Acid strengths in water

Attempts were early made to explain the relative acidities of ammonium and methyl-substituted ammonium ions (Brown, 1946 Hammett, 1940 Palit, 1947). The order of decreasing acid strength in water at 25°C is NH4+ > Me3NH+ > MeN H3+ % Me2N H2+, an order which obviously cannot be rationalized on the basis of inductive effects. There were two difficulties with the explanations given for this order. First, there is nothing magic about the temperature 25° and comparisons at other temperatures lead to inversions of the order at 25°. Second, the specific effect of the solvent was not taken into account. Since there is... 

The high acidity of sulfonic acids creates a problem in the quantitative determination of their acid strengths in water, the standard medium. Even with some of the weaker sulfonic acids like methanesulfonic acid, a 0.1 M solution of the acid is more than 99.8% ionized, and the precise determination of the components of the equilibrium can be difficult. Media other than water, of course, may be used for the determination of acid strength, and some of these will be discussed in this chapter. The loss of comparability with the vast body of acidity data for aqueous solutions, and the intervention of other problems such as ion pairing, are, however, serious disadvantages to the use of solvent media other than water, and considerable effort has been devoted to finding acceptable pK values for the aqueous system. 

H—X Bond Bond Strength (kJ/mol) Acid Strength in Water... 

Table 4. relative acid strengths in water and formamide... 

In all proton containing solvents acid-base phenomena can be described in terms of the Bronsted-Lowby theory. All of these solvents have the solvated proton in common as the solvent cation, and this determines to a considerable extent the chemistry in their solutions. Bronsted acids are usually characterized by their acidic strength in water, e.g. by the acidity constant in this solvent. Thus acetic acid and hydrofluoric acid both behave as moderately weak acids in water with at room temperature. When acetic acid is dissolved in liquid hydrogen fluoride, the former is successfully competing for the protons, so that acetic acid acts as a base ( acetic-base ) in this medium just as it does in nitric acid ... 

Acidity constant, (Section 2.8) A measure of acid strength in water. For any acid HA, the acidity constant is given by the expression... 

Stannous Sulfate. Stannous sulfate (tin(Il) sulfate), mol wt 214.75, SnSO, is a white crystalline powder which decomposes above 360°C. Because of internal redox reactions and a residue of acid moisture, the commercial product tends to discolor and degrade at ca 60°C. It is soluble in concentrated sulfuric acid and in water (330 g/L at 25°C). The solubihty in sulfuric acid solutions decreases as the concentration of free sulfuric acid increases. Stannous sulfate can be prepared from the reaction of excess sulfuric acid (specific gravity 1.53) and granulated tin for several days at 100°C until the reaction has ceased. Stannous sulfate is extracted with water and the aqueous solution evaporates in vacuo. Methanol is used to remove excess acid. It is also prepared by reaction of stannous oxide and sulfuric acid and by the direct electrolysis of high grade tin metal in sulfuric acid solutions of moderate strength in cells with anion-exchange membranes (36). 

According to the Arrhenius definitions, an acid ionizes in water to produce protons (H" ) and a base produces hydroxide ions (HO ). The strength of an acid is given by its equilibrium constant for ionization in aqueous solution ... 

Recall from the earlier discussion of acidity in Sections 2.7 through 2.11 that the strength of any acid HA in water can be expressed by an acidity constant, Ka. 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

Le Chatelier s principle When a stress is applied to a system in dynamic equilibrium, the equilibrium adjusts to minimize the effect of the stress. Example a reaction at equilibrium tends to proceed in the endothermic direction when the temperature is raised, leveling The observation that strong acids all have the same strength in water, and all behave as though they were solutions of H,Of ions. 

The strength of an acid, HA, in water, i.e. the extent to which it is dissociated, may be determined by considering the equilibrium ... 

Some cations in aqueous solution, such as Na+, K+, and Ba2+, undergo no real significant reaction with the water molecules in a solution. This is because their acid strengths are much less than the acid strength of water. In other words, these cations are such weak acids that they do not react with water. Because of this, when these cations are dissolved in water, the pH is unchanged. 

Figures 10.7 and 10.8 show the submicroscopic behavior of strong and weak acids in water. However, molecules and ions are too small to see. How then does a chemist measure the strength of an acid One way is by measuring a solution s ability to conduct an electric current, as Figure 10.9 illustrates. In pure warer there are practically no ions to conduct electricity. When a strong acid is dissolved in water many ions are generated, as indicated in Figure 10.7. The presence of these ions allows for the flow of a large electric current. A weak acid dissolved in water generates only a few ions, as indicated in Figure 10.8. The presence of fewer ions means there can be only a small electric current.

Acid Strength of Water Ligands in Dinuclear Complexes with Negligible Intramolecular Hydrogen Bond Interaction at 25°C and / = 1.0 M... 

Acid Strength of Water Ligands in Monohydroxo-Bridged Complexes Affected by Intramolecular Hydrogen Bonds at 25°C in 1.0 M (Na,H)C104... 

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