HYDROLYSIS AND ACID-BASE PROPERTIES OF SALTS (15.10)
STUDY OBJECTIVES
Hydrolysis. A salt is an ionic compound formed by the reaction of an acid and a base. In dealing with salts, it will be useful to recall that they are strong electrolytes and are completely dissociated in water. The term salt hydrolysis refers to the reaction of a cation or an anion, or both with water which splits the water molecule into two parts. Salt hydrolysis usually affects the pH of a solution.
A cation causes hydrolysis by bonding to the hydroxide part of the water molecule, causing a proton to be released. Cations that undergo hydrolysis produce acidic solutions.
M+ + HOH H+ + MOH
Certain metal ions such as Al3+, Cr3+, Fe3+, Be2+ and Bi3+ are good examples of acidic cations. NH4+ is also an acidic cation.
Anion Hydrolysis.
Anions that undergo hydrolysis produce basic solutions. An anion can cause hydrolysis by acting as a Brønsted base. A basic anion can accept a proton from water and releases a OH ion.
A + HOH HA + OH
Not all anions cause hydrolysis. You must be able to recognize those that do and those that don't. Quite simply, those anions with the ability to undergo hydrolysis are conjugate bases of weak acids. The conjugate base of any weak acid is a weak base and will raise the pH of an aqueous solution. The extent of hydrolysis is proportional to the base ionization constant, Kb.
Keep in mind that the conjugate base of a strong acid has no tendency to accept protons from H2O. Anions such as Cl, NO3-, and ClO4- do not cause hydrolysis. They do not have acid-base properties.
EXAMPLE Hydrolysis of Ions
Write net ionic equations to show which of the following ions hydrolyze in aqueous solution:
NO3-
no reaction NO3- + H2O HNO3 + OH-
Correct! Click a Hint button for help.
NO2-
no reaction NO2- + H2O HNO2 + OH-
NH4+
no reaction NH4+ + H2O NH3 + H3O+ NH4+ + H2O NH5+ + OH-
Strong acids have extremely weak conjugate bases which have no affinity for H+ ions. They have neutral properties. Weak acids have stronger conjugate bases with significant affinity for an H+ ion. The conjugate bases of weak acids have measurable basic properties.
Nitrate ion is the conjugate base of a strong acid, HNO3. Therefore has no acidic or basic properties and has no tendency to react with water to form HNO3.
NO3- + H2O ® HNO3 + OH (no reaction)
Nitrite ion is the conjugate base of a weak acid, HNO2. Therefore is a stronger base than and will accept a proton from water to form HNO2.
+ H2O HNO2 + OH
Ammonium ion is the conjugate acid of a weak base. Therefore, it is a weak acid:
+ H2O NH3 + H3O+
The Conjugate Bases of Weak Acids. Fluoride ion is the conjugate base of hydrofluoric acid (HF) a weak acid. As a weak base it can accept a proton from water. The resulting OH ions make the solution basic.
The force that makes this reaction possible is the tendency of the weak acid HF to remain undissociated.
Three realms of base strength can be distinguished:
The conjugate bases of strong acids (Ka > 1.0) are too weak to cause measurable hydrolysis. Thus Cl, Br, and NO3-ions do not hydrolyze in aqueous solutions.
Cl + H2O HCl + OH (no reaction)
The conjugate bases of acids with a Ka between 101 and 105 are weak bases. Thus F, NO2-, HCOO(formate), and CH3COO (acetate) ions hydrolyze to a small extent and produce low concentrations of OH ions in solution.
NO2 + H2O HNO2 + OH
The conjugate bases of very weak acids (Ka < 105) are moderately strong bases. Thus solutions containing CN, HS, HPO42, and CO32 ions can be quite basic.
Cation Hydrolysis. It is important to know which metal ions cause hydrolysis and which do not. Those that cause hydrolysis are small, highly charged metal ions such as Al3+, Cr3+, Fe3+, Bi3+, and Be2+. These can split H2O molecules, yielding acidic solutions. In the case of the hydrated Al3+ ion the reaction is
Al(H2O)63+(aq) + H2O(l) Al(OH)(H2O)52+(aq) + H3O+(aq)
The singly charged metal ions associated with strong bases do not cause hydrolysis. The commonly encountered ions Na+, K+, Ca2+, and Mg2+ do not have a great enough positive charge to attract an OH from a H2O molecule and bond to it. Recall that NaOH and KOH are strong bases. That is, they ionize completely in solution. These ions, K+ and OH for instance, have no tendency to stay together in solution.
KOH(aq) K+(aq) + OH(aq)
The hydrolysis reaction that would bond an OH to a K+ ion does not occur!
K+(aq)+ H2O(l) KOH(aq) + H+(aq) (no reaction)
Another acidic cation that you will need to be familiar with is the ammonium ion (NH). Solutions of ammonium ion are acidic because NH is a Brønsted acid. Recall that is the conjugate acid of the weak base NH3.
NH4+ + H2O NH3 + H3O+
or simply
NH4+ NH3 + H+ Ka = 5.6 x 1010
Table 15.5 lists the various anions and cations we have discussed according to their acid-base properties. You should review the material above, and in the textbook to see why each ion has the property described.
Table 15.5 Acid-Base Properties of Ions
Salts That Produce Acidic Solutions. Salts producing acidic solutions are salts of strong acids and weak bases. These salts contain cations that undergo hydrolysis, and anions that do not (the neutral anions). Such a salt has a cation that is a conjugate acid of a weak base. Thus solutions of these salts are acidic. The salt NH4Cl is a strong electrolyte and dissociates into ammonium ions and chloride ions when dissolved in water. The NH4+ ion is a conjugate acid of the weak base NH3 and releases H+ ion into solution.
NH4Cl(aq) ®(aq) + Cl(aq) (strong electrolyte) NH4+ NH3 + H+ Ka = 5.6 x 1010
The Cl ion is neutral having no acidic or basic properties. NH4Cl is a salt of a strong acid (HCl) and a weak base (NH3).
Solutions of certain salts such as AlCl3 and FeCl3 are also acidic. These contain small, highly charged cations, and anions that are conjugate bases of strong acids.
Salts That Produce Basic Solutions. Salts producing a basic pH are salts of weak acids and strong bases. These salts have a neutral cation and a basic anion. For example, NaF dissociates into Na+ and F ions. The Na+ ion has no acidic or basic properties, but the fluoride ion is a conjugate base of a weak acid and therefore is a basic anion.
NaF a salt of the NaOH, a strong base, and HF, a weak acid. F + H2O HF + OH
NaF a salt of the NaOH, a strong base, and HF, a weak acid.
F + H2O HF + OH
Salts That Produce Neutral Solutions. When a salt is dissolved in water the solution will acquire a pH consistent with the acid-base properties of the cation and anion making up the salt. In general, salts containing alkali metal ions or alkaline earth metal ions (except Be2+) and the conjugate bases of strong acids do not undergo hydrolysis; their solutions are neutral. Consequently, a solution of KCl, for example, is neutral. A K+ ion does not accept an OH ion from H2O, and a Cl ion does not accept a H+ ion from H2O.
EXAMPLE Acid-Base Properties of Ions
Predict whether the following aqueous solutions will be acidic, basic, or neutral.
KI acidic basic neutral
NH4I acidic basic neutral
CH3COOK acidic basic neutral
What we must decide is whether either of the ions of the salt undergoes hydrolysis. KI is a salt of a strong acid (HI) and a strong base (KOH). Neither K+ nor I have acidic or basic properties. Thus a solution containing KI remains neutral.
NH4I is the salt of a strong acid (HI) and a weak base (NH3). As explained in part (a) iodine does not hydrolyze. Since ion is the conjugate acid of a weak base it will donate protons to water.
Therefore, a solution containing NH4I will be acidic.
CH3COOK is a salt of a weak acid and a strong base. K+ ion will not hydrolyze, but CH3COO (acetate ion) is the conjugate base of a weak acid, acetic acid. Thus acetate ions will accept protons from water and form acetic acid and OH ions.
CH3COO + H2O CH3COOH + OH
Solutions containing CH3COOK will be basic.
Salts in Which Both the Anion and Cation Hydrolyze. Salts derived from a weak acid and a weak base contain cations and anions that will hydrolyze. Whether a solution containing such a salt is acidic, basic, or neutral depends on the relative strengths of the acidic and basic anions. For example, a solution of ammonium fluoride is acidic because NH4+ is a stronger acid than F is a base.
NH4F(aq) NH4+(aq) + F(aq) (strong electrolyte) NH4+ NH3 + H+ Ka = 5.6 x 1010 F + H2O HF + OH Kb = 1.4 x 1011
NH4F(aq) NH4+(aq) + F(aq) (strong electrolyte)
NH4F is a salt of a weak acid and a weak base. On the other hand, another such salt (NH4CN) is a basic salt because CN ion is a stronger base than is an acid.
The pH of Salt Solutions. The pH of a salt solution is calculated just as it is for any weak acid or base. See Example 15.20 for an illustration of this important calculation.
EXAMPLE pH of a Solution Containing a Salt
Calculate the pH of 0.25 M C6H7O6Na (sodium ascorbate) solution.
First decide what ions undergo hydrolysis. Then use the appropriate ionization constant to calculate the equilibrium concentrations. Sodium ascorbate is a strong electrolyte.
C6H7O6Na(aq) C6H7O-(aq) + Na+(aq)
Ascorbate ion is a conjugate base of a weak acid (ascorbic acid, Table 15.3 text), and so it causes hydrolysis. Na+ does not and is neutral.
C6H7O6-(aq) + H2O C6H8O6 + OH
It is important to use the correct equilibrium constant value. Here we need the Kb for ascorbate ion. From Table 15.3 (text), we need the Ka for ascorbic acid, the conjugate acid of ascorbate ion.
Calculation
The OH ion concentration in a solution that is 0.25 M is obtained from the equilibrium constant expression
Set up an ICE box and let x = moles of ascorbate ion that undergo hydrolysis per L of solution.
Substitute into the equilibrium constant expression.
Apply the approximation that [C6H7O6-] = 0.25 x 0.25 M
pOH = log [OH] = 5.24 The pH is 14.0 5.24 = 8.76.
pOH = log [OH] = 5.24
The pH is 14.0 5.24 = 8.76.
OBJECTIVE CHECK
Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.
Given that Ka for formic acid (HCOOH) is 1.7 x 104, find the Kb value for formate ion, HCOO.
Calculate Ka for ammonium ion NH given the Kb value for ammonia in Table 15.4.
Predict whether the pH is > 7, < 7, or = 7 for aqueous solutions containing the following salts.
Calculate the pH of 0.021 M NaCN solution.
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