Talk:Acid strength

Merger discussion leading to creation of this article

(Copied from Talk:Acid#Merger proposal)

I believe Strong acid should be merged into Acid. Not only does this make logical sense, as strong acids are a sub-category of acids, but it matches the format taken by bases and strong bases (strong base is a section of Base). If the consensus is that they should not be merged, I suggest moving strong base to its own article. Neonfuzz (talk) 18:37, 27 September 2011 (UTC)

One problem is that this Acid article is already very long (41K). Instead I would suggest merging the three articles Strong acid (7K), Weak acid (7.5K) and Superacid (5.6K) into one new article on Acid strength, initially with three sections. And yes, for consistency we could also merge the strong base section of Base (13K) with Weak base (7.5K) and Superbase (5K) into one new article on Base strength. Dirac66 (talk) 22:31, 27 September 2011 (UTC)

This is a great idea! I can probably get to work on it within the next few weeks. Neonfuzz (talk) 15:37, 29 September 2011 (UTC)

Agreed! Having separate (much smaller) articles makes less sense than one longer article, as readers will gain more insight into the context rather than as a stand-alone topic.--Gilderien Talk|Contribs 20:08, 17 October 2011 (UTC)

I definitely think this is a good idea, but who is going to do it? I'm not sure how, and I don't want to attampt for fear of doing something wrong. I don't see why anyone hasn't done this, yet. — Preceding unsigned comment added by W192 (talk • contribs) 18:28, 27 December 2011 (UTC)

OK, thanks for the reminder. It is complicated to decide what to do exactly, but I have now started. First I created a new article Acid strength by copy-pasting the former Acid strength section of this Acid article, and deleted its subsections from this new article. Then I copy-pasted the entire content of the former Strong acid article into Acid strength, and replaced Strong acid by a redirect. For these two actions I consider that adequate notice was given, since the Merger proposal was on both the Acid and Strong acid article for several months.

Next proposed step is to copy-paste the contents of Weak acid and Superacid into Acid strength. However since the merger proposal was not previously on these two articles, I will post one now and wait 14 days for comments before proceeding. And after the acid merger is finished, I will think about bases.

And finally I will copy this discussion to Talk:Acid strength which seems the best place for further discussion. Dirac66 (talk) 03:09, 2 January 2012 (UTC)

• comment I definitely do not think superacid should be merged into acid strength. They are quite different topics. The natural focus in an article titled superacid is on the acids (and acid systems) themselves, whereas the natural focus of an article titled acid strength is on the concept of the strength of an acid, how such strength is defined and measured. I don't see any natural fit for combining those two topics. --Trovatore (talk) 04:47, 2 January 2012 (UTC)

Conclusion There has been no objection to the consensus to merge the content of Weak acid into Acid strength, so I will do that today. There has been one objection with some merit to merging Superacid, so I will leave that article alone. The Acid strength article does have a mention of Superacids at the end of the intro, with a link to the separate article where more details are available.Dirac66 (talk) 21:27, 17 January 2012 (UTC)

Corrosivity

There is a claim that the hydrofluoric acid dissolves all metals except iridium. In my experience HF doesn't attack gold and the Handbook of Corrosion Data, Craig & Anderson (1995), p. 439 back this up. (see http://books.google.ca/books?id=KXwgAZJBWb0C&pg=RA1-PT392&lpg=RA1-PT392&dq=gold+hydrofluoric+acid&source=bl&ots=RfISDrEr2f&sig=StzO_JVWvu9OkyCwiYiJcfaWGiY&hl=en&sa=X&ei=iFxiT82CMoiD0QGEloWBCA&ved=0CEEQ6AEwBDgK#v=onepage&q=gold%20hydrofluoric%20acid&f=false). The corrosion claim has been reduced to "most metals" rather than all but iridium

Plain English?

"...one mole of a strong acid HA dissolves in water yielding one mole of H+ and one mole of the conjugate base, A−, and essentially none of the protonated acid HA. In contrast a weak acid only partially dissociates and at equilibrium both the acid and the conjugate base are in solution." As of 7/12/2012 the above is from the third sentence of the article. That does not seem like plain English. I do not know how to re-write it. (Nor do I know what it means.) — Preceding unsigned comment added by 24.205.37.180 (talk) 05:57, 13 July 2012 (UTC)

difference between strong acids and superacids

Neither this article nor superacid explain what the difference is between strong acids and superacids, and this article confusingly puts hydrochloric and other acids in the list of strong acids that are are superacids according to the definition given in both articles. What a mess.

And if they are apples and oranges, that should be explained. In any case, the article on superacids should also mention that hydroiodic, hydrobromic, perchloric, and hydrochloric acids are superacids and not only talk about the most powerful ones. --Espoo (talk) 21:08, 4 December 2014 (UTC)

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Missing links

The experimental determination of equilibrium constants is not mentioned in this article. This is a major omission. Petergans (talk) 10:24, 27 June 2018 (UTC)

It is covered quite well at Acid dissociation constant#Experimental determination. So for this article I think it will be sufficient to mention in the intro where to find the information with a wikilink. Dirac66 (talk) 23:06, 27 June 2018 (UTC)

Thanks. What about the duplication with RICE chart?Petergans (talk) 08:09, 28 June 2018 (UTC)

The calculation of concentrations and pH in this article could also be referred to another article. However I think a link to RICE chart is a poor choice since it is an obscure article with a weird name (I learned ICE!). A better plan would be to first integrate RICE chart as a section of either pH or Chemical equilibrium, and then to link this article (Acid strength) to that new section.

In support of the claim that it is obscure, I will cite the Page view statistics (Daily average for last 20 days) for the 4 articles in question: Acid strength 458, Chemical equilibrium 430, pH 3333, RICE chart 9 (!). I had never noticed the article before your mention, and it seems not many other readers have either. Dirac66 (talk) 19:01, 29 June 2018 (UTC)

My point really concerned the extent to which the two articles say the same thing (this article, section Calculating the pH of a weak acid solution). I suggest we put a redirect on RICE chart and maybe transfer anything significant that would be lost to this article.Petergans (talk) 21:44, 29 June 2018 (UTC)

Yes, I hadn't thought of merging RICE chart to here but it is probably the best way considering that the only example in RICE chart is for a weak acid. So I will say go ahead, perhaps after posting a merge notice and giving other editors a chance to object. Dirac66 (talk) 00:37, 30 June 2018 (UTC)

Duplication

This article is in effect a duplicate of Acid dissociation constant. I suggest that it be deleted as Acid dissociation constant is more rigorous and more comprehensive. Petergans (talk) 10:05, 1 July 2018 (UTC)

Here I will disagree. This article is more a simplified version of Acid dissociation constant, with more qualitative chemistry and less rigor. Wikipedia has many readers who are not all at the same level in any given subject, and it can be useful to have both introductory and advanced versions.

We could however change the title to make it less synonymous to Acid dissociation constant. Perhaps "Strong and weak acids". Dirac66 (talk) 19:10, 1 July 2018 (UTC)

Formula consistency

In the second paragraph, a molecular formula is used for carbonic acid, and a condensed formula for acetic acid. Is there a standard choice for which should be used here? Either way the inconsistency seems unhelpful.--86.169.209.11 (talk) 23:23, 1 July 2018 (UTC)

The usual practice is indeed to use the molecular formula for very simple molecules such as H₂CO₃ (1 carbon), and the condensed structural formula for somewhat more complex molecules such as CH₃COOH (2 carbons). For any readers who don't understand these formulas, the acid names do have wikilinks leading to more detailed explanations. Dirac66 (talk) 13:28, 5 July 2018 (UTC)

Lead

After extensive edits of the text I have left the Lead till last. It needs to be slimmed down to summarise, in 2 or 3 sentences, the main points covered by the article. The other stuff should go into an introductory section. Shall I do this or would user:dirac66 prefer to do it? Petergans (talk) 09:21, 10 July 2018 (UTC)

Yes, the lead is too long. I suggest the following rearrangement of the current 7-paragraph lead, without any actual deletions:

• Paragraphs 1-2 = new shorter lead.
• Paragraphs 3-5 = new introductory section with possible title Measures of acid strength. This could be divided into 2 subsections Acid dissociation constant and Hammett acidity function.
• Paragraph 6 on sulfonic acids: move to end of section Strong acids in water.
• Paragraph 7 on superacids: move to section Acids in nonaqueous solvents. Suggested placement = new third paragraph after glacial acetic acid but beIfore basic solvents such as DMSO. Dirac66 (talk) 18:39, 10 July 2018 (UTC)

Good suggestions; I have followed them through. Petergans (talk) 16:16, 12 July 2018 (UTC)

A definite improvement. I moved the Miessler-Tarr reference to the sentence which is actually found in that reference. Dirac66 (talk) 01:37, 13 July 2018 (UTC)

Final revision: Comment Suggestion

I am concerned about the section Factors determining acid strength

1. The discussion in this section contains no references. The best that I can find is Pauling's The nature of the chemical bond which gives K values for the chloro-acetic acids (1960, 3rd edn. p277).
2. The content lacks specific detail.
3. The discussion is couched entirely in terms of bond strength, which I take to mean enthalpy. Entropy is also important, particularly when comparing ions of different charges and/or sizes.

Should this section be deleted or revised? Petergans (talk) 16:46, 12 July 2018 (UTC)

Perhaps try some inorganic texts, and organic for the butanoic acid derivatives. And I would vote for revision rather than deletion, because the material is on-topic and of interest, and I think it is correct or mostly so, except for the omission of entropy effects. Dirac66 (talk) 01:42, 13 July 2018 (UTC)

I quote from Housecroft and Sharpe, Inorganic chemistry, 1st. edn. p151. In relation to the acidity of hydrogen halides. electronegativity does not (their italics) enter into the discussion... it is all too easy to conclude from electronegativity that HF is expected to be the strongest acid in the series. Table 6.2 (partially, below) shows that there are large differences in the values of both standard enthalpy and standard entropy for the formation of HF and the other hydrogen halides in solution.

	HF	HCl	HBr	HI
ΔH0 /kJmol-1	-22	-63	-71	-68
TΔS0 /kJmol-1	-30	-10	-4	+3
pKa	1.4	-9.3*	-11.7*	-12.4*

starred values are calculated from theory. Therefore the first paragraph and table of the section Polarity and the inductive effect will be removed. Petergans (talk) 09:36, 13 July 2018 (UTC)

Actually anhydrous or concentrated aqueous HF is a strong acid even though dilute aqueous HF is weak. See the detailed discussion with references at Hydrogen fluoride#Acidity. Dirac66 (talk) 10:06, 13 July 2018 (UTC)

Also I have now checked the table in Housecroft + Sharpe in my 2nd edn. p.170. Same values but the header says they are for dissociation (not formation) of the hydrogen halides in aqueous solution. The diagram on the previous page defines this as HX(aq) → H⁺(aq) + X^-(aq).

And I will agree that the paragraph on polarity you have deleted was too simplistic. Perhaps we could eventually add an improved version as per Housecroft + Sharpe. Dirac66 (talk) 12:59, 13 July 2018 (UTC)

The term "strong acid" can be used in different ways. In this article I assume it is used for acids that are completely dissociated. On the other hand, a solution of HF can be said to be strongly acidic because it attacks glass, by protonating the silicates of which glass is made. The situation with HF is complicated by the formation of oligomers, so it is not discussed in this article which is supposed to be introductory. Perhaps it can go into acid dissociation constant along with a discussion of the various species present in concentrated sulphuric acid and oleum? Petergans (talk) 17:18, 13 July 2018 (UTC)

For HF the important oligomer would be HF₂^-. A future revised section (of one or the other article) on Factors determining acid strength could perhaps include a section on the effect of atypical species (i.e. other than HA, A^- and H₃O⁺). For now there is some information in the separate articles on HF and H₂SO₄. Do you know any other acids where such effects are important? Dirac66 (talk) 19:40, 13 July 2018 (UTC)

There is an extensive range of polyphosphates and silicates. See also oxyanion.Petergans (talk) 06:03, 14 July 2018 (UTC)

Housecroft page numbers

The consolidation of references earlier today is defective because the page numbers in Housecroft and Sharpe have been suppressed, so that the reference is less useful to readers. I have not succeeded in including the page numbers (171, 217 and 171 again) in a single reference using Template:Housecroft2nd. If this is impossible, then I think it would be best to restore the two separate references including the page numbers so that readers can more easily find the pages in question. A little repeated information is less frustrating than missing information. Dirac66 (talk) 12:55, 15 July 2018 (UTC)

OK, problem solved. I have now figured out the way to put page numbers after the numbers referring to the footnotes. Dirac66 (talk) 13:51, 15 July 2018 (UTC)

The pKa requirement to be a strong acid and the reason why you do not see a solvent in the law of mass action

A strong acid is an acid that is better at donating an H+ ion than the hydronium ion is. (I know that there is no such thing as a hydronium ion, but it is a useful construct when discussing Bronsted acid/base chemistry). The value of Ka for the reaction of hydronium ion with water is 1 {H2O(l) + H3O+ (aq) <--> H3O+(aq) + H2O(l)}. Thus the pKa for H3O+ is 0. Based on the idea of the leveling effect (that no acid in water can be stronger than H3O+), any acid with a pKa les than 0 will be a strong acid, because that acid can protonate water.

Also, the concentration of the solvent should never appear in discussions about solution equilibrium. If you consider the solutions to be dilute, then the mole fraction of the solvent is essentially unity, and thus the activity of the solvent is essentially unity. Thus when developing the discussion about why you do not see the solvent in the equilibrium constant expression, it is not because the solvent was omitted. Rather, it was because the solvent has an activity of unity (1), and this "1" has been incorporated into the value of the constant. — Preceding unsigned comment added by Tneils1935 (talk • contribs) 14:22, 22 December 2019 (UTC)

Both of the above two propositions are wrong. 1) An acid can have a pK value less than zero. An acid with a pK value less than -2 is deemed to be "strong", because the concentration of undissociated acid, though not zero, is too small to be measurable. 2) When an acid AH dissociates in solution in a solvent S there exists no means of distinguishing the formation of any product of solvent-associate with the generic formula S_nH⁺ n=0,1 2... (in water H₃O⁺, H₉O₄⁺ etc.). Consequently, the solvent is not included in acid dissociation expressions, such as HA ⇌ H⁺ + A^-. Petergans (talk) 21:52, 22 December 2019 (UTC)

Some questions re the previous comment:

1. What exactly does "too small to be measurable" mean? In this age of more and more powerful analytical methods, I would think this represents a constantly moving target. So is it possible that some acid considered strong in the past would now be considered weak because the undissociated acid has now been analyzed?? I find this difficult to believe but why is it wrong?
2. Is there really no spectroscopic method which can distinguish structures as different as H₃O⁺ and H₉O₄⁺? Or no crystallographic method for the same ions in ice? Dirac66 (talk) 03:02, 26 December 2019 (UTC)

The concept of solvent levelling is more than 50 years old. but, of course, the boundary for classification of an acid as "strong" is not precise. It is equivalent to the detection limit for the products of acid dissociation. Incidentally, the value of -1.76 may be found in many sources, particularly relating to organic chemistry, but this is not based on detecability.

The spectra of solutions of acids are typically broad because of the presence of hydrogen bonding. The species, [ H₃O(H₂O)_n]⁺, cannot, at present, be characterized individually (in solution) by any spectroscopic means. Individual compounds of formula [H_2n+1O_n]X have been characterised in the solid state; the isolation of crystals depends on solubility, which is a property of the solid, not the solution. Petergans (talk) 09:43, 26 December 2019 (UTC)

Lead section

Is there some prerequisite that lead needs to be dumbed down, as much as possible? This is like high-school level at most. For starters, there's no such thing as bare proton (which "H+" does suggest) in any solution (unless you irradiate it with a particle beam!). Next, even for weak acids dissociation is near complete if diluted enough. Also oxidation states are largely irrelevant, etc. That leads to proliferation of misconceptions - students think that complete dissociation is needed for being an acid - wrong, pure H2SO4 is barely dissociated and has H0 = -12, stronger are only superacids. Mithoron (talk) 23:07, 18 February 2020 (UTC)

This article is ripe for deletion. Almost all of its content duplicates, at a more elementary level, what is in acid dissociation constant. What should be done? Petergans (talk) 09:05, 19 February 2020 (UTC)

I think it should include also stuff about Hammett acidity function and its comparison with pKa, also gas phase proton affinity. Another thing is needed is strength of Lewis acids and a bit about more exotic like Lux-Flood. Mithoron (talk) 19:17, 20 February 2020 (UTC)

I think there is a place in Wikipedia for articles of different levels on the same or related subjects. Acid dissociation constant is a good article for readers familiar with equilibrium constants, but some readers will prefer a simpler and more qualitative introduction to just understand what strong and weak acids are. The page view statistics for the last 30 days are 917 for Acid dissociation constant and 434 for Acid strength, so there is more interest in the more advanced article but still substantial interest in the elementary article. And more exotic topics like Hammett acidity can go in the advanced article.Dirac66 (talk) 02:08, 23 February 2020 (UTC)