Wikipedia:Peer review/Acid dissociation constant/archive1

Acid dissociation constant

Hello again. We're peer-reviewing again, but this time we'd like approval to promote the article to Class A in preparation for going to WP:FAC. The standards for Class A can be found at Wikipedia:Version 1.0 Editorial Team/Assessment along with a detailed rubric. We believe we have satisfactorially met the requirements of the previous peer review (see below) and are pretty sure we're ready. EagleFalconn (talk) 20:37, 2 August 2008 (UTC)

Previous Peer Review Comments

• A script has been used to generate a semi-automated review of the article for issues relating to grammar and house style; it can be found on the automated peer review page for July 2008.

This peer review discussion has been closed.
Howdy,

I'm looking to get this article up to snuff. Its been pretty idle ever since my original edits on it in 2005(!!!). Its currently listed Class-B in Wikiproject:Chemistry (which seems to be pretty idle to me...) so I'm coming here for peer review.

 Not done I can tell you right now that one thing none of you are going to like is the sourcing on the article.

 Not doneWhile the article needs to be sourced more thoroughly, before I do that I feel like the article has organizational issues. I just redid the beginning, and I feel like its decently strong, but the article delves into other issues later in the article and I'm not sure about relevance.

I'd primarily like comments regarding requests for additional material, pedagogical issues, flow of English, material that could be reorganized, moved to another part of the article, moved to another article, or removed.

Thanks, EagleFalconn (talk) 13:22, 16 July 2008 (UTC)

I did a major clean-up of this article in May 2007, which you appear to have overlooked. I am an "expert" in the field of equilibrium chemistry; see my home page. In my opinion the topic is covered reasonably well. Would you like to put specific comments on the article's talk page? Petergans (talk) 13:35, 16 July 2008 (UTC)

I have done so, and you're correct, I did miss your May 2007 edits, which form the core of the article as it currently stands, and I've commented on the talk page to be more specific about my suggestions. EagleFalconn (talk) 14:34, 16 July 2008 (UTC)

Review from Jayron32

• Well, the sourcing doesn't seem all that bad. Scientific articles have a different set of standards for sourcing; anything that wouldn't be found in say any general chemistry textbook should be cited; or any specific numbers or figures or statistics should also be cited; however if its the kind of info you'd find in every high school or college text, then its "common knowledge" and fairly uncontroversial. Somethings you probably SHOULD add inline cites for:
  • The list of pKa values of common acids. There are slightly differing techniques for determining these values, and different texts may differ slightly on which set of data they use; being data it should have a cite for where they came from.  Done
  • Same deal for the data in "Acidity in nonaqueous solutions" Done
• In some places, there seems to be some organization issue, including some places that need more info, some places that go into needless detail, and some overall flow issues.
  • Under definitions, a brief explanation of the equilibrium constant (say a paragraph or so) may be in order. Yes, someone can always click the link to get more info, but for the sake of clarity it isn't that hard to write a short introduction to the concept. Done
  • The organization is a bit confusing. For example, there should probably be some more general introduction to the basic Ka/Kb/Kw relationship at the outset, and on the concept of the equilibrium constant in general and the acid equilbrium constant specifically (see point above this) before getting into details on, say, the peculiarities of Vanadic acid, which is only trivially interesting to chemists, and probably less so to the uninitiated reader. Done
  • Consider that the article would probably read better with increasing details.
  • The lead section should probably contain enough information so an uninitiated reader could get the general concept (this article does this pretty well) Done
  • The next few sections should be written with the sort of detail one might expect in an introductory Chemistry text. Perhaps a good idea would be to dig out some to look at how actual chemistry textbooks handle the concept (Zumdahl or Brown/LeMay/Bursten are both texts I have taught from that are pretty good). This will be the audience for the first part of the article; say the freshman chemistry major. In here application and use of the Ka values is probably more important than, say, the physics behind relative proton affinities and stuff like that.
  • Following that part, then you want to go into more detail. The audience for these next sections should be the chemist himself; its here that the "nitty gritty" can be covered.
• Especially in science article, more diagrams are usually always a good thing.
  • As a specific example, under "Factors that determine the relative strengths of acids", actually having the structural diagrams of fumaric and maleic acids for comparison by the reader would be REALLY helpful. The average reader, even if an organic chemist, doesn't want to dig out a piece of paper to sketch the E and Z isomers themselves to understand what is going on here. Go ahead and put them in, it will greatly aid in readability of this example.  Done
  • It would be really neat to see a graphical representation of proton transfer (say a space-filling model or ball and stick or something like that). If one could be made and uploaded, theres at least a DOZEN articles that could benefit from it. Just an idea. These are copyrighted images, but they give you an idea on the kinds of things that I am talking about: here's one, a text based one, this is the one I like the best. Again, we can't use these, but if someone could create something like this, it would be uber-cool.  Done

I hope that give's you all a start on this; if you need more comments or need any more help, give me a buzz. --Jayron32.talk.contribs 01:47, 17 July 2008 (UTC)

Comments round 2

Got around to looking things over again since I last stopped in (been on vacation). Things are looking much better. The new graphics rock, btw. Love it. Anyhoo, more general comments, in order of how I think of them...

• Use precise language. Even chemistry has its slangy jargon that, while understood among chemists, can confuse the uninitiated. Understanding that in the context of acid-base theory a "proton" is a "H⁺ ion" is requires a bit of a coginitive leap. Chemists throw the term around rather freely, but for someone just learning the information, using the term "hydrogen ion" may be more precise and easier to understand.
• Other things like including a small key in the corner of the picture (so that everyone can undersatand that red=oxygen and white=hydrogen, etc) would greatly improve understanding for the non-chemist. Again, every chemist knows the default color scheme by heart, after many years of working with it, but others may not...
• Its a little more than random convention as to the order of the numerator and denominator. The convention exists for several reasons:
  • because K (equilibrium constant) is a ratio of the forward and reverse k's (rate constants) (K = k_f/k_r). The rate of the forward reaction R_f = k_f *{reactants} ({} = activity). The rate of the reverse reaction is the same, except it depends on the reverse rate constant, k_r and on the activity of the products. At equilibrium, R_f = R_r (by defintion), thus k_f *{reactants} = k_r *{product} and do the algebra to get k_f/k_r = {products}/{reactants}.
  • K is meant to show the extent of the reaction as written. Thus, K=1 means that the reaction goes to 50% completion. K>1 means that the reaction favors the product side (proceeds a LOT), and K<1 means the reaction favors the reactant side (proceeds very little).

You probably don't need to go into this much detail in this article, but to say it is merely "convention" implies that the opposite convention would be just as useful, and that one order was chosen arbitrarily (like with, say, the sign of delta-H or other conventions). In this case, it wasn't. The order of the numerator and the denominator is more useful in this order than the opposite...

• Concentrations of solids, liquids, etc. are left out of equilibrium calculations not merely because they are constant (at equilibrium, the values of ALL concentrations should be constant, thats the definition of equilibrium!) but because they don't effect the rate. Think of it from a mechanical point of view. The number of water molecules changed as a result of conversion to hydronium ions in the acid/base equilibrium does not significantly effect the number or rate or success of collisions; thus it does not effect the rate of the reaction, thus it does not effect the equilibrium. Saying it is because it is of a constant concentration is somewhat misleading (it is because it is of a relatively HUGE concentration compared to the other things in the reaction, but that's why the small changes made to its amount don't effect the rate of reaction). Again, you might not want to get this detailed, but this needs to be reworded to be more accurate.
• The new diagrams for the fumaric/maleic acid example REALLY help a lot. Just wanted to say well done on that...
• An explanation of the derivation of the Gibbs free energy needs explaining, and some tweaking. First I think you have confused reduction potential with Gibbs free energy. The actual equations are :
  • delta G = -RT ln K

AND

• • E⁰ = - (delta G)/nF

You seem to have confused these two equations. (also, ln = 2.303 log₁₀, but that's minor). Secondly on this issue a brief explanation as to why this is both true and important (delta G ALSO relates to extent of completion from a thermodynamic point of view, etc.) may be in order here. Also, its G zero not G theta...

• "How come?". Lets work on that... Try to use encyclopedic, as opposed to conversational language.
• The section on Importance of pKa values needs serious expansion. It sounds almost entirely like the only importance of the acid dissociation constant is in biochemistry; this section needs serious expansion.

Hope this gives you some more stuff to think about. The article is REALLY looking up, and I look forward to seeing this featured some day! Let me know if you need anything else. I'll keep an eye on things... --Jayron32.talk.contribs 04:26, 28 July 2008 (UTC)

K = 1 does not imply 50% reaction progress in general. While it is true for the simple case of A ⇌ B, it is not true for A ⇌ B + C, which is the case for acid dissociation, except under standard conditions (where all the activities = 1). With dilution, the equilibrium shifts to the right, per Ostwald's dilution law. Also, K = k_f/k_r is only valid for elementary reactions (I suppose acid dissociation can be considered an elementary reaction). As to whether the definition of K is a convention, I think it is. While it has an intuitive interpretation in terms of reaction progress, I wonder what would have happened if the opposite convention had prevailed. Perhaps we would be saying that the definition is very intuitive because it is a measure of the "resistance of the reaction to advance". ;-) Using an electrical analogy, we would think of the equilibrium constant as electrical resistance, instead of its reciprocal, electrical conductance. --Itub (talk) 09:17, 30 July 2008 (UTC)

Reply

These comments are generally very helpful, thank you.

The recommended sign for standard in thermodynamics is the plimsoll mark (Atkins, Physical Chemistry). I used $\Delta G^{\ominus }$ in math expressions in chemical equilibrium. In text I used ΔG^O.

Importance of pKa values is the only section that has not yet been revised.Petergans (talk) 08:59, 28 July 2008 (UTC)

${\frac {RT}{nF}}$: It was dementia rather than confusion (with the Nernst equation)! Petergans (talk) 07:44, 29 July 2008 (UTC)

Ruhrfisch comments

As requested, here are my comments on an interesting article. I think that what is here is generally well done, but it needs some improvements to get to FA class (I do not do Chemistry A class reviews, so I am going with FA):

• I agree with most of Jayron32's comments above, but I think this needs many more references. Model articles are often useful for ideas and examples to follow - there are about a dozen Chemistry FAs and Noble gas is one of the most recent Chemistry FAs and might be a good model. Look at how it has 69 references to eight sources and compare that to this article's current 6 refs to 5 sources.
• Per WP:CITE references come AFTER punctuation without a space, and are usually at the end of a sentence or phrase  Done
• My rule of thumb is that every quote, every statistic, every extraordinary claim and every paragraph needs a ref.
• Book references need to present the same information consistently - for example Greenwood and Earnshaw is "authors, (date), title, edition, place, publisher, ISBN, page or chapter" while March is "author, title, edition, publisher, date, place, ISBN and no page or chapter" {{cite book}} and other cite templates may be helpful. See WP:CITE and WP:V
• The lead should be an accessible and inviting overview of the whole article - this seems a bit brief. My rule of thumb is to include every header in the lead in some way - please see WP:LEAD Done
• There should be a free image in the top right corner  Done
• Article needs to be consistent on usages and symbols, for example the article uses HA and A-, but Image:Weak acid speciation.png uses HL and L (no charge). Or HA is explained with no quotes, but the symbol 'B' uses single quotes. When I read this before, I thought there were some places that used just pK and others that used PKa, but I do not see them now - I also see some edits have been made since, so this might have been caught. Done
• Always make sure to explain all symbols before they are used, so the first three equations use HA, H+, A-, B, HB+, and Ka, but only HA and B are explained. Done
• The word "annotated" seems an odd choice in When an acid with a single reactive proton, termed a monoprotic acid and annotated HA, ... elsewhere  Done
• I agree with Jayron32 that thew article shoukld start out general and become more specific and advanced, but I think I would provide context for the reader and start with a brief background section. This could explain the Arrhenius, Bronsted-Lowry (BLT sounds like a sandwich to me) and Lewis theories. I might even include information such as acids taste sour and bases taste bitter. See WP:PCR Done
• Other topics that seem to be missing (even as a brief mention) early on are the ideas of conjugate acids and bases, strong vs. weak acids and bases, and explictly comparing the p in pKa to the p in pH (which more people will be familiar with, and which is calculated the same way -log)
• I would give the equilibrium expression for the first reaction equation as well as the second. Just giving Ka = for the second is confusing.  Done
• I would also explain hopw few molecules are actually dissociated for weak acids - if I recall correctly, acetic acid is less than 5% dissociated.
• Please use my examples as just that - these are not an exhaustive list and if one example is given, please check to make sure there are not other occurrences of the same problem.

Hope this helps. I think this is a good article if you are already familiar with the chemistry, but it needs some work to make it a better article for those who do not know or are just learning. Ruhrfisch _><>°^° 19:24, 17 July 2008 (UTC)

Comments by Itub

This article is very promising and it could well be the next chemistry Featured Article! Nevertheless, I have some comments:

• "only in extreme chemical conditions does it exist for more than a few picoseconds". I suspect that not even in the most extreme "chemical conditions" can a bare proton exist. A bare proton could protonate anything that has electrons--even a helium atom or a hydrogen molecule. The only condition where a bare proton can exist is in a gas/plasma, conditions which I would not call "chemical".

• • To clarify this point, the intention of the article is to state that if an acid were to release a proton, it would only remain unpaired for picoseconds before attaching itself to something, anything, it could find. To last beyond that timeframe as a bare proton is when it would require 'extreme chemical conditions.' Your point is well taken, though. EagleFalconn (talk) 13:12, 31 July 2008 (UTC)

• I think Latex formulae are way overused in this article. Chemical equations, variable names, and simple equations can be written using HTML and Unicode, look more consistent, and don't mess up the line spacing when used inline. If you look at the rest of the chemistry articles in Wikipedia, you'll see that they generally don't use Latex for chemical equations. The style guide doesn't say it explicitly, but all the examples it gives are without Latex. I suggest reserving Latex only for the equations that need divisions, square roots, etc.

• • I didn't even know there was a chemistry style guide. I'll read it and apply it to the article. EagleFalconn (talk) 13:12, 31 July 2008 (UTC)

• With or without Latex, there are some inconsistencies with the typesetting of variables and equations. In particular, note that "operators" such as p (–log) and d (differential) are conventionally set in roman, not in italic. Same goes for chemical element symbols.

• Perhaps a little bit more detail about the speciation in polyprotic acids could be added. I think the full algebraic treatment may be better for another article, but maybe a plot would be nice.

• One factor affecting the pKa's of polyprotic acids that has not been mentioned is entropy due to having multiple deprotonated configurations. A diprotic acid with two equivalent acidic sites, for example, if the interaction between the two sites were negligible, would be more acidic than a similar monoprotic acid by 0.3 pKa units just for statistical reasons (0.3 ~ log 2). Some large dicarboxylic acids start to approach this ideal limit. For example, nonanedioic acid has pKa's of 4.550 and 5.598, whereas nonanoic acid has a pKa of 4.96. That is, nonanedioic acid has one pKa that is 0.41 lower than nonanoic acid, and one that is 0.64 higher. This means that the two acidic sites are not fully independent, but they are getting close.

• • I'm not really familiar with this argument, I don't believe I've ever heard it. I understand its physical basis, but I'm not certain how applicable it really is to the article. Since I've never heard the argument, I'm thinking of it mostly as a fun fact and less of something critical for inclusion here. Is this something commonly talked about in university courses outside the United States (or in the United States for that matter)? EagleFalconn (talk) 13:12, 31 July 2008 (UTC)

It is indeed an entropy effect. The argument is (or was) in standard inorganic texts in relation to the stepwise formation constants for formation of complexes ML_n, n= 1-6: 6 sites for the first ligand, 5 for the second etc. Citable from Beck and Nagypal, p316.Petergans (talk) 18:32, 31 July 2008 (UTC)

• Perhaps some mention of pKa prediction methods could be added. Maybe one of these days I'll start an article on pKa prediction, as I've worked on that in the past. (There is a link to protein pKa calculations, but I'm thinking in more general terms.)

• "The conclusion for this example is, perhaps, surprising: the energy of dissociation is a minor factor in determining the magnitude of the acid dissociation constant." While in the example given it is true that the entropy term is much larger than the enthalpy term, when comparing pKa's of related acids, the entropy term can remain more or less constant while the enthalpy term changes, thus determining the pKa. Also, anyone who has mixed sulfuric acid with water has seen that the enthalpy term can be quite substantial for strong acids! Finally, in some cases, such as ammonia, it is the entropy which is almost negligible. For ammonia, ΔG = 52.771 kJ/mol, with ΔH = 51.95 kJ/mol (therefore ΔS = −2.75 J/(mol K)).

We intend to enlarge the list of known pK values to include ΔH^O values and then this discussion can be extended. Petergans (talk)

• Also in the acetic acid example above, the signs of ΔG and ΔH are the opposite of those in the source. (The ΔG, in particular, has to be positive if it is a weak acid!)

• The para-toluenesulfonic acid example near the end seems a bit too specific.

• "The standard free energy change for the reaction is for the changes in concentrations of the species from the standard states to the concentrations at equilibrium." Are you sure? I thought it was the change from the reactants in their standard states to the products in their standard states.

--Itub (talk) 12:36, 30 July 2008 (UTC)}}