Talk:Hydrogen fluoride

Erroneous Picture

The bond line diagram of the HF molecules associating with one another displays the bond length of HF as 0.95 pm and the intermolecular distance as 1.55 pm. Those should be Angstroms. 143.236.35.204 (talk) 00:36, 14 December 2007 (UTC)

155 pm = 1.55 angstroms, which is why this is valid. 64.180.188.6 (talk) 07:41, 31 October 2008 (UTC)

Furthermore the ångström (symbol "Å") is not an SI unit, whereas the picometre (symbol "pm") is. See WP:UNITS. —DIV (128.250.80.15 (talk) 03:37, 29 January 2009 (UTC))

H2F2

I recall an old inorganic chemistry book that consistently notated hydrogen fluoride as H₂F₂, and similarly for other fluorides (K₂F₂, Na₂F₂). Does anyone know the reason for this choice and why it is apparently no longer used? Or is it still used somewhere? Should it be mentioned? --Trovatore (talk) 19:17, 8 May 2010 (UTC)

It should not be mentioned without a reliable source. All texts that I have ever seen just state the molecular formula as HF without comment, which almost certainly means that all experimental methods agree - molecular weight from vapour density (pV = nRT), solid-state structure by x-ray diffraction, etc, etc. You may be thinking of the fact that concentrated (liquid) hydrofluoric acid ionizes as 2 HF = H⁺ + FHF^-, but this does not mean that neutral H₂F₂ exists. As for KF it is a nonmolecular solid so there is only an empirical formula which is always written with the smallest integers. Similarly NaF. Dirac66 (talk) 20:56, 8 May 2010 (UTC)

As editor Dirac66 says, there is an anion called bifluoride (HF₂^-), the stability of which is often emphasized because you dont see that effect with chloride or bromide, etc. When NaF is produced from by treating aqueous base with HF, the initial product is apparently NaHF₂. HF forms trimers and maybe a dimer in the gas phase upon supersonic expansion: On the Aromaticity of the Planar Hydrogen-Bonded (HF)3 Trimer" Ying-Chan Lin, Dage Sundholm, Jonas Jusélius J. Chem. Theory Comput., 2006, 2 (3), pp 761–764.--Smokefoot (talk) 21:57, 8 May 2010 (UTC)

Acidity

I am confused by the discussion of acidity. What does "HF molecules remain ionized to compensate the loss of F⁻ ions." mean?

Also could someone post a reference for the H_o of HF? It seems paradoxical that HF should be a very strong acid given the greater basicity of fluoride ions. That basicity seems consistent with HF being a weaker acid than the other haloacids, its higher boiling point, and the amount of hydrogen bonding. I do not wish to argue this point until I can determine what was being measured to establish that HF is indeed a very strong acid. —Preceding unsigned comment added by Petedskier (talk • contribs) 21:07, 30 May 2010 (UTC)

First, H_o(100% HF) = -11 is cited by W.L.Jolly, Modern Inorganic Chemistry (McGraw-Hill 1984), p.203. If you want to check the original paper, his column of 11 values is referenced to 4 different articles. [see below]

As I understand it, the paradox is due to the importance of specific solvent effects, so that for this system one cannot really talk about strong/weak acidity/basicity without specifying the solvent. In dilute aqueous solution the most important solvent interaction is F---H---OH₂, which stabilizes HF and shifts the equilibrium HF(aq) + H₂O = F^-(aq) + H₃O⁺(aq) to the left, so HF is a weak acid as we teach in general chemistry.

However in concentrated HF, the solvent is effectively HF and the most important solvent interaction is [F---H---F]^- which stabilizes solute F^-. There is little effect on the activity of HF since it is the solvent. So this interaction shifts the equilibrium HF(l) + H₂O(hf) = F^-(hf) + H₃O⁺(hf) to the right [where "hf" indicates a species dissolved in HF], and so HF is a strong acid.

The sentence with "remain ionized" does need rewriting. I presume that someone meant that the ionization of HF increases due to the interaction [F---H---F]^- in HF as solvent.

These points could be made clearer in the article. However I will wait a few days for comments before proceeding.

Finally, it would probably be most logical to start by considering gas-phase acidity, if someone can find the information. Dirac66 (talk) 02:22, 31 May 2010 (UTC)

Re source of H_o for HF: The value in Jolly's table is from G.A.Olah et al. Science 206, 13 (1979). This is not the original source - Olah et al. just mention on the first page that anhydrous HF has an H_o of -11.0, after having listed 3 previous reviews which might help trace it further. Dirac66 (talk) 21:30, 31 May 2010 (UTC)

H0 experimental paper

On reading the reference by Iftimie et al. (2008), I note that they refer the "weak-to-strong acid behaviour observed in HF aqueous solutions" to their ref.17, which is H.H.Hyman et al., JACS 79, 3668 (1957) "The Hammett acidity function H0 for HF aqueous solutions". This paper by Hyman et al gives experimental details of an infrared study using indicators, from dilute solutions up to 40% HF where H0 = -10.2. It seems to be the original quantitative paper as it cites only qualitative previous observations on the HF system.

Next point is to revise the Acidity section of the article. The large variation of acidity (H0) is the fundamental fact and should be mentioned first, followed by an improved version of the spectroscopic interpretations of Giguere et al and Iftimie et al. Dirac66 (talk) 01:40, 16 June 2010 (UTC)

I have just noticed these aspects presented here and I think that more quantitative details should be added from sources, especially by Paul-Antoine Giguère and Sylvia Turell, JACS 1980.--5.2.200.163 (talk) 16:24, 10 September 2018 (UTC)

Homoconjugation? Homoassociation?

This morning Smokefoot described my edit as being about "homoconjugation", which surprised me since I thought that word meant something else entirely. I checked this IUPAC Gold Book site and found the two meanings:

(1)Association between a base and its conjugate acid through a hydrogen bond (B ... HB+ or AH ... A−). This is presumably what Smokefoot is referring to, but IUPAC describes this use as "obsolete" (I never learned it), and states that "Homoassociation is a more appropriate term for this phenomenon."

(2)"The orbital overlap of two π-systems separated by a non-conjugating group, such as CH2" (as in CH2=CH-CH2-CH=CH2), which is clearly not relevant here. This is the meaning of the word which I learned, and according to IUPAC it is the current (non-obsolete) meaning.

So we could follow IUPAC and write "homoassociation". However there are two interactions involved: H3O+.F- in dilute solution and F-.HF in concentrated solution. Only the second obeys the definition above. So I think we should just say "owing to the formation of hydrogen bonds", which includes both interactions. Dirac66 (talk) 13:42, 4 July 2010 (UTC)

Oh, I am okay with any changes that you might choose. The idea that homoconjugation is obsolete term was not known to me. My goal was to connect the article with a searchable term (or jargon). There is a short section on "homoconjugation" in acid dissociation constant, which may need revision. The concept probably deserves a separate article, even with homoassociation/homoconjugation constants. The main thing to be careful about is whether IUPAC's recommended term is actually followed, since many of its recommendations are almost completely ignored. The sources available to me, all use the term homoconjugation. And I have never seen "homoassociation" in journals or books for acids, but I am not a super, super expert. IMHO, the overlapping definitions with electronic structure of organic cmpds is not problematic. --Smokefoot (talk) 14:14, 4 July 2010 (UTC)

It is true that IUPAC is sometimes ignored, and I don't know how extensively "homoconjugation" or "homoassociation" is actually used to mean an A-.HA interaction. However in this case neither describes the H3O+.F- interaction, so I have changed the wording. Thanks for your other edits. Dirac66 (talk) 16:50, 4 July 2010 (UTC)

merge with hydrofluoric acid?

I think lots of chemists use the words interchangeably (although understanding the distinction). Similar situation with HCl. Also, some of the content (e.g. uses of HF) is mixed into the HF acid article.TCO (talk) 18:37, 12 May 2013 (UTC)

See Edgar181's explanation below61.14.175.114 (talk) 08:46, 24 July 2015 (UTC)

acid section

If we are going to insist on having separate articles for the compound and acid (maybe not ideal way of organizing things, would Ullmans do that), maybe we should talk about details of hydrated acids in the other article.TCO (talk) 19:01, 12 May 2013 (UTC)

Well I used to be outspoken about keeping the anhydrous HX and hydrohalic acid articles separate. I am only one vote and anyway I would follow consensus. The issue is broad enough and has been discussed so often that we might want to bring it to the chemicals talk page. So maybe it is time to re-discuss since you have some insights and recommendations. Kinda awkward (actually impossible, it seems) to discuss (aqueous) pK_a of anhydrous anything. Also because of homoconjugation, the following statement might be incorrect "Unlike other hydrohalic acids, such as hydrochloric acid, hydrogen fluoride is only a weak acid in water solution" The pK_a depends on concentration. Most importantly, thank you for your editing. --Smokefoot (talk) 20:31, 12 May 2013 (UTC)

I'm cool with whatever, also.TCO (talk) 22:16, 12 May 2013 (UTC)

I have changed the "might be incorrect" statement from "weak acid in water solution" to "weak acid in aqueous dilute solution". As for one article or two, I don't think it matters very much, but integrating the two means much work so it is easier to leave the status quo in this case. Dirac66 (talk) 02:23, 13 May 2013 (UTC)

Sources for H2F+ ion in anhydrous HF

Today editor 85.211.100.255 wrote the autoionization reaction in anhydrous HF as 3 HF = H₂F⁺ + HF₂^-, which Material Scientist and later Smokefoot were quick to revert to the previous 2 HF = H⁺ + HF₂^-. However Bronsted taught us 90 years ago that H⁺ does not exist in liquids, and no one has given any sources in this debate, so I opened my books and found that:

1. Housecroft and Sharpe, Inorganic Chemistry (2nd ed, p.221) writes this equation with H₂F⁺
2. Cotton and Wilkinson, Advanced Inorganic Chemistry (5th ed, p.111) writes it with H₂F⁺

Yes, some more elementary books write the equation with H⁺, but they are simplifying in the same way that some books still write the autoionization of water with H⁺ instead of H₃O⁺. So I will restore the correct equation with H₂F⁺, plus the references.

Also: the reaction of HF + F- can be called homoassociation or homoconjugation, but the first term is preferred because the second has another meaning in organic chemistry; this is explained at homoassociation. And the reaction of 2 HF is (molecular) autoionization. Dirac66 (talk) 21:00, 27 May 2013 (UTC)

Well I am glad you caught this mistake on my and Materials's parts. I will go back and revise this thing (or you can) in a few hours. Thank you, I also dont like naked protons (but my response was overruled by my not liking to protonate fluorides). --Smokefoot (talk) 21:47, 27 May 2013 (UTC)

OK, I can wait a few hours for your revision. We might also make the reaction 3 HF → clearer by writing it in two steps plus the overall reaction, as Plasmic Physics has just done at Homoassociation. Dirac66 (talk) 23:48, 27 May 2013 (UTC)

Show them as two equations for pedagogical reasons or because the process occurs in separate steps? Plasmic did it the former way in homoconjugation article and I did it the second way in this one. No big deal to me. Again, thanks for paying attention.--Smokefoot (talk) 00:20, 28 May 2013 (UTC)

I meant for pedagogical reasons since I don't really know that the steps are separate. In any case my two sources just write it as one equation which they both call self-ionization, so I think I will just leave it that way. Dirac66 (talk) 15:18, 28 May 2013 (UTC)

Etching silicon wafers

To Gareth Griffith-Jones: could you please explain why you reverted today's edit by 24.73.247.6 stating that HF is used "in the electronics industry to etch silicon wafers". This agrees with the article Etching (microfabrication), which says that "buffered hydrofluoric acid (BHF) is used commonly to etch silicon dioxide over a silicon substrate". Is that article wrong too? Please explain. Dirac66 (talk) 01:12, 16 October 2013 (UTC)

Separate page for Hydrogen fluoride

There really should be separate pages for hydrofluoric acid and hydrogen fluoride, just as there are for other hydrogen halides. Until there is, I'm going to redirect to hydrofluoric acid. There are dozens of articles pointing here and they're better of going to hydrofluoric acid than to nowhere. Edgar181 22:47, 7 December 2005 (UTC)

Resolved 61.14.175.114 (talk) 08:46, 24 July 2015 (UTC)

Density of Hydrogen Fluoride

Density was listed as .922 kg/m^3, pretty sure it is 922.0 kg/m^3 http://pubs.acs.org/cgi-bin/article.cgi/jceaax/1998/43/i06/html/je980122v.html

I can't read the article you posted because it is behind a pay wall. However, the article is titled "Density of Liquid Hydrogen Fluoride between 258 K and 373 K at Pressures up to 200 MPa". Normal room temperature is 298 K and sea level air pressure is about 0.101 MPa, thus the conditions in the article is far from normal. For reference purposes, the density of water at room temperature is about 1000 kg/m^3. 61.14.175.114 (talk) 08:46, 24 July 2015 (UTC)

Is HF an electrolyte

Is HF an electrolyte? —The preceding unsigned comment was added by Yugor (talk • contribs).

An electrolyte is a substance that ionizes when dissolved in suitable ionizing solvents such as water. Since HF dissociates in water, it is an electrolyte. 61.14.175.114 (talk) 08:49, 24 July 2015 (UTC)

Ionic or covalent

This article describes calcium fluoride as covalent, while the main Calcium Fluoride article describes it as ionic! Which is closer to the truth? Dajwilkinson 23:54, 9 May 2007 (UTC)

Both are true. Simply put, there is no such thing as a truly covalent bond or a truly ionic bond. They are just convenient descriptions for the distribution of electronics (or electron orbital shape if you have a basic understanding of quantum mechanics) in a bond.61.14.175.114 (talk) 08:49, 24 July 2015 (UTC)

Solution vs mixture?

This statement was disputed "solution of HF and potassium bifluoride". I am unsure what the problem is. Anhyd HF is an excellent solvent. --Smokefoot (talk) 12:05, 10 November 2016 (UTC)

A solution is a homogeneous mixture, so I think the question is whether the HF-KHF₂ system used to produce F₂ is in fact homogeneous. If yes, then I think solution is a better word than mixture because it implies homogeneity. But if the production takes place in a 2-phase (solid + liquid) system, then I would prefer molten mixture as in the recent revision. Does the actual process take place in a 1-phase or 2-phase system? Dirac66 (talk) 21:58, 10 November 2016 (UTC)

Agreed. It just seems simpler to say solution, and I was challenging the change. --Smokefoot (talk) 22:36, 10 November 2016 (UTC)

Smokefoot, you know WAY more about this sort of thing than I do. Reversion is absolutely ok with me. I was assuming that at RT, you're talking about a dry gas and a solid. In order to mix them at all you're going to need to melt the KF2, at which point you begin to form the semi-KH2F3 that actually gets electrolyzed. The bulk of the HF is still HF - it is dissolved and electrolyzed. That is why I swapped the terminology - intuition, not scholarship. If you think it's better this way, I'm all for it.

Riventree (talk) 16:58, 11 November 2016 (UTC)

I dont know much more other than HF is a superb polar solvent and easily handled by experts. Qualitatively, its essentially water-like. It is the solvent I recall for electrofluorination.--Smokefoot (talk) 18:00, 11 November 2016 (UTC)

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