Talk:Gibbs free energy

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[edit] Archives

[edit] Definitions

I fixed the definitions, they were wrong. I kept all the links, I think. PAR 16:41, 20 January 2007 (UTC)

Could you please elaborate on the word "wrong"? Also, what do you think of the use of "main equation" boxes? I saw them on another page somewhere and tried them out here. In your last contrib, the main change you seemed to make was to remove three biological-relevant work terms? I'll add a generic work function, and cleaned your last addition a bit. --Sadi Carnot 10:56, 22 January 2007 (UTC)
Hi Sadi - The definition for a closed system was
G = H-TS \,
That's correct but it kind of begs the question to define G in terms of H since H (which equals U+PV) is just another thermodynamic potential. The internal energy U is the most theoretically fundamental, so I think we should use G=U+PV-TS, showing each conjugate variable (PV and TS). Thats a theoretical viewpoint. I think an experimental chemist will probably want to stress the H. Anyway, using the Euler integration, we can then say G=∑ μi Ni.
The definition for open systems was:
dG = SdT + VdP + fdl + \Psi de - \sum_{i=1}^{k} \mu_{i} dn_{i} + ... \,
  • The sign on SdT is wrong, it should be negative
  • This is not the equation defining G, it defines an infinitesimal change in G
  • The sign on the sum term involving μ and n is wrong. Also, that term should use N (number of particles) not n (particles per unit volume).
  • The added terms, while correct, should be added on an as-needed basis. The -SdT and VdP terms have wide applicability and must be considered in almost every situation. Other terms may be added for the specific application. To add every conceivable term is impossible, but adding e.g. a term for a contractile fiber makes it sound like this term has a wide range of applicability while it does not.
I think the edits you made since improve things, definitely. For familiarity I wouldn't include the bounding box, I'd aim for something that looks like a journal article, but they don't look bad. PAR 21:17, 23 January 2007 (UTC)
Good eye. I think that when I originally typed that in, I was looking at the expression for dU, and trying to do the rest in my head? We should probably add in, or discuss the use of, the mgh term, to explain how changes in height effect the change in dG. As to the boxes, I’m rather ambivalent…we’ll have to see if anyone else has a comment? Talk later: --Sadi Carnot 01:26, 24 January 2007 (UTC)

[edit] IUPAC recommendations, consensus, etc.

Related to the discussion on the IUPAC recommendation to use Gibbs energy rather than Gibbs free energy, it should be noted that for the text "Its symbol is ΔGfO", the actual IUPAC recommended notation would be "Its symbol is ΔfGO". This is easy to understand as it then "reads": the change (delta) for formation (f) in Gibbs energy (G) under standard conditions (O). For further details, see p.51&52 of reference 6e. Keeping the apparent consensus in mind, I will not change this now, but should it be decided that IUPAC recommendations are to be followed in the (hopefully near) future, this would have to be corrected as well. Buurma 18:56, 23 January 2007 (UTC)

I understand that it's important to be uniform throughout the world in terms of names and symbols, but this suggestion is an even worse idea than the last one. Hardly anyone uses that notation. Robert Alberty, e.g. in his 2003 Thermodynamics of Biochemical Reactions, uses it, but he is in the minority, this letter after the delta format is probably used in less than 5% of written material. We have to remember that we are writing an encyclopedia not a chemistry journal. Interesting point though. --Sadi Carnot 01:12, 24 January 2007 (UTC)
I'm thinking that this article should get a name change? Principle Chemistry for the Biosciences (Raymond Chang, 2004)is referring to Gibbs free energy as Gibbs energy already. Major textbook for physical chemistry, guys. —The preceding unsigned comment was added by 154.20.164.19 (talk) 02:38, 2 February 2007 (UTC).
See the archive where this point was discussed at some length. Your example is interesting since it was suggested that while all recent straight Physical Chemistry books (i.e. those for chemistry students) used "Gibbs energy", those for the biosciences might not. The problem remains that the term is not only used by chemists, but by engineers and physicists and they still seem to prefer "Gibbs free energy". --Bduke 03:06, 2 February 2007 (UTC)
While there are some situations where Wikipedia should be prescriptive instead of descriptive (for example, insisting that all pictures of experiments display proper safety equipment), nomenclature is not one of them (does Wikipedia use [[β-(benzoylamino)-α-hydroxy-,6,12b-bis-(acetyloxy)-12-(benzoyloxy)-2a,3,4,4a,5,6,9,10,11,12,12a,12b-dodecahydro-4,11-dihydroxy-4a,8,13,13-tetramethyl-5-oxo-7,11-methano-1H-cyclodeca(3,4)benz(1,2-b)oxet-9-ylester,(2aR-(2a-α,4-β,4a-β,6-β,9-α(α-R*,β-S*),11-α,12-α,12a-α,2b-α))-benzenepropanoic acid]] as the primary article name or paclitaxel)? Whether or not textbooks use the word "free" or not, I'd say the majority of actual researchers still use the word "free" in everyday conversations, presentations, etc, so Wikipedia should respect this. Same goes for that delta nonsense.—The preceding unsigned comment was added by 137.131.130.118 (talk • contribs).
I see that I can reply to a few people in one go here.
To Sadi: I obviously checked encyclopaedia and nowhere does it state that an encyclopaedia is based on consensus rather than on definition ;-) I also take slight offence with the sentence "... this suggestion IS an even worse idea than ...". It only IS in your opinion, but your opinion is not necessarily the truth. In my opinion (...) you should have written something along the lines of "I think this is an even worse idea than ...". In any case, going back to the discussion, I just believe that it is important to clearly state on the relevant pages that Gibbs free energy and ΔGfO are the terms/notations that are currently in general use but that it should be noted that these are not the IUPAC recommended terms. By not stating these things clearly, we are actively withholding information from Wikipedia-users which is an activity that I strongly disagree with!
To the previous anonymous poster: I'm taking offence with the notion of "this delta nonsense" and will therefore reply accordingly. Whereas you are probably very proud of your analogy, unfortunately, it's misplaced here. Unless there is an IUPAC recommendation in favour of systematic nomenclature and against trivial names, these cases are simply not comparable. Also, I would like to point out that the full name of paclitaxel is actually given on the page and not withheld from the reader (also see reply to Sadi above).
To Jheald: Agreed - that's what we need to find out (I actually gave finding out a quick try but unsuccessfully so far ...)!
Buurma 19:26, 16 February 2007 (UTC)


Question: did the IUPAC committee ever say why they came to their recommendation? I can't help but feel it seems completely wrong-headed. There are all sorts of different energies out there, but the idea behind the Gibbs energy and the Helmholtz energy is so distinctive, that to me it seems to make enormous sense to identify them together as variants of the same thermodynamic free energy concept.

Rather than just being told "it's the IUPAC recommendation", I'd be more persuadable if somebody could explain to me what the reasoning was that IUPAC put forward.

Otherwise I can't shake a horrible suspicion that IUPAC decided that if there was no way to stop physicists disagreeing with the chemists as to which was the true free energy, to be awarded the prize of the letter F, then IUPAC would take the ball right out of the arena and say that neither concept was to be called a free energy, so neither should get the letter.

And two fingers to the value lost by losing the word "free".

No doubt that isn't what happened at all, but can anyone point to any good reasons given by IUPAC for their recommendation ? Jheald 14:25, 2 February 2007 (UTC)

I don't know of any "official" explanation. My speculation is that it is due to an ambiguity in the French translation (IUPAC nomenclature has to be easily translated). Energie libre can only refer to the Helmholz variety, while the Gibbs free energy is referred to as enthalpie libre ("free enthalpy"). Physchim62 (talk) 10:36, 8 February 2007 (UTC)
  • Comment: I'm responding to the RFC request. I am much more familiar with the term "Gibbs Free Energy" than "Gibbs Energy". However, my guess is that the IUPAC recommendation is likely to be based on not getting confused with Helmholzt Free Energy. To be honest, I think we should stick with the more common Gibbs Free Energy (unless evidence is shown that it's not more common in which case Gibbs energy would be fine). In general though, it's not that important... -SocratesJedi | Talk 22:16, 2 February 2007 (UTC)
Agreed. Thanks for commenting. We reached the same consensus in the last archive. --Sadi Carnot 17:16, 3 February 2007 (UTC)

[edit] Units

You can't have it both ways. Either the chemical potential has units of joules/particle and N has units of particle, or the chemical potential has units of joules and N is dimensionless. I am very much in favor of the first option, so I have changed it accordingly. If there is disagreement, we should at least agree that the product μN should have units of joules. PAR 22:03, 2 February 2007 (UTC)

What do the standard bodies say? What you are "very much in favor of" is not a reason. A chemist would say that the Gibbs Energy for an arbitary amount of material has units of Joule (extensive), but results would be published as the intensive unit, Joule/mole. --Bduke 22:32, 2 February 2007 (UTC)
Yes, right. If you have the chemical potential in joules/mole, then you multiply by N in moles to get the energy in joules. In exactly the same way, if you have the chemical potential in joules/particle, then you multiply by the number of particles to get the energy in joules. Chemists like the first way, physicists the second. (The molar Gibbs free energy is just a different name for the chemical potential in joules/mole)
When I said I preferred μ in joules/particle and N in particles, I meant that I preferred it over μ in joules and N dimensionless. When it comes to μ in joules/particle or joules/mole, I have no formal objection to either, although being a physicist, I am more comfortable with particles than moles.
My point was that the μ, N, and their product must be dimensionally consistent. You can't have μ in joules/particle, N dimensionless, and μN in joules. PAR 00:09, 3 February 2007 (UTC)
Short on time, but off the top of my head, according to Gibbs (from memory), N is particle number (crossing the system boundary), μ is the differential change in the internal energy U of the system when one particle crosses the boundary. --Sadi Carnot 17:15, 3 February 2007 (UTC)
Right, but to be maniacally rigorous, the chemical potential is the (partial) derivative of internal energy with respect to particle number, which is slightly different. The slight difference is unimportant for everyday size objects since a single particle is such a small fraction of the whole, but for a smaller number of particles, it may be significant. PAR 17:53, 3 February 2007 (UTC)

I think essentially that the definition is “energy change per amount of substance”; however, “amount” can be defined as mass, moles, particles, etc.; moreover, the “energy” can change per application as well, such as chemical potential defined in terms of U, H, F, or G. Per “amount”, however, I checked a few sources and it’s usually either per unit “particle” or per unit “mols”. I added two sources to clarify this. --Sadi Carnot 02:00, 10 February 2007 (UTC)

The SI unit of "amount" is the mole. Amount and mass are not the same. You can talk about the energy change per kilogram, but it is not “energy change per amount of substance”. --Bduke 02:31, 10 February 2007 (UTC)

[edit] Help!!

I know this may not be the correct place but I need some help with calculating the Enthalpy (ΔH) of a reaction given the following data:

  • Mass of salt that dissolved in 5ml water
  • Temperature at which it completely dissolves
  • Mass of Salt in solution of water
  • Mole of Salt in 100 g solution water
  • Density of solution at 20 Degrees celcius
  • Density of solution at Temperature in which it dissolves
  • Volume of 100g of solution
  • Molarity of salt
  • Equilibrium constant

Tourskin 19:51, 23 March 2007 (UTC)

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