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Metal ion hydrolysis
I have started a new section for this on my talk page. As I do not follow all articles which include metal ion hydrolysis, may I ask you to post a simple message there, so that I can follow your edits. Petergans (talk) 10:33, 6 January 2017 (UTC)
- I don't see anything on your talk page since my edit there on Dec.18. Are you referring to your recent edits at Talk:Acid dissociation constant?
- In any case my recent relevant edits are at (1) the article Acid dissociation constant, and (2) the infoboxes for LiOH, NaOH, KOH, RbOH, CsOH and Mg(OH)2. To come: Ca(OH)2, Sr(OH)2 and Ba(OH)2. Also note that you can follow my (or any user's) edits by going to my page or my talk page, and clicking on User contributions (in the left-hand column, at least in my setup). Dirac66 (talk) 17:18, 6 January 2017 (UTC)
- Sorry, wrong number - I put it at Talk:Acid_dissociation_constant#Metal_ion_hydrolysis. Petergans I have to say that I am very skeptical about the (unreferenced) values for NaOH and KOH given by Chem Buddy. The value for Li of pKb = -0.36 is perhaps credible, but it needs verification to a primary source. The problem is that a pKa> 14 is out of range for determination by potentiometry. It's just possible that it was determined by nmr; a very concentrated solution of LiOH may, perhaps, show evidence of ion-association in a 7Li nmr signal. I'll see if I can find anything in the literature. (talk) 10:55, 12 January 2017 (UTC)
I have searched the SC-database and found these entries for LiOH (lg K = pKa)
|Method||Medium||Temp.||Ionic str.||Lg K values||Comment||reference|
|nmr||R4N.X||25°C||3.4M||K1=-0.1||method: NMR Li-7||K Popov,L Lajunen,A Popov et al; Inorg.Chem.Comm.,5,223 (2002)|
|EMF||NaClO4||25°C||3.00M||K1=-0.18||At I=0 corr.: K1=-0.1||H Ohtaki; Acta Chem.Scand.,18,521 (1964)|
|EMF||NaClO4||25°C||3.0M||K1=-0.2||*K1=-14.4 Method: H electrode||Personal Communication etc; Chem.Soc.Spec.Publ.,no.17 (1964)|
|EMF||none||25°C||0||K1=0.17||Method: H electrode||F Gimblett,C Monk; Trans.Faraday Society,50,965 (1954)|
Both Popov and Ohtaki are usually reliable. The last entry should also be reliable, but the value appears to have been extrapolated to zero ionic strength.
Inorganic Chemistry Communications, Volume 5, Issue 3, March 2002, Pages 223–225
7Li 23Na 39K 133Cs NMR comparative equilibrium study of alkali metal cation hydroxide complexes in aqueous solutions. First numerical value for CsOH formation
K. Popova, L.H.J. Lajunenb, A. Popovc, H. Rönkkömäkib, M. Hannu-Kuureb, A. Vendiloc
NMR spectrometric comparative study of 0.05 M alkali metal chloride hydrolysis in aqueous solutions
at 25 °C is performed. It is found that chemical shifts of all nuclei involved are sensitive to variations of ionic strength and pH.
Stability constants for Li+, Na+, K+ and Cs+ are found to be −0.04 (0.05), −0.93 (0.04), −0.7 (0.2) and −0.8 (0.1), respectively.
The stability of CsOH is estimated to be the same as that of NaOH and KOH within an experimental error. No evidence of chloro-complexes formation was found.
Metal ion Hydrolysis, continuation
I agree that Chem Buddy is not really a reliable source, as it does not cite or evaluate the primary sources. This is also true of the” Das Periodensystem online” source cited in the infoboxes for hydroxides of Rb, Cs, Mg, Ca, Sr and Ba. I cited Chem Buddy because it was better than no source at all as was the case previously. However (some or all of) the primary sources you have found would be preferable, especially if the articles actually specify the equilibria to which the infobox values refer.
- For LiOH as the first case I have some questions on the values you cite:
- I have searched the SC-database and found these entries for LiOH (lg K = pKa). Since the values are close to 0 and not 14, should this be pKb?
- Column Lg K values: Does K1 here mean Kb? Or pKb since the heading says Lg.
- Why are the stability constants different from Kb? Or pKb?
- Clearly I am still confused on the exact meaning of the constants here. I think it will help readers if you can specify in the text the exact reaction to which the cited value corresponds for the case of LiOH: Li+(aq) + OH-(aq) = LiOH(aq)? LiOH(s)?? LiOH(OH2)5(aq)?
- Also since you have read these sources and understand them better than me, it seems best that you edit the infobox and add explanatory text in the LiOH article. Dirac66 (talk) 01:12, 13 January 2017 (UTC)
- The relation between pKa and pKb is derived at Acid_dissociation_constant#Bases_and_basicity: . The problem with making the conversion is that the value of pKw is ionic strength and temperature dependent. Its value should be taken from the original publication but the value is not in the abstract. Using a value of 14 will introduce an error of unknown magnitude.
- lg stands for log10. pK(x) = -log10(x) = log10(1/x)
- Ideally each value in an infobox should be accompanied by the relevant temperature and ionic strength values. In the case of lithium hydroxide the phrase "(dissociation of OH–)" next to the value is meaningless. The equilibrium expression relating to pKa is
- Li+ + H2O ⇌ LiOH + H+
- which identifies the reaction as an hydrolysis reaction. pKb refers to the association of Li+ and OH-
- Li+ + OH-⇌ LiOH
- and is more clearly related to base strength of LiOH. I don't want to get involved in editing infoboxes...Petergans (talk) 11:27, 13 January 2017 (UTC)
- Thank you for finding primary sources which I agree are better than ChemBuddy. For the infoboxes I think it is simpler to use one value without the experimental uncertainty, so I propose to use the Popov values which are the more recent of the two which you say are usually reliable. I’ll start by inserting a value for LiOH. If you agree with my way of doing it, I will continue to NaOH, KOH and CsOH. And then we can worry about Mg(OH)2 etc.
- Just to be certain we agree on the interpretation: I take lg K = −0.04 as referring to Li+ + OH− = LiOH(aq), since stability constants are usually defined for solution species.
- Above you wrote that pKb refers to the same reaction, but I think that is backward, as Li+ is a Lewis acid and not a base. It should be LiOH ⇌ Li+ + OH−, which leads to KaKb = Kw as it should. So the basicity of LiOH(aq) = Li+ + OH− is lg Kb = +0.04, and the infobox parameter pKb = −0.04. This is what I meant by “dissociation of OH−” but perhaps it would be clearer to just write Li+ + OH−.
- Also I don’t know why the SC database quotes Popov as saying pKb = −0.1. I found the Popov abstract with Google and it definitely says −0.04. Dirac66 (talk) 03:00, 21 January 2017 (UTC)
- There is extensive data on stability constant values for Mg(OH)+ at Donald A. Palmer and David J. Wesolowski, "Potentiometric measurements of the first hydrolysis quotient of magnesium(II) to 250°C and 5 molal ionic strength (NaCl)",Journal of Solution Chemistry, February 1997, Volume 26, Issue 2, pp 217–232. Absolutely no evidence for a second constant due to low solubility of Mg(OH)2. c.f. the infobox for magnesium hydroxide. Petergans (talk) 22:50, 13 January 2017 (UTC)
- If I understand correctly, the term stability constant is used for solution species only. I agree that the paper of Palmer and Wesolowski considers only Mg2+ + H2O = MgOH+ + H+ in solution, while referring to Mg(OH)2 in the solid state only so that its stability constant cannot be obtained from the data in the paper.
- However basicity (as opposed to stability constant) can also be a property of a solid; Mg(OH)2 (s) is usually considered a strong base which dissolves to give OH– ion. Perhaps the best entry for basicity is just Strong base with no numerical value, and citing a general chemistry text. Similarly for Ca, Sr, and Ba hydroxides.
- Actually if Mg(OH)2(aq) is not observed (in this or any other experiment – I have not checked), its stability constant would be (close to) zero and therefore its basicity constant Kb = 1/K(stab) is very large, so that it is a strong base in solution as well. The situation is analogous to that of HCl(aq), which I doubt has actually been observed in aqueous solution, so again its stability constant is (close to) zero and here Ka is very large so we call it a strong acid in both solution and gas. In effect it is too strong an acid to exist in solution.
- The Dasperiodensystem online source does say that its pKb value is for Mg(OH)2, but this compilation may not be reliable. In addition to not citing primary sources, it lists about 700 reactions with pKb values accompanied by a pKa (pKs in German, found by clicking on the molecule name which links to a data page for the element), and in all cases pKb + pKa is 14.00 exactly! As you have said this is not valid, and I presume that in each case the authors just took one value (pKb or pKa) from some primary source and calculated the other assuming pKw = 14.00. Dirac66 (talk) 01:13, 23 January 2017 (UTC)
Bond angle in water
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