Weak bases Ammonia

Weak bases, like weak acids, react with water to establish an equilibrium system. Ammonia is a typical weak base. It reacts with water to form the ammonium ion and the hydroxide ion  

Like a weak acid, a weak base is only partially ionized. There s a modified equilibrium constant expression for weak bases — the K. You use it exactly the same way you use the (see Acetic acid and other weak acids for the details), except you solve for the [OH-]. 

Acid Base Reactions Usinq the Brpnsted LovUrif System 

The Brpnsted-Lowry theory states that acid-base reactions are a competition for a proton. For exeunple, take a look at the reaction of ammonia with water  

Ammonia is a base (it accepts the proton), and water is cin acid (it donates the proton) in the forward (left to right) reaction. But in the reverse reaction (right to left), the ammonium ion is an acid and the hydroxide ion is a base. 

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Strong acids (the acids listed in Table J.l) are completely deprotonated in solution weak acids (most other acids) are not. Strong bases (the metal oxides and hydroxides listed in Table J.l) are completely protonated in solution. Weak bases (ammonia and its organic derivatives, the amines) are only partially protonated in solution. 

Similarly, the net ionic equation for the reaction of the weak base ammonia with the strong acid HC1 in water is... 

Just as there are weak acids, there are also weak bases. A weak base does not readily accept protons from water molecules but does quantitatively accept protons from hydronium ions. Ammonia is the most common weak base. Ammonia exists predominantly as NH3 molecules in aqueous solution, but it undergoes quantitative proton transfer with hydronium ions to generate ammonium ions ... 

Consider a solution of ammonium chloride, NHjCl". This is the salt of the strong acid, hydrochloric acid, and the weak base, ammonia. 

A basic buffer consists of a solution of a weak base and one of its salts, such as a solution of ammonia and ammonium chloride. The weak base, ammonia, removes any added hydrogen ions. The conjugate acid, the ammonium ions from the ammonium chloride salt, replaces any hydrogen ions removed when the alkali was added. 

Table 13.2 illustrates the presence of hydrogen in acids. It is also apparent that bases contain hydroxide ions, but the weak base ammonia seems to be an exception. Ammonia illustrates one of the shortcomings of the Arrhenius definition of acids and bases specifically, bases do not have to contain the hydroxide ion to produce hydroxide in aqueous solution. When ammonia dissolves in water, the reaction is represented by ... 

There are many different buffer systems useful for maintaining particular pH values. The acetic acid—sodium acetate system is good for maintaining a pH around 4.8. Buffer solutions containing equal mixtures of a weak base and a salt of that weak base maintain alkaline pH values. For example, a buffer solution of the weak base ammonia, NH3, and ammonium chloride, NH4C1, is useful for maintaining a pH about 9.3. 

Bases, like acids, can also be either strong or weak, depending on the extent to which they dissociate and produce OH- ions in aqueous solution. Most metal hydroxides, such as NaOH and Ba(OH)2, are strong electrolytes and strong bases, but ammonia (NH3) is a weak electrolyte and a weak base. Ammonia is weakly basic because it reacts to a small extent with water to yield NH4+ and OH- ions. In fact, aqueous solutions of ammonia are often called "ammonium hydroxide," although this is really a misnomer since the concentrations of NH4+ and OH ions are low. 

Notice that water is not included in the Ka it is a pure liquid and is omitted, as discussed in Chapter 16. In a way similar to (17-3), we can write the ionization of the weak base, ammonia, and its Kb. 

So far, we ve been looking at the equilibria of weak acids, from which we developed the acid-dissociation constant, Ka. There is a similar process for weak bases. Let s use the weak base ammonia. In solution, ammonia establishes the equilibrium shown below ... 

The reaction is a reversible one (compare Section 1.28), and any reagent which will reduce the hydroxyl-ion concentration sufficiently should cause the reaction to proceed from right to left with the consequent precipitation of aluminium hydroxide. This may be effected with a solution of ammonium chloride (the hydroxyl-ion concentration is reduced owing to the formation of the weak base ammonia, which can be readily removed as ammonia gas by heating) or by the addition of acid in the latter case a large excess of acid causes the precipitated hydroxide to redissolve. 

It may be added that, according to Bredig this law of dissociation holds for weak bases (ammonia and its derivatives) as well as for weak acids hence in solutions of bases (e. g. ammonia) we may assume the state of equilibrium expressed by... 

Similarly, water will react slightly with the weak base ammonia and completely with the strong base sodium oxide, forming hydroxide ion in both cases, together with the ammonium ion and the sodium ion ... 

Beginning NH3 and a very small amount of NH4 and OH are present. As a weak base, ammonia dissociates very little. 

Weak Bases. Ammonia, or ammonium hydroxide, is a weak alkali base used in many cleaning and disinfecting products. Foods that contain weak alkalis often taste bitter. So do some things that are not edible. Soap bubbles taste bitter because of the alkalis in them. 

An example of this type of buffer solution is one that contains the weak base ammonia, NH3, and its soluble ionic salt ammonium chloride, NH Cl. The reactions responsible for the operation of this buffer are... 

The ammonium ion is the conjugate acid of the weak base, ammonia, and hence expected to be moderately acidic (relative to water). Accordingly, the equation below explains the observed acidity of ammonium ions. 

Buffer solutions are prepared by using a weak acid with one of its salts or a weak base with one of its salts. For example, a buffer solution can be prepared by using the weak base ammonia, NH3, and an ammonium salt, such as NH4CI. If an acid is added, NH3 reacts with the H+. 

In a similar fashion the ionization of the weak base, ammonia, is written ... 

These reactions are very similar, but only the first reaction would be considered an acid-base reaction in the Arrhenius system. In each of the reactions, an H" " is transferred from one reactant to another, but only the first is a reaction between an Arrhenius acid and an Arrhenius base. In the first reaction, an is transferred from the Arrhenius weak acid acetic acid, HC2H302( ), to the Arrhenius weak base ammonia, NH3(<3 ). 

We have already mentioned the most common weak base, ammonia. Many organic compounds function as weak bases. Several examples of weak bases follow ... 

Knoevenagel900 applied weak bases (ammonia or an amine) as catalysts for this type of reaction, thereby enabling the condensation and the decarboxylation of the intermediate arylidene- or alkylidene-malonic acids to be effected at lower temperatures and thus achieving higher yields. 

Note that water, a pure liquid, does not appear in the equilibrium constant expression. The Bronsted-Lowry ionization process for the weak base ammonia is written... 

Ammonium chloride is the salt of a weak base (ammonia). It will react with strong aqueous hydroxide to form ammonia (Le Chatelier s principle). 

There are two weak bases, ammonia and polyaniline (emeraldine base) competing for the acid. When ammonia is present, the reaction moves to the right and polyaniline is dedoped. When the ammonia vapor is turned off (removed), the reaction may move back to the left and polyaniline becomes redoped, releasing ammonia. This process involves a reversible change in the mass of the film [85] in addition to the changes in color and resistance. 

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