Weak acids hydrocyanic acid

We end this chapter with a brief discussion of two sample preparation methods. The first is to determine mercury (Hg) and the second method determines cyanide (CN ). Both parameters are important with respect to the evaluation of a sample as a hazardous waste. The presence of either Hg or cyanide renders a sample hazardous due to the acute toxicity of both species. Mercury exists either in elemental form, as the dimer ion Hg2 in which the oxidation state of the element is +1, or as the divalent ion Hg. The elemental form is the familiar liquid silver metal obtained by roasting cinnabar, HgS (106). The cyanide ion is a moderately strong base derived from the weak acid hydrocyanic acid, HCN. HCN is a gas at room tem-... 

Hydrogen cyanide (hydrocyanic acid) is a colorless liquid (b.p. 25.6°C) that is miscible with water, producing a weakly acidic solution. It is a highly toxic compound, but a very useful chemical intermediate with high reactivity. It is used in the synthesis of acrylonitrile and adiponitrile, which are important monomers for plastic and synthetic fiber production. 

Another example of an acid is hydrogen cyanide, HCN, which transfers its proton to water when it dissolves to form the solution known as hydrocyanic acid, HCN(aq). However, only a small fraction of the HCN molecules donate their protons, and so we classify HCN as a weak acid in water. We write the proton transfer reaction with equilibrium half-arrows ... 

A number of small molecules that are acids do not fall into the categories mentioned above. These acids have no clear patterns in their structures, so it is best simply to learn their names and stmctures. Three hydrogen halides, HCl, HBr, and HI, are strong acids, but the fourth, HF, is a weak acid. Hydrogen sulfide, H2S, and hydrocyanic... 

Hydrogen cyanide (Table 15.1) is a colorless, flammable liquid or gas that boils at 25.7°C and freezes at minus 13.2°C. The gas rarely occurs in nature, is lighter than air, and diffuses rapidly. It is usually prepared commercially from ammonia and methane at elevated temperatures with a platinum catalyst. It is miscible with water and alcohol, but is only slightly soluble in ether. In water, HCN is a weak acid with the ratio of HCN to CN about 100 at pH 7.2, 10 at pH 8.2, and 1 at pH 9.2. HCN can dissociate into H+ and CN. Cyanide ion, or free cyanide ion, refers to the anion CN derived from hydrocyanic acid in solution, in equilibrium with simple or complexed cyanide molecules. Cyanide ions resemble halide ions in several ways and are sometimes referred to as pseudohalide ions. For example, silver cyanide is almost insoluble in water, as are silver halides. Cyanide ions also form stable complexes with many metals. 

Cyanide ion, CN , reacts with Fe " to form the blue dye that is used in blueprint paper. Hydrocyanic acid, HCN(aq), is a weak acid, with... 

Caution. The cyanide anion is a strong respiratory inhibitor. Ai such, it is a strong base, thus producing the weak, highly toxic, hydrocyanic acid on protonation. Hence, alkali metal cyanide salts must be handled with great care. Toxic CO is evolved in Section 37.A. All operations should be conducted in an efficient fume hood. 

Chromium in the trivalent state forms a variety of salts, the most important and the simplest being the violet salts, which liberate in aqueous solution chromium cation Cr" A green series of chromic salts, isomeric with the violet salts, liberate in aqueous solution some chromium cation, whilst part of the chromium is present as a complex ion. With weak acids, sulphurous, hydrocyanic, or thiocyanic acids, the chromic ion forms complex ions of great stability. Finally, a very large group of salts exists where chromium associated with ammonia forms the complex ion, the chromi-ammines. 

Nitrates and carbonates of vanadium, niobium, and tantalum have not been prepared. Niobium and tantalum salts of other weak acids, e.g. boric add, hydrocyanic add, phosphoric acid, are also unknown, and in the case of vanadium are not well defined. 

Aqueous solutions of HCN, known as hydrocyanic acid, are very weakly acidic (Ka = 4.9 X 1CT10). 

The upper half of Figure 9.9 demonstrates that acetone can also be transformed into acetone cyanohydrin (A) by the combined treatment with sodium cyanide and ammonium chloride. Ammonium chloride is a weak acid. Consequently, the protonation of the (nonvolatile) sodium cyanide to the (volatile) hydrocyanic acid occurs to a lesser extent than with the NaCN/H2S04 method (see above). However, ammonium chloride is not acidic enough to activate acetone as the carboxonium ion to the same extent as sulfuric acid. This changes the addition mechanism. As shown in Figure 9.9, it is the cyanide anions that react with the unactivated acetone. Only when the cyanohydrin anions have thus been formed does the ammonium chloride protonate them to yield the neutral cyanohydrin. 

Sodium cyanide is the salt of the weak hydrocyanic acid, HCN (ionization = 0.01 per cent in 0.1 equivalent solution), and the strong base, sodium hydroxide. A solution of this salt shows an alkaline reaction to litmus, thus demonstrating that the solution contains an appreciable quantity of OH ions. This is the result of hydrolysis, and the process may be explained as follows ... 

If we consider the case of a salt of a much weaker acid than hydrocyanic acid, or of a salt of both a very weak acid and a very weak base, it is fairly obvious that hydrolysis will be much more extensive. 

The action is referable to the effect of the ionisation of the water in the presence of one of its salts a weak acid such as hydrocyanic is dissociated to an extent so slight as to be comparable in dissociation with water itself. Under such conditions an appreciable competition will occur between the acid of the salt and the water for possession of the metallic radicle. 

Any acid, which is stronger than carbonic acid (Kt = 4-31 x 10 7) will displace it, especially on warming. Thus, even acetic acid (K = l-76x 10"5) will decompose carbonates the weak boric acid (Kl = 5-8 xlO-10) and hydrocyanic acid (K = 4-79 x 10"10) will not. 

Sodium cyanide forms colourless crystals, very soluble in water, the weak acidic character of the hydrocyanic acid inducing hydrolytic dissociation, thus imparting to the solution a strong alkaline reaction, and an odour of hydrocyanic acid.10 The anhydrous salt is converted by boiling with 75 per cent, alcohol into the dihydrate, a substance converted by slow evaporation over lime into the yellow, crystalline monohydrate.11... 

For the heat of formation of sodium cyanide from its elements, Berthelot12 gives 22-6 Cal., and Joannis13 23-1 Cal. The heat of neutralization of hydrocyanic acid by sodium hydroxide is between 2 77 and 2-9 Cal., its low value being due to the heat absorbed by the ionization of the weak acid. At 9° C. the heat of solution of the anhydrous salt is 0-5 Cal., and of the dihydrate 4-4 Cal.13 It is very poisonous. 

We know that x is going to be much smaller than 1, since this weak acid is only very slightly ionized. Hence we replace 1 — x by 1 (neglecting the small difference between un-ionized hydrocyanic acid and the total cyanide concentration), obtaining... 

It dissolves in water to form a solution which has a weak acid reaction and is unstable in time. The dissolved hydrocyanic acid partly forms a brown flocculent substance and partly... 

Acid ionization constants As you have learned, a weak acid produces an equilibrium mixture of molecules and ions in aqueous solution. Thus, the equilibrium constant, provides a quantitative measure of the degree of ionization of the acid. Consider hydrocyanic acid (HCN), a deadly poison with applications in the steel industry and in the processing of metal ores. See Figure 19-9. The ionization equation and equilibrium constant expression for hydrocyanic acid are... 

HCN is soluble in water, ethanol, chloroform, and benzene. Unlike sulfuric acid, hydrochloric acid, and so forth, hydrocyanic acid (i.e., aqueous HCN) is a rather weak acid with pKj = 9.32 (Clifford, 1961b). An aqueous solution of HCN barely turns litmus red. While forming salts with strong bases, it does not with carbonates. 

Write the ionization equations for acetic acid, HC2H302(aq), and Hydrocyanic acid, HCNfaq), both weak acids. 

Hydrocyanic acid (HCN) is a weak acid and a deadly poisonous compound—in the gaseous form (hydrogen cyanide) it is used in gas chambers. Why is it dangerous to treat sodium cyanide with acids (such as HCl) without proper ventilation ... 

Because HCN (in solution, called hydrocyanic acid) is a very weak acid (K = 4.9 X 10 °), most of the HCN produced in this reaction is in the nonionized form and leaves the solution as hydrogen cyanide gas. For this reason acids should never be mixed with metal cyanides in the laboratory without proper ventilation. 

In aqueous solution, HCN behaves as a weak acid (p.Ka = 9.31) and is slowly hydrolysed (equation 13.77). An older name for hydrocyanic acid is prussic acid. 

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