Weak acids hydrogen fluoride

When ionic fluoride enters the acidic environment of the stomach lumen, it is largely converted to weak acid hydrogen fluoride (HF) with a pKg of 3.45 [17], The higher acidity of the stomach speeds up the process of absorption by passive diffusion from both the stomach and the small intestine, suggesting that fluoride is absorbed from the stomach as undissociated hydrogen fluoride rather than as fluoride ion [63,72]. Most of the fluoride that is not absorbed from the stomach will be rapidly absorbed from the small intestine. There is no convincing evidence that active transport processes are involved [17]. 

The double arrow implies that this reaction does not go to completion. Instead, a mixture is formed containing significant amounts of both products and reactants. With the weak acid hydrogen fluoride... 

Most protic solvents have both protogenic and protophilic character, i.e. they can split off as well as bind protons. They are called, therefore, amphiprotic. These include water, alcohols, acids (especially carboxylic), ammonia, dimethylsulphoxide and acetonitrile. Solvents that are protogenic and have weak or practically negligible protophilic character include acid solvents, such as sulphuric acid, hydrogen fluoride, hydrogen cyanide, and formic acid. 

Although the hydrogen halides all dissolve in water to give acidic solutions, there is a great difference in the acidity. Hydrogen fluoride is a weak acid, whereas all the others are strong. The pKa values are 2.92, -7, -9, and -9.5 for HF, HC1, HBr, and HI, respectively. The weakly acidic character of HF is due in part to the fact that F is a hard base and competes effectively with H20 for the protons (see Chapter 5). Consequently, the reaction... 

Hydrofluoric acid, hydrogen fluoride and fluorine are less corrosive to many metals and alloys than their own halide counterpart. The nickel-copper alloys, typified by Monel alloy 400 have excellent resistance to hydrofluoric acid corrosion. Stainless steels, such as 316, suffered severe transgranular corrosion. Table 9.23 summarizes the corrosion resistance of nickel alloys and stainless steels in anhydrous hydrogen fluoride [37]. The weakness of stainless steel to anhydrous hydrogen fluoride corrosion is shown in Table 9.23. 

Consider first two substances which have very similar molecules. He, hydrogen fluoride and HCl. hydrogen chloride the first is a Weak acid in water, the second is a strong acid. To see the reason consider the enthalpy changes involved when each substance in water dissociates to form an acid ... 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

Aqueous hydrogen fluoride is a weak acid (see above) and dissolves silica and silicates to form hexafluorosilicic acid hence glass is etched by the acid, which must be kept in polythene bottles. 

In dilute aqueous solution hydrogen fluoride is a weak acid but the acid strength increases with the concentration of hydrogen fluoride. 

Writing the equation in the usual way directs too much attention to the atoms and not enough to the electrons We can remedy that by deleting any spec tator ions and by showing the unshared electron pairs and covalent bonds that are made and broken Both sodium hydroxide and sodium fluoride are com pletely ionized in water therefore Na" which ap pears on both sides of the equation is a spectator ion Hydrogen fluoride is a weak acid and exists as undissociated HF molecules in water... 

Because of the small size of the fluoride ion, F participates in coordination stmctures of high rank. Tantalum and niobium form stable hexafluorotantalate and hexafluoroniobate ions and hydrogen fluoride attacks these usually acid-resistant metals. Hydrogen fluoride in water is a weak acid. Two dissociation constants are... 

Trifluoromethylpyridine can be prepared ia 54% yield from picolinic acid and sulfur tetrafluoride—hydrogen fluoride (434). 2-Trifluoromethylpyridine is a weak base no hydrochloride salt is formed. However, 2-trifluoromethylpyridine 1-oxide [22253-71-0] (bp 132—133°C/2.7 kPa (20 mm Hg)) is prepared ia 81% yield usiag 30% hydrogen peroxide—acetic acid (438). 

Effective precipitators are relatively weak acids, such as Co2+, Ni2+, Cu2+, Zn2+ etc. Such precipitators were ranked by Pirson to form a contiguous group of cations [65]. However, in order to synthesize compounds of either Mn(TaF6)2 or Mn(NbF6)2 type, it is necessary either to use cations with a higher acidity or anhydrous hydrogen fluoride, HF [64]. 

When sodium acetate, CH3COONa, is added to an aqueous solution of hydrogen fluoride, HF, a reaction occurs in which the weak acid HF loses H+. 

Protogenic solvents are acidic in nature and readily donate protons. Anhydrous acids such as hydrogen fluoride and sulphuric acid fall in this category because of their strength and ability to donate protons they enhance the strength of weak bases. 

The problem states that hydrogen fluoride is a weak acid, so the major species in solution are H2 O and HF molecules. In aqueous solution, HF transfers protons to H2O ... 

The noteworthy advantages of the allyl ester are (a) it is readily introduced into amino acids (b) after isomerization (to 1-propenyl) by a palladium(O) catalyst it may be removed under weakly acidic or basic and neural conditions (32), even if sulfur-containing amino acids are present (34) (c) it shows orthogonal stability to the tert-butyloxycarbonyl and 9-fluorenylmethoxy-carbonyl groups (10) and (d) it is not affected by the hydrogen fluoride-pyridine complex (35). 

Dinitrofluorobenzene (DNFB) reacts with phenols and, which is not of interest here, with amino groups. Hydrogen fluoride is eliminated. DNFB does not react w ith carboxylic acids. Alcohols, if they react at all, form dinitrophenyl ethers very slowly. Very weakly dissociated phenolic hydroxyl groups, e.g., in salicylic acid (pK = 13.4), are inert towards DNFB. 

Although anhydrous hydrogen fluoride is a very strong acid, its aqueous solution, hydrofluoric acid, is weakly acidic, particularly when dilute. The Ka value of aqueous acid at 25°C is 6.46x10- mol/L. It is an excellent solvent for many inorganic fluorides, forming bifluoride anion ... 

Hydrogen fluoride in aqueous solution is a weak acid, characterized by its pKa value of 3.2. By comparison, the other hydrogen halides are extremely strong acids in aqueous solution all three are fully dissociated in dilute solution, and their pA", values may be estimated by thermochemical cycle calculations. The thermochemical cycle shown in Figure 3.1 represents the various processes as the aqueous hydrogen halide, HX, is converted to a solution containing hydrated protons and hydrated halide ions. The enthalpy of acid dissociation of the HX(aq) compound is given by ... 

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