced it with the following. Let’s have a technical discussion about Count Iblis’s additions:

A process in which a system is kept at constant pressure and thermally insulated so that it does not exchange heat with the environment, will be isenthalpic. There will usually be significant changes in temperature during the process. There are also isenthalpic processes in which the pressure does change.
  • Firstly it talks about a system at constant pressure but then says there will usually be significant changes in temperature. (How does a thermally insulated system at constant pressure change temperature?)
  • Secondly it says the pressure can change!

Count Iblis’s additions make no sense to me. Unfortunately Count Iblis has not cited any material to give confidence that his additions are verifiable. What do others think? Dolphin51 (talk) 23:58, 17 June 2008 (UTC)


Ok, I see that there is a problem here, which I'll correct. However, the original text was:

"Process is isenthalpic if there is no transfer of heat to (or from) the surroundings and no work done on (or by) the surroundings."

Which is wrong as I pointed out above. It may be the case that the sentence is true if you take "process" to mean some very specific class of processes, but then that should be mentioned. I'm sure that the source doesn't claim that the sentence taken in isolation is true. Count Iblis (talk) 00:08, 18 June 2008 (UTC)


Count Iblis, thank you for conceding that you introduced a problem. You have drawn attention to what you call the original text "Process is isenthalpic ..." and made the claim that it was wrong. However, this is not the text you most recently deleted. The text you most recently deleted was supported by a legitimate citation and said:
Significant changes in pressure and temperature can occur in a process and yet the process will be isenthalpic if there is no transfer of heat to (or from) the surroundings and no work done on (or by) the surroundings.

Let's now talk about why you deleted that statement. Dolphin51 (talk) 00:53, 18 June 2008 (UTC)

Ok, my point is that without a proper context this sentence can be interpreted to say something that the source did not intend to say. And it is not unreasonable to assume that someone reading this will indeed interpret it in that unintended way.
The words "surroundings" and "work" have not been defined. So, I as a physicist reading this, will just interpret this as the standard textbook case I teach to my students. No work being done on a system which is kept isolated, well I then imagine a system kept insulated in a closed box with fixed volume. But then enthalpy won't be conserved, internal energy will be conserved.
Now, this is not the intended interpretation. What is meant is the following (correct me if I am wrong):
Work is not work done by the gas, it is "useful work extracted from the system". Surroundings is not meant to be the immediate surroundigs but what is outside some system boundary.
Example: in the throttling process, when the gas moves through the valve we mean by no work done that no useful work is extracted from the gas as it moves through the valve.
To point out that such clarifications are really necessay and it's not just me seeking trouble, read this quote. The first part is about free expansion, the second is about Throttling effect. And then you do see that if a physicst looks at this he'll say that the gas performs work, because a physicsist doesn't have turbines in mind and doesn't define the useful work extracted by such a turbine as useful work that leaves the system boundary. Count Iblis (talk) 01:21, 18 June 2008 (UTC)
  • I see. The terms surroundings and work are not defined in Isenthalpic process so rather than define them or add a Wikilink you deleted the sentence because it was not consistent with what you teach your students. And you replaced it with a sentence of your own that you now concede contained a problem.
Let’s now talk about why you deleted the reference I used to prepare my additions:
G.J. Van Wylen and R.E. Sonntag, (1985), Fundamentals of Classical Thermodynamics, John Wiley & Sons, Inc., New York ISBN 0-471-82933-1 You deleted the only general reference contained in the article Isenthalpic process is again without a general reference. Dolphin51 (talk) 01:41, 18 June 2008 (UTC)
Well, we'll put the ref back as soon as this is sorted out. Anyway, the source had to be removed, otherwise it is seen to back up a sentence that is not manifestly correct. Sure, the sentence is correct in its intended meaning which is not trivial at all. Why didn't I correct the problem by writing down the intended interpretation? Because this is a highly non-trivial exercise.
E.g. If I let a gas undergo free expansion in a vacuum, then the internal energy and not the enthalpy will be conserved. So, the sentence, correctly formulated, must automatically imply that it doesn't apply to free expansion. But I don't see how you can have a generic definition of "environment" and "useful work" so that one can exclude free expansion as a case covered by the sentence. So, I have to add a new condition. Perhaps requiring the process to be a steady state process will work?
Anyway, you have your source, so why don't you to fix the problem, e.g. by writing down all the missing small print that accompanies the statement explicitely? Looks more effective than asking me to guess what it is.

Count Iblis (talk) 02:36, 18 June 2008 (UTC)

Suggestion for first paragraph[edit]

I suggest to write a statement in the first paragraph that is manifestly true. I.e. when I read this article I should not have to assume that internal work within some (undefined) system boundary does not count. So, all the small print should be spelled out exactly and then the statemnt which is then rigorously true should be formulated.

A physics student who reads this article will not, by default, have a typical engineering setting in mind and won't be able to make sense of the statement if we don't do this. Count Iblis (talk) 00:18, 18 June 2008 (UTC)


So let's take a close look at the original paragraph:

Significant changes in pressure and temperature can occur in a process and yet the process will be isenthalpic if there is no transfer of heat to (or from) the surroundings and no work done on (or by) the surroundings.[2]

I don't have the ref [2] handy, but it is clear that this cannot be taken to be valid in general. What is meant, as pointed out by Dolphin above, is a situation like in the throttling case. However, in physics, we would interpret the surroundings by the immediate suroundings of the gas that moves through the valve. The gas does in fact perform work on its "surroundings". See quote from a source here: Talk:Joule–Thomson effect which explicitely says that the gas performs work. So, one really does need to clarify that by surroundings we mean something different than is usually assumed by default by physicists here.

Count Iblis (talk) 00:45, 18 June 2008 (UTC)

  • The current first paragraph is entirely satisfactory. It says An isenthalpic process is one that proceeds without any change in enthalpy, H, or specific enthalpy, h.
Here is my suggestion of a suitable sentence to form the second paragraph of Isenthalpic process.
In a steady-state, steady flow process, significant changes in pressure and temperature can occur to the fluid and yet the process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings, and no change in the kinetic energy of the fluid.

The citation I can quote to show that this statement is verifiable is G.J. Van Wylen and R.E. Sonntag, (1985), Fundamentals of Classical Thermodynamics, Section 5.13 (3rd edition), John Wiley & Sons, Inc., New York ISBN 0-471-82933-1

What do people think of this? Dolphin51 (talk) 13:15, 18 June 2008 (UTC)

Yes, this looks correct to me. We can put this in the article for now. We do need to explain what we mean by "surroundings", "work done by surrounding", to make sure people interpret it correctly. That can be done later. Count Iblis (talk) 14:05, 18 June 2008 (UTC)

I have inserted into Isenthalpic process the text suggested above. I believe the term surroundings can be explained using a link to Control volume. The following wording may be suitable:
Steady-state, steady-flow processes can be analysed using a control volume. Everything outside the control volume is considered to be the surroundings.
What do people think of this? Dolphin51 (talk) 13:06, 19 June 2008 (UTC)
Looks ok. to me. Count Iblis (talk) 18:27, 19 June 2008 (UTC)
  • Isenthalpic process now has some clarifying information, and the example of throttling, about steady-state, steady-flow processes. It would be good to add information about isenthalpic processes that are not steady-state, steady-flow. My knowledge of the subject is limited to steady-state, steady-flow. Is there a volunteer to add information about some other kind of process? Dolphin51 (talk) 03:31, 21 June 2008 (UTC)

Processes that are not isenthalpic[edit]

Are endoenthalpic and exoenthalpic the correct terms to describe situations of decreasing and increasing enthalpy? Jeff Knaggs (talk) 08:28, 23 August 2008 (UTC)