Ammonia Bronsted-Lowry theory

At the microscopic level, acids are defined as proton (H ) donors (Bronsted-Lowry theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors (Bronsted-Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase reaction between hydrogen chloride and ammonia ... 

Unlike the Arrhenius theory, the Bronsted-Lowry theory of acids and bases can explain the basic properties of ammonia when it dissolves in water. See Figure 8.4. 

The Arrhenius theory accounts for the properties of many common acids and bases, but it has important limitations. For one thing, the Arrhenius theory is restricted to aqueous solutions for another, it doesn t account for the basicity of substances like ammonia (NH3) that don t contain OH groups. In 1923, a more general theory of acids and bases was proposed independently by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. According to the Bronsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton (H + ion) to another substance, and a base is any substance that can accept a proton. In short, acids are proton donors, bases are proton acceptors, and acid-base reactions are proton-transfer reactions ... 

The basic properties of ammonia are clearly accounted for by the Bronsted-Lowry theory. Ammonia accepts a proton from the solvent water, producing OH . An equilibrium mixture of NH3, HjO, NH4+, and OH results. 

The Bronsted-Lowry theory expands on this notion beyond solvents of just water, to cover others such as liquid ammonia, glacial acetic acid, anhydrous sulfuric acid, and all solvents containing hydrogen. Bases can accept protons but don t necessarily contain OH. ... 

The HCN molecule donates a proton, so it is clearly an acid. But the water molecule accepts that proton. Does this make water a base By the Bronsted-Lowry theory, it does. But by the same theory, water can also react as an acid, as you can see in the following reaction between ammonia and water ... 

One of the advantages of the Bronsted-Lowry theory is that it can describe acid-base reactions in a nonaqueous medium like liquid ammonia. In liquid ammonia, HCl(g) donates a proton to NH3, forming NH4 and CL. No water is involved, so there are no H30 or OH ions formed. Ammonium and chloride ions are solvated by ammonia molecules, so instead of the usual aq , we might use am to show solvation by ammonia molecules. 

In the Bronsted—Lowry acid—base theory, there is competition for an H+. Consider the acid—base reaction between acetic acid, a weak acid, and ammonia, a weak base ... 

Explain the differences between the Bronsted-Lowry and the Lewis acid-base theories, using the formation of the ammonium ion from ammonia and water to illustrate your points. 

The concepts of acid-base equilibria were developed in this chapter for aqueous solutions (in aqueous solutions, water is the solvent and is intimately involved in the equilibria). However, the Bronsted-Lowry acid-base theory can be extended easily to other solvents. One such solvent that has been investigated in depth is liquid ammonia, NH3. 

Bronsted-Lowry and Lewis theory Chemical reactions with proton and or electron transfer 1. the term acid is used for both the substance and the particle. 2. Chemical reaction between hydrochloric acid and ammonia (gaseous phase) does not involve aqueous solutions with H3O and OH present... 

Lewis proposed his stiU broader and more useful definition of acids and bases in the late 1920s and early 1930s. Classifying acids as electron-pair acceptors and bases as electron-pair donors, he thereby liberated acid—base theory entirely from its former dependence on the presence of hydrogen. The advantage of the Lewis definition is that a larger number of reactions can be classified as acid-base than under either the Arrhenius or Bronsted-Lowry definitions. The classic example used to demonstrate the more general nature of the Lewis definition is the gas-phase reaction between boron trifluoride and ammonia, as represented in Equation (4.1) ... 

A major problem with Arrhenius s acid-base theory is that some substances, like ammonia, produce basic solutions and react with acids, but do not contain hydroxide ions. In 1923 Johannes Bronsted, a Danish chemist, and Thomas Lowry, an English chemist, independently proposed a new way to define acids and bases. An acid donates hydrogen ions (also called a proton donor) a base accepts hydrogen ions (also called a proton acceptor). These definitions not only explain all the acids and bases covered by Arrhenius s theory, they also explain the basicity of ammonia and ions such as carbonate, and phosphate, P04 ... 

One important aspect of the Lowry-Bronsted theory is that, because it involves proton transfers, it does not necessarily have to involve water. It is possible to describe reactions in nonaqueous solvents, such as liquid ammonia, in terms of acid-base reactions. 

---