Acid dissociation constant weak adds

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

You can determine the value of for a particular acid by measuring the pH of a solution. In the following investigation, you will add sodium hydroxide to acetic acid, which is a weak acid. (See Figure 8.8.) By graphing pH against the volume of sodium hydroxide that you added, you will be able to calculate the concentration of the acetic acid. Then you will be able to determine the acid dissociation constant, Ka, for this acid. 

This table can be helpful in estimating the pATs of other weak acids from their structures. In using this table it is important to remember that -log [H ] is used in the expression for the acid dissociation constant in terms of pH. To obtain pKs based on -log y (H ) [H ], add 0,0.08, 0.11,0.12, and 0.14 at ionic strengths of 0, 0.05, 0.10,0.15, and 0.25 M, respectively, at 298.15 K as indicated by Table 1.3. PaddedForm rounds the output to two figures to the right of the decimal point. There is a list of full names of reactants in the Appendix of this book. The reactants bpg, nmn, pep, and prpp are bisphosphoglycerate, nicotinamidemononucleotide, phosphoenolpyruvate, and 5-phosphoribosyl-alpha-pyrophosphate, respectively. 

Addity equilibrium constants vary over a wide range. Acids with Ka>i are referred to as strong adds adds with Kaweak acids. The addity equilibrium constant is usually defined as an acid dissociation constant and... 

Acids and bases are essential substances in home, industry, and the environment. In aqueous solution, water combines with the proton released from an acid to form the hydrated species represented by H3O (aq). In the Arrhenius definition, adds contain H and yield H3O in water, bases contain OH and yield OH" in water, and an acid-base reaction (neutralization) is the reactfon of H and OH to form HgO. Acid strength depends on [HsO j relative to [HA] in aqueous solution. Strong acids dissociate completely and weak acids slightly. The extent of dissociation is expressed by the acid-dissociation constant, K. Weak acids have values ranging from about 10 to Many acids and bases can be classified qualitatively as strong or weak based on their formulas. 

Section 16.6 Weak adds are weak electrolytes only part of the molecules exist in solution in ionized form. The extent of ionization is expressed by the acid-dissociation constant, which is the equilibrium constant for the reaction HA(fl ) H (aq) + A (aq), which can also be... 

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. 

Confining attention to the case in which the concentrations of the acid and its salt are equal, i.e. of a half-neutralised acid then pH = pKa. Thus the pH of a half-neutralised solution of a weak acid is equal to the negative logarithm of the dissociation constant of the acid. For acetic (ethanoic) acid, Ka = 1.75 x 10 5 mol L 1, pKfl = 4.76 a half-neutralised solution of, say 0.1M acetic acid will have a pH of 4.76. If we add a small concentration of H + ions to such a solution, the former will combine with acetate ions to form undissociated acetic acid ... 

Since this is the equilibrium constant associated with a weak acid dissociation, this particular Kc is the weak add dissociation constant, Ka. The IQ expression is ... 

Section 19.1 discusses the Brpnsted theory of acids and bases, which extends the concepts of add and base beyond aqueous solutions and also explains the acidic or basic nature of solutions of most salts. Dissociation constants, the equilibrium constants for the reactions of weak acids or bases with water, are introduced in Section 19.2. The concept of the ionization of covalent compounds is extended to water itself in Section 19.3, which also covers pH, a scale of acidity and basicity. Section 19.4 describes buffer solutions, which resist change in their acidity or basicity even when some strong acid or base is added. Both the preparation and the action of buffer solutions are explained. Section 19.5 discusses the equilibria of acids containing more than one ionizable hydrogen atom per molecule. 

A titration curve of a dibasic add is essentially a composite of the titration curves of an equimolar mixture of two weak acids with dissociation constants and Ki- In the two buffer regions, where Equation (3-30) applies, the shape can be calculated directly if Aj. The hydrogen ion concentration at the first end point is given approximately by Vj, and at the second end point it is nearly the same as that for the titration of a monobasic weak acid with = 2- If is much larger than K2, the above approximations are no longer valid then the precision of the intermediate end point is so low that it is no longer of analytical value. The titration curve is too shallow for quantitative use unless KJK2 exceeds about 10 or 10 . ... 

The degree of dissociation of any given acid (HA) in water is expressed in terms of the distinct value of its dissociation constant, K, defined by the formula K = [H" ][A"]/[HA]. When comparing weak and strong acids, the strength of the add conventionally is expressed by its pK, defined as pK = -log K. 

According to the Bronsted—Lowry deHnition (Section 2.7), an cicid is any substance that donates H. Altiioi h we usually think of oxyacids (H2SOJ, HNOj) or halogen acid.s (HC1, HBr) in this context, any compound containing a hydrc en atom can be an add under the ri t circumstances. By measuring dissociation constants of dilFerent adds and expressii the results as pX, values, an acidity order can be established. Recall from Section 2.8 that a low pK corresponds to a strong add, and a high pK corresponds to a weak acid. 

This test method covers procedures for the determination of acidic constituents in petroleum products and lubricants soluble or nearly soluble in mixtures of toluene and propan-2-ol (Note I). It is applicable for the determination of acids whose dissociation constants in water are larger than 10 extremely weak adds whose dissociation constants are smaller than I0 do not interfere. Salts react if their hydrolysis constants are larger than 10 . 

Aggett and Kadwani [13] report the development and application of a relatively simple anion exchange mettiod for the speciation of arsenate, arsenite, monomethylarsonic acid and dimethylarsinic acid. As these four arsenic species are weak adds the dissociation constants of which are quite different it seemed that separation by anion exchange chromatography was both logical and possible. 

PK. A measurement of the complete ness of an incomplete chemical reaction. It is defined as the negative logarithm ito the base 101 of the equilibrium constant K for the reaction in question. The pA is most frequently used to express the extent of dissociation or the strength of weak acids, particularly fatty adds, amino adds, and also complex ions, or similar substances. The weaker an electrolyte, the larger its pA. Thus, at 25°C for sulfuric add (strong acid), pK is about -3,0 acetic acid (weak acid), pK = 4.76 bone acid (very weak acid), pA = 9.24. In a solution of a weak acid, if the concentration of undissociated acid is equal to the concentration of the anion of the acid, the pAr will be equal to the pH. 

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