The hydrolysis of salts

Salts may be divided into four main classes  

When any of these from classes (2) to (4) is dissolved in water, the solution, as is well known, is not always neutral in reaction. Interaction may occur with the ions of water, and the resulting solution will be neutral, acid, or alkaline according to the nature of the salt. 

With an aqueous solution of a salt of class (1), neither do the anions have any tendency to combine with the hydrogen ions nor do the cations with the hydroxide ions of water, since the related acids and bases are strong electrolytes. The equilibrium between the hydrogen and hydroxide ions in water  

Consider, however, a salt MA derived from a weak acid HA and a strong base BOH class (2). The salt is completely dissociated in aqueous solution  

A very small concentration of hydrogen and hydroxide ions, originating from the small but finite ionisation of water, will be initially present. HA is a weak acid, i.e. it is dissociated only to a small degree the concentration of A- ions which can exist in equilibrium with H+ ions is accordingly small. In order to 

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The Dissociation Constant of Nitric Add. Alodcrately Weak Acids. The Variation of J with Temperature. Proton Transfers between Solute Particles. A Proton Transfer in Methanol Solution. Proton Transfers with a Negative Value for. / . The Hydrolysis of Salts. Molecules with Symmetry. Substituted Ammonium Ions. Deuteron Transfers in D2(). The Dissociation of Molecular Ions. 

The Hydrolysis of Salts. As shown in Table 41, the NH,i molecule has a dipole moment almost as large as that of the H20 molecule. When ammonia has been dissolved in water the electrostatic interaction of an NH3 dipole with an adjacent solvent dipole is of the same order of magnitude as the interaction between two adjacent H20 dipoles. At the same time, when ammonia has been dissolved in water, the solution has a feeble electrical conductivity, due to the fact that a certain... 

Neutralisation reactions, or addimetry and alkalimetry. These include the titration of free bases, or those formed from salts of weak acids by hydrolysis, with a standard acid (addimetry), and the titration of free acids, or those formed by the hydrolysis of salts of weak bases, with a standard base (alkalimetry). The reactions involve the combination of hydrogen and hydroxide ions to form water. 

Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base the wash liquid must therefore be basic. Thus Mg(NH4)P04 may hydrolyse appreciably to give the hydrogenphosphate ion HPO and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron(III), chromium(III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid ... 

The first case corresponds to the hydrolysis of salts of strong acids and weak bases. This can generally be shown as ... 

Hydrazine salts have been prepared by the action of hypochlorites on ammonia1 or urea 2 by the hydrolysis of salts of sulfohydrazimethylene disulfonic acid 3 by the hydrolysis of triazoacetic acid 4 by the reduction of diazoacetic ester 5 by the reduction of nitroguanidine followed by hydrolysis 6 by the reduction of the nitroso derivatives of hexamethylene tetramine 7 by the reduction of nitrates or nitrites with zinc in neutral solution 8 by the action of sodium bisulfite on hyponitrous acid... 

Aqueous solutions of the +2 and +3 metal salts of strong acids are always slightly acid. This would probably seem reasonable to you if you reasoned from our general statement about the hydrolysis of salts on p 358, assuming that the metal ions come from weak bases (though they are actually insoluble ). A more satisfactory explanation is that hydrated aquo metal ions can act as weak acids. For example, hexaaquo iron(III) could dissociate to give... 

Hydrolysis and ammonolysis.—The hydrolysis of salts has previously been described. Bismuth chloride hydrolyzes in a series of stages which can be symbolized ... 

H. N. Stokes prepared salts of triimidotetraphosphoric add, HN PO(OH).NH. PO(OH)2 2, by the hydrolysis of salts of pentametaphosphimic acid—e.g. a hot acetic acid soln. of sodium pentametaphosphate furnishes sodium triimidotetra-phosphate, NH PO(OH).NH.PO(ONa)2 2 and the addition of silver nitrate to the soln. furnishes the silver triimidotetraphosphate, HN PO(OH).NH.PO(OAg)2 2. The acid is so unstable that it cannot be made by heating tetrametaphosphimate as might have been anticipated by the formation of diimidotriphosphoric acid by the action of heat on trimetaphosphimic acid. 

Arrhenius arrived at the value i i x lO from Shields measurements of the hydrolysis of salts. 

Very many problems in solution chemistry are solved with use of the acid and base equilibrium equations. The uses of these equations in discussing the titration of weak acids and bases, the hydrolysis of salts, and the properties of buffered solutions are illustrated in the following sections of this chapter. 

THE TITRATION OF WEAK ACIDS AND BASES. THE HYDROLYSIS OF SALTS... 

The Hydrolysis of Salts of Metals Other than the Alkalis and Alkaline Earths. The metal hydroxides other than the alkalis and alkaline earths are weak bases. Accordingly metal salts of strong acids, such as FeClg, CuSO, KA SOJo 12HoO (alum), etc., hydrolyze to produce acidic solutions the sour taste of these salts is characteristic. It is interesting that the hydrolysis of a metal salt need not produce the hydroxide of the metal, but may produce a soluble complex cation thus the hydrolysis of alum or of aluminum sulfate or nitrate takes place primarily according to the following equation ... 

With use of these rules we can answer questions as to the hydrolysis of salts or the choice of indicators for titration without referring to tables of acid constants. 

Capillary phenomena may arise from other causes. Thus a solution of ammonium acetate turns both red and blue litmus papers violet. The drops are more bluish in the center and more red at the edge. It appears that the paper hinders the diffusion of ammonium hydroxide to a greater extent than it does the diffusion of acetic acid. The effect is even more evident when the reaction is produced with lead acetate. Around the center of the drop is found a blue region (adsorption of lead hydroxide) which is surrounded by a red region due to the diffusion of the acetic acid. These observations account for the discrepancy found in medical books regarding the reaction of lead acetate. It is impossible to determine accurately the reaction of this salt with litmus paper. It can be done, however, with methyl red solutions. The reaction resulting from the hydrolysis of salts such as sodium acetate and ammonium chloride is demonstrated readily by means of indicator papers. 

Hydrazine salts have been prepared by the action of hypochlorites on ammonia (1) or urea (2) by the hydrolysis of salts of sulfohydrazimethylene disiilfonic acid (3) by the hydrolysis of triazoacetic acid (4) by the reduction of diazoacetic ester (5) by the reduction of nitroguanidine followed by hydrolysis (6) by the reduction of the nitroso derivatives of hexamethylene tetramine (7) by the reduction of nitrates or nitrites with zinc in neutral solution (8) by the action of sodium bisulfite on hyponitrous acid followed by reduction (9) by the reduction of K2S03N202 (10) by the action of ammonia on dichlorourea (11) by the reduction of nitrosoparaldimin (12) by the action of copper sulfate on ammonia at high temperatures (13) by the reduction of methylene diisonitrosoamine (14) by the hydrolysis of the addition product of diazoacetic ester and fumaric or cinnamic esters (15). 

The formation of hydrosols by the hydrolysis of salts is quite common in nature and has practical importance. The process of salt hydrolysis is used in wastewater treatment. The high specific surface area of colloidal... 

In general, a hydrolysis reaction is a reaction with water. Many types of hydrolysis reactions are known, but at this point we will discuss only one, the hydrolysis of salts. In Section 9.10, we said that the pH at the equivalence point is not 7 for all acid-base... 

Explain why the hydrolysis of salts makes it necessary to have available in a laboratory more than one acid-base indicator for use in titrations. 

Adsorption by Clays. — Owing to the possibility of chemical reactions between the clay and the adsorbed substances, the phenomena here are much more complicated than is ordinarily the case with many colloidal systems. According to Sullivan changes between the radicals are often involved. For instance when acid or neutral salts are adsorbed, sodium, potassium, and magnesium from the clay may be released or dissolved, while an equivalent amoimt of the adsorbed basic radical remains with the clay. The addition of alkaline solution is still more complicated. Not only may there be free alkali but basic solutions may be formed because of the hydrolysis of salts of a strong base and a weak acid, e.g., carbonates and phosphates. Three different reactions are now possible. First, the free alkali may react with the colloidal silica. Second, the silicate radical from the clay may form insoluble salts with the adsorbed base. Third, the sodium, potassium, or magnesium displaced from the clay may form soluble carbonates and phosphates, and these salts in turn be adsorbed by the clay constituents. These reactions are of great importance in the study of the fertilization of the soil. It has been claimed that the addition of lime not only neutralizes the undesirable acids, but also renders the potassium of the clay available for the plant. 

Acidity may also arise due to the presence of mineral acids produced by the hydrolysis of salts of certain heavy metals such as FeCl3 or Al2(S04)3 ... 

One striking illustration of the simplification of chemistry made possible by the Lewis theory occurs in the hydrolysis of salts. According to the electronic theory, any sufficiently strong acid or base will increase the concentration of cation or anion in an ionizing solvent. When a test with litmus shows a salt solution to be acidic or basic, a single simple equation is often all that is necessary by way of explanation. For example, the hydrolysis of zinc chloride may be represented by the equation... 

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