Hydrofluoric acid constant

The fluoboric acid may be prepared by adding 92 g. of A.R. boric acid slowly and with constant stirring to 250 g. of hydrofluoric acid (40-48 per cent.) in a copjier, lead or a waxed-lined beaker. A lead rod may be used for stirring. All operations should be carried out in a fume cupboard. 

The equihbrium constant for this reaction at 25°C is 3.4 x 10 (92). The effects of two hydrofluoric acid solutions of different concentrations on various sihca phases are shown in Table 4. 

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. 

To determine the exact Si02 content of the residue, moisten it with 1 mL water, add two or three drops of concentrated sulphuric acid and about 5 mL of the purest available hydrofluoric acid. (CARE ) Place the crucible in an air bath (Section 3.21) and evaporate the hydrofluoric acid in a fume cupboard (hood) with a small flame until the acid is completely expelled the liquid should not be boiled. (The crucible may also be directly heated with a small non-luminous flame.) Then increase the heat to volatilise the sulphuric acid, and finally heat with a Meker-type burner for 15 minutes. Allow to cool in a desiccator and weigh. Re-heat to constant weight. The loss in weight represents the weight of the silica (Note 2). 

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. 

C16-0105. Write the equilibrium reaction and equilibrium constant expression for each of the following processes (a) Trimethylamine, (CH3)3 N, a weak base, is added to water, (b) Hydrofluoric acid, HF, a weak acid, is added to water, (c) Solid calcium sulfate, CaSOq, a sparingly soluble salt, is added to water. 

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. 

Exists in two adotropic modifications. Crystalline sihcon is made up of grayish-black lustrous needle-hke crystals or octahedral platelets cubic structure Amorphous sdicon is a brown powder. Other physical properties are density 2.33g/cm3 at 25°C melts at 1,414°C high purity liquid silicon has density 2.533 g/cm at its melting point vaporizes at 3,265°C vapor pressure 0.76 torr at 2,067°C Mohs hardness 6.5. Brinell hardness 250 poor conductor of electricity dielectiric constant 13 critical temperature 4°C calculated critical pressure 530 atm magnetic susceptibility (containing 0.085%Fe) 0.13x10 insoluble in water dissolves in hydrofluoric acid or a mixture of hydrofluoric and nitric acids soluble in molten alkalies. 

A. A. Bineau9 found that during the boiling of a cone. aq. soln. of hydrofluoric acid an excess of hydrogen fluoride escapes until there remains a soln. with a constant b.p., 130°, and a constant composition corresponding with about 36 per cent. HF. In an analogous way, if dil. soln. are distilled, the first fractions are particularly rich in water, this continues until there remains an acid with the same constant b. temp, and the same constant composition. Similarly, if the cone, or dil. acid is allowed to stand over slaked lime, the former becomes more dil. and the latter more cone. 

Streitwieser40 pointed out that the correlation which exists between relative rates of reaction in deuterodeprotonation, nitration, and chlorination, and equilibrium constants for protonation in hydrofluoric acid amongst polynuclear hydrocarbons (cf. 6.2.3) constitutes a relationship of the Hammett type. The standard reaction is here the protonation equilibrium (for which p is unity by definition). For convenience he selected the 1 -position of naphthalene, rather than a position in benzene as the reference position (for which er is zero by definition), and by this means was able to evaluate p -values for the substitutions mentioned, and cr -values for positions in a number of hydrocarbons. The p -values (for protonation equilibria, x for deuterodeprotonation, 0-47 for nitration, 0-26 and for chlorination, 0-64) are taken to indicate how closely the transition states of these reactions resemble a er-complex. 

In dilute aqueous solutions, the three halides, HC1, HBr, and HI, are essentially fully ionized, but in the poorer ionizing solvent, methanol, ionization is partial. In the latter solvent, HC1 is the most weakly ionized of the three whereas HI is the most strongly ionized this order of acidities has already been discussed (p. 90 see also Exercise 14). The ionization constant for hydrofluoric acid in dilute solutions is 7.2 X 10 4, but in very concentrated solutions the degree of ionization appears to rise sharply, a behavior opposite to that of almost all other weak electrolytes. This increase in acid strength is almost certainly due to the increase in concen-... 

Nonsiliceous Substances Transfer about 200 mg of sample, accurately weighed, into a tared platinum crucible, add 5 mL of hydrofluoric acid and 2 drops of 1 2 sulfuric acid, and evaporate gently to dryness. Cool, add 5 mL of hydrofluoric acid, evaporate again to dryness, and then ignite to constant weight. 

Transfer the filter paper and precipitate into a tared platinum crucible, char, and ignite at 900° to constant weight. Moisten the residue with a few drops of water, add 15 mL of hydrofluoric acid and 8 drops of sulfuric acid, and heat on a hot plate in a hood until white fumes of sulfur trioxide evolve. Cool add 5 mL of water, 10 mL of hydrofluoric acid, and 3 drops of sulfuric acid and evaporate to dryness on the hot plate. Heat cautiously over an open flame until sulfur trioxide fumes cease to evolve, and ignite at 900° to constant weight. The weight loss after the addition of hydrofluoric acid represents the weight of Si02 in the sample taken. 

Non-stoichiometric association can lead to linear complexes or large rings, as in hydrofluoric acid and phenol, or to a three-dimensional spatial ordering as in water and the alcohols. This association gives rise in both cases to a greatly increased dielectric constant as in water, HF, H202, HCN and alcohols, to a much smaller extent in NH3 and CH3NH2. 

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