Acidity of ammonia

When an acid and a base react, the products are a new acid and base. For example, the acetate ion, C1T3COO-, in reaction 6.7 is a base that reacts with the acidic ammonium ion, N1T45", to produce acetic acid and ammonia. We call the acetate ion the conjugate base of acetic acid, and the ammonium ion is the conjugate acid of ammonia. 

A range of amines and alcohols has been selected for calculating 8 and Tg of fully capped EPM-g-MA as a function of the MA graft content. Examples of these amines and alcohols included ammonia, 2-aminoethanol, 2-(methylamino)ethanol, diethanolamine, diisopropanolamine, urea, glycine, 1,4-dia-minobutane, and the amide acid of ammonia-capped MA. Typical results are presented in Figure 13.7. 

Salts that contain cations of weak bases are acidic. For example, the ammonium cation Is the conjugate acid of ammonia. When ammonium salts dissolve in water, NH4 ions transfer protons to H2 O molecules, generating H3 O and making the solution slightly acidic NH4" ((2 q) + H2 0(/) NH3(c2 q) + H3 O (a q) The equilibrium constant for this reaction can be calculated from Equation and for ammonia (Example ) ... 

The conjugate acids of ammonia and methylamine are the ammonium ion, NH44" (pKa = 9.2) and the methylammonium ion, CH3NH3+ (pKa = 10.6) respectively. Since methylammonium ion is a weaker acid than ammonium ion, methylamine is a stronger base than ammonia. 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

The conjugate acid of a base is formed when the base acquires a proton from the acid. In this reaction, water acts as an acid because it donates a proton to the ammonia molecule. The ammonium ion (NH4+) is the conjugate acid of ammonia (NH3), a base, which receives a proton from water. The hydroxide ion (OH ) is the conjugate base. 

Alternatively it is noted that the conjugate acids of ammonia and methoxide are the ammonium ion and methanol, respectively, and the equations for their ionization in water are... 

According to the Bronsted-Lowry definition of acids and bases, what is the conjugate acid of ammonia ... 

When a strong base such as NaOH is added to the original buffer solution, it is neutralized by the more acidic component, NH4CI, or NH4+, the conjugate acid of ammonia. 

The acid and base of a half-reaction are called conjugate pairs. Free protons do not exist in solution, and there must be a proton acceptor (base) before a proton donor (acid) will release its proton. That is, there must be a combination of two half-reactions. Some acid-base reactions in different solvents are illustrated in Table 7.1. In the first example, acetate ion is the conjugate base of acetic acid and ammonium ion is the conjugate acid of ammonia. The first four examples represent ionization of an acid or a base in a solvent, while the others represent a neutralization reaction between an acid and a base in the solvent. 

Water doesn t have to be one of the participants—if we replace water in the reaction we have been discussing with ammonia, we now have ammonia as the conjugate base of NH (the ammonium cation) and the ammonium cation as the conjugate acid of ammonia. What is different is the position of equilibrium ammonia is more basic than water and now the equilibrium will be well over to the right. As you will see, piC will help us assess where equilibria like these lie. 

Biological fixation is done by specialized microbes that convert the N2 into ammonia (NHj"), nitrate ions (NOj"), or urea ((NH2)2CO). Although ammonia can be taken up by plants through the roots, it s more often the ammonium ion (NH ), that s the conjugate acid of ammonia, that is used by biological systems. (See Chapter 5 for a quick recap on conjugate acids and bases.)... 

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