Hydrogen ions from water

Basicity nearly always arises from the presence of unshared electron pairs. Consequently, amines produce an alkaline reaction in aqueous solution by functioning as an electron donor (or Lewis base), withdrawing hydrogen ions from water and leaving an excess of hydroxyl ions in the solution. 

FIGURE 10.4 In this molecular portrayal of the structure of a solution of ammonia in water at equilibrium, we see that NH, molecules are still present because only a small percentage of them have been protonated by transfer of hydrogen ions from water. In a typical solution, only about 1 in 100 NH, molecules is protonated. The overlay shows only the solute species. 

This is a simple double replacement reaction (see Chapter 8 for an introduction to these types of reactions). A hydrogen ion from water switches places with the sodium of sodium carbonate to form the products carbonic acid and sodium hydroxide. By the Bronsted-Lowry definition, water is the acid because it donates its hydrogen to Na2COj. This makes Na2C03 the base because it accepts the hydrogen from H2O. 

When added to water, ammonia behaves as a base, as its nonbonding electrons (see Figure 6.16) accept a hydrogen ion from water, which, in this case, behaves as an acid ... 

Ethylamine acts as a base and accepts a hydrogen ion from water to become the eth-ylammonium ion. This reaction generates a hydroxide ion, which increases the pH of the solution. 

Sodium hydroxide, NaOH, accepts a hydrogen ion from water to form water and sodium hydroxide In solution, of course, this sodium hydroxide is dissolved as individual sodium ions and hydroxide ions. 

Let s write an equation for the carbonate ion behaving as a base. To write this equation, remember that bases take hydrogen ions from water, giving hydroxide ions and the conjugate acid of the base. 

We found it convenient to compare acidities of carboxylic acids by measuring the extent to which they give up hydrogen ion to water the equilibrium constant for this reaction was called the acidity constant, Ka. In the same way, it is convenient to compare basicities of amines by measuring the extent to which they accept hydrogen ion from water the equilibrium constant for this reaction is called a basicity constant, K. ... 

The answer is that many salts react with water in a process known as salt hydrolysis. In salt hydrolysis, the anions of the dissociated salt accept hydrogen ions from water or the cahons of the dissociated salt donate hydrogen ions to water. Does a reaction occur when potassium fluoride dissolves in water Potassium fluoride is the salt of a strong base (KOH) and a weak acid (HF) and dissociates into potassium ions and fluoride ions. 

Because 10 for NH/ (equivalently, K, > 10 for NH3), the net reaction is production of OH ions. Therefore, the equilibrium is written to show the net transfer of hydrogen ions from water to NH3 ... 

An aqueous solution of K2[Pt(OH)5] has a pH greater than 7. Explain this fact by writing an equation showing the Pt(OH) ion acting as a Bronsted-Lowry base and accepting a hydrogen ion from water. 

A few bases are covalent compounds that produce hydroxide ions by an ionization process when dissolved in water. The ionization involves the transfer of a hydrogen ion from water to the base. The most common example of this type of base is ammonia, NH3. 

An approximate equation for the ionization of a weak acid, including consideration of the hydrogen ions from water is ... 

CA is the well-studied classic Zn enzyme. CA catalyzes the reversible formation of bicarbonate and a hydrogen ion from water and carbon dioxide. So far, seven human CA isozymes have been identified CA I-VII, which are expressed in varying amounts in different tissues and cell locations. All the isozymes have at least 31% amino-acid homology and CA I-III share 58-60% amino-acid identity. 

When ammonia gas is bubbled into water, a few of the NH3 molecules remove hydrogen ion from water and produce ammonium ion, NH +, and hydroxide ion, as shown by the following reaction ... 

A schematic view of the general glass leaching process is given in Figure 2. There is an exchange of the metal ions with the hydrogen ions from water. This is typical of an acidic solution [4]. 

The standard molar Gibbs energies of transfer of ions from water to nonaqueous solvents are dealt with in Section 4.3.2.1 and those for transfer into mixed aqueous-organic solvents in Section 6.1. Specifically, the standard molar Gibbs energies of transfer of hydrogen ions from water to solvents S, A G"(H, W S), are available in Table 4.2 for nonaqueous solvents, in Table 6.1 for equimolar mixtures of water with cosolvents, and in the compilations by Kalidas et al. [17] and by Marcus [18] for other compositions. The pH scale is a universal one, because it refers to the same standard state, infinite dilution of hydrogen ions in pure water, where its activity coefficient is unity. The acidity in other solvents, pH, is related to this universal one by Equation 8.8. 

Exercise 5.2. For hydrocyanic acid (HCN), Kg = 4.9 x 10 ° at 25 C.Find [H ] if0.1000 mol of hydrocyanic acid is dissolved in enough water to make 1.0001. Assume that activity coefficients are equal to unity and neglect hydrogen ions from water. 

For each of the following weak acids, find the pH of a solution made from 0.100 mol of the acid and 1.000 kg of water at 298.15 K. Do each calculation twice once assuming that y equals unity, and once using the Davies equation to estimate y . In each case, decide whether hydrogen ions from water must be included ... 

In water, amines act as Br0nsted-Lowry bases because the lone electron pair on the nitrogen atom accepts a hydrogen ion from water and produces alkylammonium and hydroxide ions. 

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