Conjugate base, of a weak acid

To determine whether the solution of a salt will be acidic, basic, or neutral, we must consider both the cation and the anion. First we examine the anion to see whether it is the conjugate base of a weak acid. If the anion is neither acidic nor basic, we examine the cation to see whether it is an acidic metal ion or the conjugate acid of a weak base. If one ion is an acid and the other a base, as in NH4F, then the pH is affected by the reactions of both ions with water and both equilibria must be considered, as in Section 10.19. 

A strong base has a pronounced tendency to accept protons. Therefore, the conjugate acid of a strong base is a weak acid and conversely, the conjugate acid of a weak base is a strong acid ... 

Hydrolysis also occurs when the conjugate acid of a weak base is placed in water. For example, when NH4C1 is dissolved in water, the hydrolysis reaction that takes place is... 

The salt forms NH4 (aq) and Cl (aq) in solution. NH4 (aq) is the conjugate acid of a weak base, so it reacts with water. Cl (aq) is the conjugate base of a strong acid, so it does not react with water. The pH of the solution is therefore determined by the extent of the following reaction. 

The conjugate acid of a weak base has acid strength. Ammonia is a weak base, so the ammonium ion behaves as an acid in water. Therefore, a solution of ammonium chloride will be acidic. 

The carboxylate-ion-end of the zwitterion is the conjugate base of a weak acid, and the ammonium-ion-end of the zwitterion is the conjugate acid of a weak base. Thus, the zwitterion is a buffer, and solutions of amino acids (and proteins) resist changes in pFl. 

This type of behavior is not limited to amine hydrochlorides, and amine hydrobromides and hydrothiocyanates undergo similar reactions. Some of the reactions of amine hydrochlorides and hydrothiocyanates are useful in the preparation of coordination compounds containing the amines. The important characteristic is that ammonia or a protonated amine is a conjugate acid of a weak base and is able, therefore, to react as an acid. 

In general, a salt whose cation is the conjugate acid of a weak base / -duces an acidic solution. 

The ionization constant for the cationic acid is thus KJKi, the classical value for the hydrolysis constant of the cation (conjugate acid) of a weak base. Thus for a solution containing ammonium ions the acid-base reaction can be written... 

The conjugate acid of a weak base is a weak acid. 

Y ou already know that the conjugate base of a strong acid is extremely weak. The conjugate acid of a weak base, however, is relatively strong. Therefore, the conjugate ion from the weak base will hydrolyze to produce an acidic solution. The weaker the base, the stronger the conjugate acid will be. An example would be NH Cl. The NH ion, the cation from abase, hydrolyzes water as shown below ... 

For example, will an aqueous solution of ammonium cyanide, NH4CN, be acidic or basic First, we write equations for any reactions that occur between the separated ions and water. Ammonium ion is the conjugate acid of a weak base, so it acts as a weak acid ... 

If the salt contains a cation that reacts with water to produce hydronium ions and an anion that does not react with water, we expect the pH to be acidic. Such is the case when the cation is a conjugate acid of a weak base or a small cation with a charge of 2 + or greater. Examples NH4NO3, AICI3, Fe(N03)3. 

ACID-BASE PROPERTIES OF SALTS (SECTION 16.9) The acid-base properties of salts can be ascribed to the behavior of their respective cations and anions. The reaction of ions with water, with a resultant change in pH, is called hydrolysis. The cations of the alkali metals and the alkaline earth metals as well as the anions of strong acids, such as Cl , Br , F, and NO3 , do not undergo hydrolysis. They are always spectator ions in acid-base chemistry. A cation that is the conjugate acid of a weak base produces H upon hydrolysis. 

Ammonium chloride, NH4CI, dissociates in water to produce NH4 ions and Cl ions. Chloride ions are the conjugate base of the strong acid HCl, so they do not hydrolyze in water. Ammonium ions, however, are the conjugate acid of a weak base, NH3. Ammonium ions donate protons to water molecules. Equilibrium is established with an increased [H3O+], so pH is lower than 7. 

When the cation of a salt is the conjugate acid of a weak base, a solution of the salt will be acidic. For example, when ammonium chloride dissolves in water, it dissociates to give a solution of ammonium ions and chloride ions ... 

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