The Hammett acidity function (H0) is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. It was proposed by the physical organic chemist Louis Plack Hammett and is the best-known acidity function used to extend the measure of Brønsted–Lowry acidity beyond the dilute aqueous solutions for which the pH scale is useful.
In highly concentrated solutions, simple approximations such as the Henderson–Hasselbalch equation are no longer valid due to the variations of the activity coefficients. The Hammett acidity function is used in fields such as physical organic chemistry for the study of acid-catalyzed reactions, because some of these reactions use acids in very high concentrations, or even neat (pure).
The Hammett acidity function, H0, can replace the pH in concentrated solutions. It is defined using an equation analogous to the Henderson–Hasselbalch equation:
where log(x) is the common logarithm of x, and pKBH+ is −log(K) for the dissociation of BH+, which is the conjugate acid of a very weak base B, with a very negative pKBH+. In this way, it is rather as if the pH scale has been extended to very negative values. Hammett originally used a series of anilines with electron-withdrawing groups for the bases.
Hammett also pointed out the equivalent form
where a is the activity, and the γ are thermodynamic activity coefficients. In dilute aqueous solution (pH 0–14) the predominant acid species is H3O+ and the activity coefficients are close to unity, so H0 is approximately equal to the pH. However, beyond this pH range, the effective hydrogen-ion activity changes much more rapidly than the concentration. This is often due to changes in the nature of the acid species; for example in concentrated sulfuric acid, the predominant acid species ("H+") is not H3O+ but rather H3SO4+, which is a much stronger acid. The value H0 = -12 for pure sulfuric acid must not be interpreted as pH = −12 (which would imply an impossibly high H3O+ concentration of 10+12 mol/L in ideal solution).
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Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions. HA -> H+ + A- Examples of strong acids are hydrochloric acid (HCl), perchloric acid (HClO4), nitric acid (HNO3) and sulfuric acid (H2SO4). A weak acid is only partially dissociated, with both the undissociated acid and its dissociation products being present, in solution, in equilibrium with each other.
vignette|350px|L'acide acétique, un acide faible, donne un proton (ion hydrogène, ici surcoloré en vert) à l'eau dans une réaction d'équilibre donnant l'ion acétate et l'ion hydronium (code couleur : rouge=oxygène, noir=carbone, blanc=hydrogène). En chimie, une constante d'acidité ou constante de dissociation acide, Ka, est une mesure quantitative de la force d'un acide en solution. C'est la constante d'équilibre de la réaction de dissociation d'une espèce acide dans le cadre des réactions acido-basiques.
L’acide fluorhydrique est une solution aqueuse très corrosive et toxique de fluorure d'hydrogène. Le fluorure d'hydrogène pur est souvent appelé acide fluorhydrique anhydre. L'acide fluorhydrique est un acide faible en solution aqueuse diluée. En effet, bien que l'acide se dissocie totalement en solution aqueuse, c'est-à-dire qu'il y a formation d'ions H3O+ et F−, les ions fluor sont tellement électronégatifs qu'ils vont se lier très fortement aux ions hydronium (H3O+) formés, les rendant par le fait même inactifs, car très stables, HF + H3O+•••F− où les deux espèces résultantes (produits) sont fortement liées par une liaison ionique entre l'ion fluorure très électronégatif et l'ion hydronium.
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