The Relative Acidities of Water and Methanol

Abrash, Henry I.

doi:10.1021/ed078p1496

Cited by 6 publications

(9 citation statements)

References 8 publications

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“…Our analysis of textbooks above shows that organic chemists have been the main supporters − of the Starkey et al contention that Brønsted’s “rational” p K a values are the “correct” ones. Their primary supporting argument ,− mirrors that originally stated by Ballinger and Long,who compared their experimentally determined alcohol p K a s (14.4–15.5 for methanol, glycerol, and ethylene glycol) to the “rational” value of 15.74 for water, and concluded that the alcohols were stronger acids than water (Figure ).…”

Section: Introductionsupporting

confidence: 62%

“…Literature values tabulated in Section III of the Supporting Information show that K ′ averages to 0.6 ± 0.6, ranging from 0.1 to 2.0. Although it seems statistically reasonable to conclude that K ′ is essentially 1, previous authors used the overall average K ′ value to conclude that methanol is a stronger acid than water by a factor of 1.5–3 …”

Section: Discussionmentioning

confidence: 99%

“…In eq , a single water molecule acts as the base; analogously, Brønsted presented the autoionization of water in terms of two water molecules, one behaving as acid and the other as base (eq ). …”

Section: Discussionmentioning

confidence: 99%

“…4 In this paper, Ballinger and Long concluded (erroneously, as we shall prove) that methanol (pK a 15.54) is a stronger acid than water; hence, the pK a of water must be 15.74 and not 14.00 (see Figure 1). Soon after Ballinger and Long published their work in 1960, a split developed in the K a and pK a tables published in first-year and organic chemistry textbooks: 11 First-year chemistry texts either used Brønsted's "conventional" values of 14 and 0, or omitted water and the hydronium ion from the table; on the other hand, organic chemistry textbooks uniformly used Brønsted's "rational" values of 15.74 and −1.74. For example, Cram listed the "rational" values in his 1965 monograph Fundamentals of Carbanion Chemistry, 12 citing Ballinger and Long; from there, they made their way into the seminal 1970 edition of Hendrickson, Cram, and Hammond's Organic Chemistry (3rd ed.)…”

Section: Two Different Pk a Values For Watermentioning

confidence: 99%

“…Although it seems statistically reasonable to conclude that K′ is essentially 1, previous authors used the overall average K′ value to conclude that methanol is a stronger acid than water by a factor of 1.5−3. 11 This raises two important questions: (1) Is K′ (or K b (MeO − )) a reasonable proxy for the spontaneity of the methanol/hydroxide acid−base reaction? And (2) how can the same reaction have two different equilibrium constants, one of which is greater than 1, and the other less than 1?…”

Section: Ballinger and Long: Experimental Systems And Calculationsmentioning

confidence: 99%

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pK_a Values in the Undergraduate Curriculum: What Is the Real pK_a of Water?

Silverstein

Heller

2017

J. Chem. Educ.

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Since at least the 1960s, organic chemistry textbooks have featured pK a tables for organic acids that include values for H2O and H3O+ (15.74 and −1.74, respectively) that are thermodynamically and chemically indefensible. Here we trace this error back to Brønsted’s early contributions in the 1920s to the Brønsted–Lowry Theory of acids and bases. Organic chemists generally defend the use of these values by citing measurements of the equilibrium constant for the water + methoxide acid–base reaction that suggested that methanol (pK a = 15.54) is a stronger acid than water; from this, organic chemists have concluded that pK a of water must be 15.74 rather than 14.00. Here we discuss the problems that invalidate this conclusion, the most important being that it is based on the use of the pure liquid standard state (mole fraction = 1) that is quite different from the standard state for acidities determined in dilute solution (molality = 1). Using the latter standard state, the equilibrium constant for the water/methoxide reaction ranges from 4 to 70, showing water to be a stronger acid than methanol, and justifying the use of the thermodynamically correct value, pK a(H2O) = 14.00.

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Section: Introductionsupporting

confidence: 62%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Two Different Pk a Values For Watermentioning

confidence: 99%

Section: Ballinger and Long: Experimental Systems And Calculationsmentioning

confidence: 99%

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pK_a Values in the Undergraduate Curriculum: What Is the Real pK_a of Water?

Silverstein

Heller

2017

J. Chem. Educ.

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Impact of Methanol Photomediated Surface Defects on Photocatalytic H₂ Production Over Pt/TiO₂

Jiang

Zheng-wen

et al. 2020

Energy & Environ Materials

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Co‐catalysts play a critical role in enhancing the efficiency of inorganic semiconductor photocatalysts; however, synthetic approaches to tailoring co‐catalyst properties are rarely the focus of research efforts. A photomediated route to control the dispersion and oxidation state of a platinum (Pt) co‐catalyst through defect generation in the P25 titania photocatalyst substrate is reported. Titania photoirradiation in the presence of methanol induces long‐lived surface defects which subsequently promote the photodeposition of highly dispersed (2.2 ± 0.8 nm) and heavily reduced Pt nanoparticles on exposure to H2PtCl6. The optimal methanol concentration of 20 vol% produces the highest density of metallic Pt nanoparticles. Photocatalytic activity for water splitting and associated hydrogen (H2) production under UV irradiation mirrors the methanol concentration employed during the P25 photoirradiation pretreatment, and resulting Pt loading, resulting in a common mass‐normalized H2 productivity of 3800 ± 130 mmol gPt−1 h−1. Photomediated surface defects (arising in the presence of a methanol hole scavenger) provide electron traps that regulate subsequent photodeposition of a Pt co‐catalyst over P25, offering a facile route to tune photocatalytic efficiency.

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Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators

Meister

Willeke

Angst

et al. 2014

Helvetica Chimica Acta

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In chemistry textbooks, the pK a,H2O value of water in the solvent water at 25 8C is sometimes given as 14.0, sometimes as 15.7. This is confusing. The particular chemical reaction considered is the one in which water as BrønstedÀLowry acid reacts with water as BrønstedÀLowry base in water as solvent to yield equal concentrations of hydrated oxonium and hydroxide ions, H 3 O þ (aq) and HO À (aq), respectively. This reaction is also known as the self-ionization of water for which the equilibrium constant is abbreviated as K w with its known value of 10 À14.0 at 25 8C, i.e., pK w (25 8C) ¼ 14.0. Identical values for pK a,H2O and pK w at a fixed temperature appear reasonable, since K a,H2O and K w refer to one and the same reaction. Therefore, reasons for the apparent disagreement between the thermodynamically correct pK a value for water (14.0 at 25 8C) and the value reported in most organic chemistry textbooks (15.7) should be discussed when teaching acidÀbase chemistry. There are good arguments for introducing, from the very beginning, the concepts of activity and thermodynamic standard states when teaching quantitative aspects of chemical equilibria. This also explains in a straightforward way why all thermodynamic equilibrium constants, including K w , are dimensionless, and why pK H3O þ(25 8C) ¼ 0.

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The Relative Acidities of Water and Methanol

Cited by 6 publications

References 8 publications

pK_a Values in the Undergraduate Curriculum: What Is the Real pK_a of Water?

pK_a Values in the Undergraduate Curriculum: What Is the Real pK_a of Water?

Impact of Methanol Photomediated Surface Defects on Photocatalytic H₂ Production Over Pt/TiO₂

Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators

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The Relative Acidities of Water and Methanol

Cited by 6 publications

References 8 publications

pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?

pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?

Impact of Methanol Photomediated Surface Defects on Photocatalytic H2 Production Over Pt/TiO2

Confusing Quantitative Descriptions of BrønstedLowry AcidBase Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators

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Product

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pK_a Values in the Undergraduate Curriculum: What Is the Real pK_a of Water?

pK_a Values in the Undergraduate Curriculum: What Is the Real pK_a of Water?

Impact of Methanol Photomediated Surface Defects on Photocatalytic H₂ Production Over Pt/TiO₂