Ions as Acids and Bases

Ions as Acids and Bases 16-9 Qualitative Aspects of Acid-Base Reactions... 

Ions as Acids and Bases—In reactions between ions and water—hydrolysis reactions—the ions react as weak acids or weak bases. The pH of salt solutions depends on the anions and/or cations present. Anions from weak acids act as bases whereas cations from weak bases act as acids. 

Nucleophiles can also act as acids and bases, and this behavior substantially alters their nucleophilicity. At pH 5, trimethylamine exists mainly as its conjugate acid, trimethylammonium cation. First draw a Lewis structure, and then examine the electrostatic potential for trimethylammonium ion. On the basis of the map, which is the better nucleophile, the cation or the corresponding neutral amine At pH 12, phenol exists mainly as its conjugate base, phenoxide anion. First draw a Lewis structure (or series of Lewis structures), and then examine the electrostatic potential map for phenoxide anion. Which is the better nucleophile, phenoxide or phenol ... 

At the microscopic level, the Arrhenius theory defines acids as substances which, when dissolved in water, yield the hydronium ion (H30+) or H+(aq). Bases are defined as substances which, when dissolved in water, yield the hydroxide ion (OH). Acids and bases may be strong (as in strong electrolytes), dissociating completely in water, or weak (as in weak electrolytes), partially dissociating in water. (We will see the more useful Brpnsted-Lowry definitions of acids and bases in Chapter 15.) Strong acids include ... 

Zeffren and Hall (1973) have commented that, since reactions with polar transition states in nonpolar solvents can be accelerated by several orders of magnitude by the presence of low concentrations of salts (Winstein et al., 1959), the rate enhancement of tetramethylglucose mutarotation provided by the presence of acid-base pairs such as phenol and pyridine may be due to formation of ion pairs in benzene solution. Salts which do not act as acids and bases catalyse mutarotation of tetramethylglucose in aprotic solvents (Eastham et al., 1955 Blackall and Eastham, 1955 Pocker, 1960). The efficiency of enzymatic catalysis could arise largely from electrostatic catalysis... 

The substances classified as acids and bases affect the and OH concentrations in solutions, but because the product of the two values must equal K, increasing one ion must depress the other. An acidic solution has more hydrogen ions than hydroxide ions, whereas a basic—or alkaline— solution has more hydroxide ions than hydrogen ions. (See Table 10-1.)... 

The positive ion is a hydrated hydrogen ion and the negative ion is a water molecule minus one hydrogen ion. The water molecules act equally as acids and bases this type of behaviour is termed amphiprotic. The extent of the autoionization is very slight. The autoprotolysis constant,... 

What if an acid-base reaction was reversible Could the products act as acids and bases in a reversible reaction Look at the following HF + NH3 <—> F1 + NH41+. In the forward reaction the HF is the acid and the NH3 is the base. The fluoride ion is what is called the conjugate base because it was formed from the loss of a proton and in the reverse reaction the fluoride ion is what accepts a proton. HF and F1 are what we call a conjugate pair, a pair of substances that differ by an H1+ ion. 

Brpnsted theory (19.1) A theory of acids and bases that is broader than the Arrhenius theory in that it does not limit acids and bases to water solutions and it defines ions as acids or bases depending on their tendency to react with water. 

III. Salt of Weak Acid and Weak Base.-Hf both the acid and base from which a given salt is made are weak, the resp( ctive conjugate base and acid will have appreciable strength and consequently will tend to interact with the amphiprotic solvent water. When a salt such as ammonium acetate is dissolved in water, it dissociates almost completely into NH " and Ac" ions, and these acting as acid and base, respectively, take part in the hydrolytic equilibria... 

Similar conclusions can be drawn for ionic determinations. Pure salts or elements can be obtained as well as highly pure reagents such as acids and bases, ions can be produced in a very reliable manner. It is another story when dealing with organic substances. 

Heterogeneous catalysis is made much more understandable by considering the interactions between the surface atoms of the catalyst and the adsorbed reactants as acid-base reactions. The two main classes of catalysts are typified by the transition metals and by the acid clays. The bulk metals have atoms in the zero-valent state and are all soft acids. They are also soft bases, since they can donate electrons easily. Catalysts such as Al2O3-SiO2 mixtures contain hard metal ions as acids, and hard oxide, or hydroxide, ions as bases. 

Ions must be present and they come from ionic compounds or from electrolytes such as acids and bases. 4.5 (2)... 

In aqueous solution, free metal ions are complexed with water. The metal ions are said to be hydrated. The interaction of these hydrated metal ions with acids and bases is a ligand exchange reaction that is commonly called hydrolysis, or piotolysis. These terms describe the general reaction in which a proton is transferred from an acid to water, or from water to a base. This type of reaction involving hydrated metal cations as the proton-donors or acids occurs readily and is of extreme importance in natural waters. For example, the stepwise hydrolysis of the aquoaluminum(III) ion can be represented by the following series of equations ... 

A buffer resists changes in pH because it contains both an acid to neutralize added OH ions and a base to neutralize added H ions. The acid and base that make up the buffer, however, must not consume each other through a neutralization reaction. ooo (Section 4.3) These requirements are fulfilled by a weak acid-base conjugate pair, such as CH3COOH/CH3C00 or NH4 /NH3. The key is to have roughly equal concentrations of both the weak acid and its conjugate base. 

Mobile phase pH is a very effective optimization parameter for analyte molecules that exhibit a pH-dependent net charge, such as acids and bases. Beyond the pH, the type of buffer and the column temperature is of great relevance for selectivity of such analytes. The use of ion-pair-forming additives is also often helpful. 

In Section 12.1, we discussed the effect of a common ion on acid and base ionizations. Here we will examine the relationship between the common ion effect and solubility. As we have noted, the solubihty product is an equilibrium constant precipitation of an ionic compound from solution occurs whenever the ion product exceeds for that substance. In a saturated solution of AgCl, for example, the ion product [Ag ][Cr] is, of course, equal to K p. Furthermore, simple stoichiometry tells us that [Ag ] = [Cl ]. But this equality does not hold in all situations. 

Broad classes of organic compounds, such as acids and bases, can be separated by pH control, and trace metal ions complexed with organic reagents can be separated or concentrated prior to spectrometric analysis (Topic ElO). 

Bipolar membranes (BPM) have found many applications in the last decades, such as acid and base production from corresponding salt, separation of ions with different valence, recovery or purification of acid and bases from the spent liquors, and production of organic acids [28]. Properties and performance of some commercial BPMs are tabulated in Table 11.2. 

Describe how complex ions may behave as acids and bases. 

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