Ammonia autoionization

A point of this reaction is the release of H+ ions—the Bronsted-Lowry approach considers this the appearance of a proton from the acid. A Bronsted-Lowry acid is a proton donor. Note that the hydrogen ion is a proton, hydrogen s nucleus without the electron found in the atom. Then, a Bronsted-Lowry acid must contain a hydrogen. Of course, if the solvent were not to be water, this statement may not work because the cation released could be other than the hydrogen ion, but there might be other ions performing the same service (liquid ammonia autoionizes, Problem 17.3). 

On the planet Baseacidopolous, the major solvent is hquid ammonia, not water. Ammonia autoionizes much like... 

On the planet Baseacidopolous, the major solvent is liquid ammonia, not water. Ammonia autoionizes much Uke water (2NH3 NH4 + NH2 ). If instead of water, ammonia is used as a solvent (a) What is the formula of the cation that would indicate that a compound is an acid (b) What is the formula of the anion produced if a compound is a base (c) Look at the way that NaCl is formed from an acid-base reaction on earth and determine if NaCl can be a salt on Baseacidopolous. 

The conductivity of liquid ammonia is sufficiently high to indicate a very slight degree of autoionization. In order for ions to be produced, something must be transferred from one molecule to another, and in solvents such as water or ammonia it is proton transfer that occurs. Accordingly, the ionization of liquid ammonia can be shown as... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

The "equilibrium boxes" for the solvents (Fig. 10-1) indicate the range over which differentiation occurs outside the range of a particular solvent, all species are leveled. For example, water can differentiate species (i.e., they are weak adds and bases) with pKa s from about 0 to 14 (such as acetic acid). Ammonia, on the other hand, behaves the same toward acetic acid and sulfuric acid because both lie below the differentiating limit of —12. The extent of these ranges is determined by the autoionization constant of the solvent (e.g, —14 units for water). The acid-base behavior of several species discussed previously may be seen to correlate with Fig. 10.1.11... 

Vibrational mode selectivity can also be used to promote electronic processes. Vibrational autoionization is a process whereby a bound electron acquires sufficient energy to escape by extracting one quantum of vibrational energy from the ionic core of the molecule. For such an energy transfer to occur, the electron must first collide with the core. Scattering of the electron with the core can be promoted if the amplitude of the nuclear motion overlaps the electronic charge density. An example of this process studied by Steven Pratt is vibrational autoionization of the 3d Rydberg electrons of ammonia, which is enhanced... 

However, generating doubly charged ions would be highly unfavorable compared to autoionization of water, ammonia, or hydrogen fluoride in which proton transfer occurs. 

A. Label each substance in the equation for the autoionization of liquid ammonia as Brpnsted acid or base ... 

Autoionization can occur in other liquids as well. For example, in liquid ammonia the autoionization reaction is... 

You travel to a distant, cold planet where the ammonia flows like water. In fact, the inhabitants of this planet use ammonia (an abundant liquid on their planet) much as earthlings use water. Ammonia is also similar to water in that it is amphoteric and undergoes autoionization. The K value for the autoionization of ammonia is 1.8 X 10 12 at the standard temperature of the planet. What is the pH of ammonia at this temperature ... 

Ammonia undergoes autoionization with the formation of ammonium and amide... 

Liquid ammonia (bp — 33.4°C) is used as a solvent for some chemical reactions. It is hydrogen bonded, just as H2O is, but NH3 is a much more basic solvent. Its weak autoionization produces the ammonium ion, NH4+, and the amide ion, NH2. This is similar to H2O, which ionizes to produce some H3O+ and OH ions. 

Like water, liquid ammonia undergoes autoionization ... 

Ammonia, amines, and anions of weak acids behave as weak bases in a process associated with a base-dissociation constant, K. The reaction of HA with H2O added to the reaction of A" with H2O gives the reaction for the autoionization of wata thus. Kg x Kb = Ky,. 

Another acid-base theory that is useful for solvents other than water was postulated by American chemist Fdward Franklin in 1905. It makes use of the autoionization of solvents, and defines an acid as a solute that produces the positively charged species of the solvent and a base as a solute that produces the negatively charged species of the solvent. In the case of the autoionization of water (equation 18) HjO is the acid and OH is the base. For the nonaqueous solvent, liquid ammonia, the autoionization gives... 

After all, we have no problem in calling water, ethanol and ammonia molecular liquids, in spite of the fact that they all have autoionization reactions that lead to measurable concentrations of ions at room temperature. However, a comparison of the ion concentration between a saturated KQ solution in water and the typical ionic liquid [BMIM][PFe] immediately shows the unsuitability of such an approach. While a saturated KCl solution in water at 22 °C has an ion concentration of 4.6 mol 1 , the ion concentration of [BMIMjjPFs] is only 0.0054 mol 1 at the same temperature [32]. This means that a general ion concentration at which a system can be considered to be an ionic liquid certainly does not exist. Again, we find ourselves in a position where exploring the limits of a definition leads to it breaking down. 

The autoionization of liquid ammonia is given by Equation (14.5). Thus, any species that is capable of increasing the concentration of ammonium ion in liquid ammonia is considered to be an acid and any species that generates NH2 is a base ... 

Strat y The procedure here is similar to the one used for a weak acid (see Example 11.11). For the ionization of ammonia, the major species in solution at equilibrium are NH3, NH4, and OH. The hydrogen ion concentration is very small as expected for a basic solution, so it wdl only be a minor species in solution. As in the case of weak acids, we can usually ignore contributions from the autoionization of water, unless the solution is very dilute. 

Liquid ammonia, NH3, undergoes autoionization, much like water does ... 

Like water, ammonia undergoes autoionization in liquid ammonia ... 

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