Salt Hydrolysis

Acid-base neutralization reactions are defined as an acid + base reacting to give water and a salt written generally as

\[ \mathrm{\textcolor{Red}{HA}}(aq) + \mathrm{\textcolor{green}{BOH}}(aq) \rightleftharpoons \mathrm{H_2O}(l) + \mathrm{\textcolor{Peach}{B}\textcolor{TealBlue}{A}}(aq)\]

where AB is the salt written in molecular form.

In the ionic form, the salt would be written as

\[\mathrm{\textcolor{Peach}{B}\textcolor{TealBlue}{A}} (aq) \longrightarrow \textcolor{Peach}{\mathrm{B^+}}(aq) + \textcolor{TealBlue}{\mathrm{A^-}}(aq)\]

The goal of this article is to address the question: “Will the components of a salt (the cation and/or the anion), react with water to change the pH of solution?”

Acidic Salt

Consider the reaction of the conjugate acid, B⁺, with water.

\[\textcolor{Peach}{\mathrm{B^+}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{green}{\mathrm{B}}(aq)\] This reaction is equivalent to writing the following equation

\[\textcolor{Peach}{\mathrm{HB^+}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{green}{\mathrm{B}}(aq)\]

The proton is now explicitly shown to illustrate the proton being donated to water. B⁺ and HB⁺ are the same (think of it as a NH₄⁺ cation). The positive charge on B is due to the extra proton that is bound to it.

If the conjugate acid, HB⁺, is strong enough to react with water, it will produce H₃O⁺ thereby lowering the pH of solution.

A salt that, when added to water, lowers the pH of the solution, is known as an acidic salt.

Basic salt

Consider the reaction of the conjugate base, A^–, with water

\[ \textcolor{TealBlue}{\mathrm{A}^-}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-} + \textcolor{Red}{\mathrm{HA}}\]

If the conjugate base, A^–, is strong enough to react with water, it will produce OH^–, thereby raising the pH of the solution.

A salt that, when added to water, raises the pH of the solution, is known as a basic salt.

Strong acid + strong base

A reaction between a strong acid and strong base gives a neutral solution.

\[\mathrm{H\textcolor{TealBlue}{Cl}}(aq) + \mathrm{\textcolor{Peach}{Na}OH}(aq) \rightleftharpoons \mathrm{H_2O}(l) + \mathrm{\textcolor{Peach}{Na}\textcolor{TealBlue}{Cl}}(aq)\]

Group 1A and 2A metals do not react with water to produce hydroxide. Na⁺, a group 1A metal, therefore will not undergo acid ionization and no OH^– is produced and the pH does not change.

\[\mathrm{\textcolor{Peach}{Na^+}}(aq) + \mathrm{H_2O}(l) \longrightarrow \mathrm{no~reaction}\]

The anion, Cl^–, does not undergo base ionization as it is too weak to react with water (its K_b << 1). The chloride anion is a conjugate base to a strong acid, and therefore, will never become HCl (strong acids dissociate completely). To illustrate the reaction that will not take place, we write the following

\[\mathrm{\textcolor{TealBlue}{Cl^-}}(aq) + \mathrm{H_2O}(l) \longrightarrow \mathrm{no~reaction}\]

Because neither component of the salt, NaCl, react with water, NaCl is a neutral salt.

Summary

Group 1A and 2A metals do not react with water to change the pH (neutral cations)
Conjugate bases of strong acids do not react with water to change the pH (neutral anions)
Neutral salts do not affect the pH of a solution

Strong acid + weak base

A reaction between a strong acid and weak base gives a slightly acidic solution.

\[\mathrm{H\textcolor{TealBlue}{Cl}}(aq) + \mathrm{NH_3}(aq) \rightleftharpoons \mathrm{\textcolor{Peach}{NH_4}\textcolor{TealBlue}{Cl}}(aq)\]

The above reaction can be written out in two steps to illustrate the explicit production of both water and salt from this neutralization reaction.

\[\begin{align} \mathrm{NH_3}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{\textcolor{Peach}{NH_4}OH}(aq) \end{align}\] \[\begin{align} \mathrm{HCl}(aq) + \mathrm{\textcolor{Peach}{NH_4}OH}(aq) \rightleftharpoons \mathrm{H_2O}(l) + \mathrm{\textcolor{Peach}{NH_4}\textcolor{TealBlue}{Cl}}(aq) \end{align}\]

We have already discussed that Cl^– will not react with water.
However, NH₄⁺ will since it is a conjugate acid to a weak base. The K_a of ammonium ion is less than 1. Since the following reaction will occur

\[\mathrm{\textcolor{Peach}{NH_4^+}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \mathrm{\textcolor{green}{NH_3}}(aq)\]

NH₄⁺ will cause the pH of the solution to decrease giving a slightly acidic solution.

Summary

Conjugate acids to weak bases will react with water to lower the pH of solution
Salts that lower the pH of solution are called acidic salts
Cations that lower the pH of solution are conjugates to a weak base or small, high-charge metals (such as Al³⁺, Fe³⁺, Cr³⁺ and Sc³⁺) and are collectively called acidic cations.

Weak acid + strong base

A reaction between a weak and strong base gives a slightly basic solution.

\[\mathrm{H\textcolor{TealBlue}{F}}(aq) + \mathrm{\textcolor{Peach}{K}OH}(aq) \rightleftharpoons \mathrm{H_2O}(l) + \mathrm{\textcolor{Peach}{K}\textcolor{TealBlue}{F}}(aq)\]

We have already discussed that Group 1A and 2A metals do not react with water to change the pH of solution. Therefore, K⁺ does not affect the pH of solution. However, F^– can react with water as it is a conjugate base to a weak acid, thereby raising the pH of the solution.

\[\mathrm{\textcolor{TealBlue}{F^-}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{\textcolor{red}{HF}} + \mathrm{OH^-}(aq)\]

Summary

Conjugate bases to weak acids will react with water to raise the pH of solution
Salts that raise the pH of solution are called basic salts
Anions that are conjugates to weak acids (e.g. CN^–, NO₂^–, CH₃COO^–) are called basic anions

Weak acid + weak base

In this scenario, one must examine the K_a and K_b values of the acid and base being mixed. Consider the following reaction

\[\mathrm{\textcolor{red}{HF}}(aq) + \mathrm{\textcolor{green}{NH_3}}(aq) \rightleftharpoons \mathrm{\textcolor{Peach}{NH_4}\textcolor{TealBlue}{F}}(aq)\]

Both components can react with water to change the pH of solution.

F^– is a conjugate base to the weak acid, HF. F^– can react with water to produce OH^– (raise pH). * NH₄⁺ is a conjugate acid to a weak base, NH₃. NH₄⁺ can react with water to produce H₃O⁺ (lower pH).

Since both components can raise/lower the pH of solution, which one will win? You have to compare the K_a of the acid to the K_b of the base.

K_a(HF) = 3.5 × 10^–4; therefore, K_b(F^–) = 2.86 × 10^–11
K_b(NH₃) = 1.76 × 10^–5; therefore, K_a(NH₄⁺) = 5.68 × 10^–10

Since the K_a of NH₄⁺ is greater than the K_b of F^–, the acid (NH₄⁺) will react slightly more with water to produce slightly more H₃O⁺ giving a slightly acidic solution!

Summary

If K_a of conjugate acid is greater than the K_b of the conjugate base, the solution will be slight acidic * If K_a of conjugate acid is less than the K_b of the conjugate base, the solution will be slight basic

Practice Problems

Question 1

Ammonium chloride, NH₄Cl, is dissolved in pure water. Is the resulting solution acidic, basic, or neutral?

Solution

Acidic

NH₄⁺ is an acidic cation that is a conjugate acid to a weak base, NH₃. It is able to react with water to produce H₃O⁺.

\[\mathrm{NH_4^+}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \mathrm{NH_3}(aq)\]

Question 2

Sodium cyanide, NaCN, is dissolved in pure water. Is the resulting solution acidic, basic, or neutral?

Solution

Basic

CN^– is a basic anion that is a conjugate base to a weak acid, HCN. It is able to react with water to produce OH^–.

\[\mathrm{CN^-}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-}(aq) + \mathrm{HCN}(aq)\]

Question 3

Potassium nitrate, KNO₃, is dissolved in pure water. Is the resulting solution acidic, basic, or neutral?

Solution

Acidic

NO₃^– is an basic anion that is a conjugate base to a strong acid, HNO₃ It will not react with water to produce OH^–.

Question 4

What is the pH and pOH (to two decimal places) of a 50.0 mL 0.300 M KF aqueous solution (at 25 °C)? pK_b(F^–) = 10.83

Solution

Salt dissociates and delivers a weak base (F^–).

\[\mathrm{NaF}(s) \longrightarrow \mathrm{Na^+}(aq) + \mathrm{F^-}(aq)\] This base is a conjugate to a weak acid (HF) and can therefore react with water to produce OH^–.

\[\mathrm{F^-}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-}(aq) + \mathrm{HF}(aq)\] Make ICE table. Proceeds right.

	F^–(aq)	OH^–(aq)	HF(aq)
I	0.300	0	0
C	-x	+x	+x
E	0.300 – x	x	x

\[\begin{align*} K_{\mathrm{b}} &= 10^{-\mathrm{p}K_{\mathrm{b}}} \\ &= 10^{-10.83} \\ &= 1.48\times 10^{-11}\\[3ex] \dfrac{[\mathrm{OH^-}][\mathrm{HF}]}{[\mathrm{F^-}]} = K_{\mathrm{b}}\\[1.5ex] \dfrac{(x)(x)}{0.300-x} &= 1.48\times 10^{-11}\\[1.5ex] \dfrac{x^2}{0.300} &= 1.48\times 10^{-11}\\[1.5ex] x &= 2.11\times 10^{-6}\\[3ex] \% x &= \dfrac{2.11\times 10^{-6}}{0.300}\times 100\% = 7\times 10^{-4}\% \\[3ex] \mathrm{pOH} &= -\log(2.11\times 10^{-6}) \\ &= 5.68 \\[3ex] \mathrm{pH} &= \mathrm{p}K_{\mathrm{w}} - \mathrm{pOH} \\ &= 14 - 5.68 \\ &= 8.32 \end{align*}\]

Timed Assessments

Acid/Base Equilibrium

VIDEO

Answers

pH = 8.27; pOH = 5.73; basic
pH = 8.39; pOH = 5.61; basic
pH = 4.73; pOH = 9.07; acidic

Last updated on Jul 19, 2021