JACS Video Bytes: What Is the Real pKa of Water?
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- čas přidán 11. 01. 2023
- JACS Video Bytes are short video summaries of articles published in the Journal of the American Chemical Society. In this special tutorial video, Editor-in-Chief Professor Erick Carreira speaks with Professors Tom Neils and Todd Silverstein to address persistent misconceptions regarding the pKa of water, which emanated from errors in a JACS communication published in the 1960s. Watch this JACS Video Bytes as they set the record straight, discuss the historical roots of the error, and set out some thermodynamically sound corrections.
Read the related articles in ACS journals:
pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?
Journal of Chemical Education:
bit.ly/pka-of-water
Acid Ionization Constants of Alcohols. II. Acidities of Some Substituted Methanols and Related Compounds 1,2
Journal of the American Chemical Society:
bit.ly/acid-ionization-consta...
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Literally had this discussion during my PhD with my supervisor. Activity goes to 1, the activity estimation to concentration only applies to dilute solutions.
A second note in added proof:
It is common practice to ignore the change in pH caused by the addition of NaCl to aqueous solutions. This practice is valid for Cl- ions (pKb ≈ 20)a because it is a much weaker base than water is. But is this practice valid for the Na+ ion, which has a pKa = 13.9?b
If we were to take the pKa of water to be 15.7, the Na+ ion would be over 60 times as strong an acid as water itself is. If that were the case, a 0.1 M solution of NaCl would have a decrease in pH of about 0.5 pH units below the pH of pure water. Ignoring the change in pH caused by the addition of Na+ ions would not be valid in this scenario.
However, if we take the pKa of water to be 14.0, then the Na+ ion is only 1.3 times as strong an acid as water itself is, and the decrease in pH caused by the added Na+ can be ignored in most situations. This is the common experience of everyone who has measured the pH of a solution of NaCl and the expectation of anyone who has made a solution of NaCl.
a A. Trummal, L. Lipping, I. Kaljurand, I. A. Koppel, and I. Leito, J. Phys. Chem. A 2016, 120, 20, 3663-3669
b R.M. Smith, A.E. Martell, and R.J. Motekaitis, NIST Critical Stability Constants of Metal Complexes Database 46 (Gaithersburg, MD: NIST, 2001)
I’m glad we can learn from the mistakes of others. Never thought I’d find pKa this exciting.
I often ask my students to calculate the molar water concentration of water in water to be ~54M. Is that still correct? And should it be included in the Kw? Thank you for your insights. Thanks.
The molar concentration of water does not belong in the Kw. Equilibrium constants are based on activities. The activity of the water, as the solvent, is approximated as unity (1). This approximation is based on the application of Raoult's Law to the solvent in ideal solutions.
A Note in Added Proof:
Chemists that employ the "Bronsted Equation" (as described in Bronsted, J. Acid and Basic Catalysis. Chem. Rev. 1928, 5 (3), 231-338.) to relate the acid/base strength of a compound with its catalytic abilities commonly use the incorrect pKa values of 15.7 for water and -1.7 for the aqueous proton. However, Bell (Bell, R.P. Rates and Equilibria in the Ionisation of C-H Bonds. Trans. Farad. Soc. 1943, 39, 253-259.) noted that many results for water, the hydroxide ion, and the hydronium ion were poor, and that there were objections to using the 15.7 and -1.7 pKa values. He went so far as to propose that a reactive "free" water species existed in low concentration within the bulk of the "associated" water molecules. In his 1978 review, (Bell, R.P. (1978). The Brönsted Equation-Its First Half-Century. In: Chapman, N.B., Shorter, J. (eds) Correlation Analysis in Chemistry. Springer, Boston, MA. doi.org/10.1007/978-1-4615-8831-3_2) Bell again describes the poor results for water, the hydroxide ion, and the hydronium ion when using the 15.7 and -1.7 pKa values.
This video is bound to become a meme
So your response is an ad hominem attack? That is not a rebuttal, just a logical fallacy. But that is what most proponents of the 15.7 value have to offer so far here and on Twitter- logical fallacies such as appeal to authority (famous chemists have used this number, so it must be true), the sunk cost fallacy (it has been used for almost 100 years, so it must be true), and many ad hominem attacks. But if you talk to a physical chemist, or read about the difference between an ideal solution and an ideal-dilute solution, you will see that the pKa of water is 14.0. You might also want to read Bronsted's 1928 paper carefully to see that he did not invoke two different types of water molecules, and read the reviews by Bell and Kresge to see that the 15.7/-1.78 values have long been an issue.
@@thomasneils2319 To answer your question: I pointed out my observation that the video was instantly iconic and predicted it would be a source of memes. Given the wealth of jokes and memes (e.g. the pKa of water is 2) on Twitter the next day, that prediction was correct. I wouldn't misconstrue that as me trying to engage with your argument through logical fallacy or personal attack, I'm making on observation on the video. I don't see a logical fallacy in making that observation, and I can make that observation irrespective of whether or not I agree with your argument.
I've rarely seen such a flawed and embarassing presentation of arguments. But suppose for a moment that the conclusion is nevertheless valid: if there's really a misconception to be repaired, why confine it to the organic chemistry community? Contact IUPAC and have them form a physical chemistry division to decide on the matter.
Why flawed? There are no experimental results or thermodynamic calculations that support the 15.7/-1.7 values. Proponents of these values misapply/misunderstand Raoult's Law and Henry's Law, and also ignore the difference between an ideal solution and an ideal-dilute solution. They require the existence of "solute" water molecules (which do not exist), and then they claim the solution is an ideal-dilute solution and apply Henry's Law to these "solute" water molecules to claim that the standard state for these "solute" water molecules should be 1 M and that their activity should be approximated by their molarity. But if you did have a solution of water "solute" molecules dissolved in water, you would have an ideal solution. In an ideal solution, the solute obeys Raoult's Law and the solvent obeys Raoult's law. Thus, you must approximate the activity of the "solute" water molecules by their mole fraction, not by their molarity. Using the mole fraction for both solute and solvent gets you a pKa of 14.0 at 25 C. If that is too complicated to follow, ask yourself this: If there are two types of water molecules in a sample of water, what would the composition of the vapor phase above that sample be? The only way to get the answer is to apply Raoult's Law to both types of water molecules. Thus, the activity of all the water molecules in such a mixture is correctly described by Raoult's Law.
Based af
If there are flaws, please point them out. Meanwhile, ponder the issue of the pKa of the Na+ ion:
It is common practice to ignore the change in pH caused by the addition of NaCl to aqueous solutions. This practice is valid for Cl- ions (pKb ≈ 20)a because it is a much weaker base than water is. But is this practice valid for the Na+ ion, which has a pKa = 13.9?b
If we were to take the pKa of water to be 15.7, the Na+ ion would be over 60 times as strong an acid as water itself is. If that were the case, a 0.1 M solution of NaCl would have a decrease in pH of about 0.5 pH units below the pH of pure water. Ignoring the change in pH caused by the addition of Na+ ions would not be valid in this scenario.
However, if we take the pKa of water to be 14.0, then the Na+ ion is only 1.3 times as strong an acid as water itself is, and the decrease in pH caused by the added Na+ can be ignored in most situations. This is the common experience of everyone who has measured the pH of a solution of NaCl and the expectation of anyone who has made a solution of NaCl.
a A. Trummal, L. Lipping, I. Kaljurand, I. A. Koppel, and I. Leito, J. Phys. Chem. A 2016, 120, 20, 3663-3669
b R.M. Smith, A.E. Martell, and R.J. Motekaitis, NIST Critical Stability Constants of Metal Complexes Database 46 (Gaithersburg, MD: NIST, 2001)
I do not understand a word, but I can say that the quality of evidence presenting is awful.