Hydrofluoric acid dissociation

Since ATHf >s a larger number than ZfCH,cooll, hydrofluoric acid dissociates in water to a larger extent than does acetic acid. Though HF is a weak acid (only partially dissociated), it is a stronger acid than is acetic acid. 

O Hydrofluoric acid dissociates in water to form fluoride ions. 

Qua.driva.Ient, Zirconium tetrafluoride is prepared by fluorination of zirconium metal, but this is hampered by the low volatility of the tetrafluoride which coats the surface of the metal. An effective method is the halogen exchange between flowing hydrogen fluoride gas and zirconium tetrachloride at 300°C. Large volumes are produced by the addition of concentrated hydrofluoric acid to a concentrated nitric acid solution of zirconium zirconium tetrafluoride monohydrate [14956-11-3] precipitates (69). The recovered crystals ate dried and treated with hydrogen fluoride gas at 450°C in a fluid-bed reactor. The thermal dissociation of fluorozirconates also yields zirconium tetrafluoride. 

The most common hydrogen halides are HF (U.S. production = 3 X 108 kg/yr) and HC1 (3 X 109 kg/yr). They are most familiar as water solutions, referred to as hydrofluoric acid and hydrochloric acid, respectively. Recall (Chapter 13) that hydrofluoric acid is weak, incompletely dissociated in water, whereas HCl is a strong acid. 

The strong attraction of fluorine for protons shows up in another way. In aqueous solution, HF is a weak acid whereas HC1, HBr, and HI are strong acids. The dissociation constant of HF is 6.7 X 10-4, so hydrofluoric acid is less than 10% dissociated in a 0.1 M HF solution. 

Complexes with the Fluoride Ion. For the compilation of stability constants (Table IV) of complexes with F , we have used, when needed, thermodynamic parameters (K, AH) pertaining to the dissociation of hydrofluoric acid as given by Smith and Martell (77) or extrapolated from their selection. 

Recall that, because of the strong H-F bond, hydrofluoric acid is a weak acid with a small acid dissociation constant Ka of 6.8 x 10-4. In contrast, the other binary acids of the halogen family—HC1, HBr, and HI—are strong acids that completely dissociate in water.) The fluorosilicic acid produced, H2SiF6, is a water-soluble substance with a structure as in Fig. 6.3.1. 

If anhydrous hydrofluoric acid is used as the acid phase, a further fact about one variable in (12b) can easily be introduced. It is known from the investigations of Fredenhagen (1939 Fredenhagen and Cadenbach, 1930) that alkali fluorides are practically completely dissociated in anhydrous HF. The acidity of HF can therefore be varied over a wide range by the addition of alkali fluoride or BF3, with the acidity being reduced by the addition of alkali fluoride but increased by the addition of BFs as a result of the reaction ... 

The fluorides of what metals are soluble in water How can you explain the low degree of dissociation of hydrofluoric acid What is... 

A weak acid is one that is only partially dissociated in water. Only a small fraction of the weak acid molecules transfer a proton to water, and the solution therefore contains mainly undissociated HA molecules along with small amounts of H30+ and the conjugate base A-. Typical weak acids are nitrous acid (HNO2), hydrofluoric acid (HF), and acetic acid (CH3C02H). In the case of very weak acids such as NH3, OH-, and H2, the acid has practically no tendency to transfer a proton to water, and the acid-dissociation equilibrium lies essentially 100% to the left. It follows from this definition that very weak acids have strong conjugate... 

As the electrostatic potential maps show, the C-H bond is relatively nonpolar, and methane has no tendency to dissociate in water into H30+ and CH3- ions. The N-H bond is more polar, but dissociation of NH3 into H30+ and NH2 ions is still negligibly small. Water and hydrofluoric acid, however, are increasingly stronger acids. 

Whenever there is a chemical erosion of glass, water or its dissociation products, H+ or OH ions will be involved. Because of this, a distinction is made between the resistance of glass to water (its hydrolytic resistance) and to alkali or acid. Under attack from water or acids, small numbers of cations, particularly monovalent and divalent, are released. On resistant types of glass, a very thin layer of silica gel forms in this way on the glass surface and usually inhibits further erosion. In contrast, hydrofluoric acid, alkaline solutions and, under certain circumstances, phosphoric acid will slowly remove the inhibitor layer and thus the entire surface. Nonaqueous solutions (organic solutions), however, are practically nonreactive with glass. 

In dilute solutions HF(aq) behaves as a weak acid with piweak acids, however, it becomes more and more dissociated with increasing concentration and is a considerable strong acid in pure state. This is because, irrespective of the concentration, hydrofluoric acid is mostly ionized in water. Nevertheless, the fluoride anion has strong tendency to form ion pairs like H30+ F in dilute solutions. Provided the solution is more concentrated, the molecules of water that stabilize the ion pairs are replaced by the ions of dissociated HF, increasing the number of charged species [v]. 

A few acids contain only a single hydrogen ion that can dissociate. These acids are called monoprotic acids. (The prefix mono- means one. The root -protic refers to proton. ) Hydrochloric acid, hydrobromic acid, and hydroiodic acid are strong monoprotic acids. Hydrofluoric acid, HF, is weak monoprotic acid. 

The association of water, hydrofluoric acid, the alcohols, the organic acids, etc., is a consequence of the formation of strong intermolecular hydrogen bonds with a dissociation energy of about 6 kcal/mol. As a result association can occur which leads to the formation of stoichiometrically constituted complexes, such as double molecules. This is the case with the organic... 

The acid dissociation constant is 6.7 X 10 . (The fluoride ion from the sodium fluoride shifts this equilibrium to the left it does not react directly with the hydrofluoric acid.) Start by assuming that no hydronium ion is present ... 

Hydrohalides dissolve easily in water to form hydrohalic acids and undergo virtually complete ionic dissociation (equation 84). All the hydrohalic acids, except HF, are very strong acids. Although AHF is one of the most acidic liquids, as measured by its Hammett function, hydrofluoric acid is a weak acid. 

The first step, as always, is to write the major species in the solution. From its small Ka value, we know that hydrofluoric acid is a weak acid and will be dissociated only to a slight extent. Thus when we write the n jor... 

Since hydrofluoric acid is a weak acid and only slightly dissociated, the major species in the solution are HF, Na+, F , and H20. The common ion in this solution is F, since it is produced by both hydrofluoric acid and sodium fluoride. What effect does the presence of the dissolved sodium fluoride have on the dissociation equilibrium of hydrofluoric acid ... 

To answer this question we compare the extent of dissociation of hydrofluoric acid in two different solutions, the first containing 1.0 M HF and the second containing 1.0 M HF and 1.0 M NaF. According to Le Chatelier s principle the dissociation equilibrium for HF... 

Uranium hexafluoride rapidly dissociates into hydrofluoric acid and uranyl fluoride on contact with moisture in the air. 

From a weakly acid solution AlFe, SO/ , P04 , and NO3 can be retained on anion-exchangers, while boric acid, being only very slightly dissociated, is eluted. Borate ions can be absorbed on anion-exchangers only in neutral or alkaline media. Strongly basic anion-exchangers retain traces of boron as BF4 from dilute solutions of hydrofluoric acid [18]. The... 

Hydrofluoric acid is the most basic common precursor of most fluorochemicals. Aqueous hydrofluoric acid is prepared by reaction of sulfuric acid with fluorspar (CaF2). Because HF etches glass with formation of silicon tetrafluoride, it must be handled in platinum, lead, copper. Monel (a Cu-Ni alloy developed during the Manhattan Project), or plastic (e.g. polyethylene or PTFE) apparatus. The azeotrope contains 38 % w/w HF and it is a relatively weak acid (pfC 3.18, 8 % dissociation), comparable with formic acid. Other physicochemical properties of hydrofluoric acid are listed in Table 1.2. 

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