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7.22: Hydrolysis of Salts- Equations

Hydrolysis of Salts: estimation of pH for a salt solution

A salt is an ionic compound that is formed when an acid and a base neutralize each other. While it may seem that salt solutions would always be neutral, they can frequently be either acidic or basic.

Consider the salt formed when the weak acid hydrofluoric acid is neutralized by the strong base sodium hydroxide. The molecular and net ionic equations are shown below.

\begin{align} &\ce{HF} \left( aq \right) + \ce{NaOH} \left( aq \right) \rightarrow \ce{NaF} \left( aq \right) + \ce{H_2O} \left( l \right) \\ &\ce{HF} \left( aq \right) + \ce{OH^-} \left( aq \right) \rightarrow \ce{F^-} \left( aq \right) + \ce{H_2O} \left( l \right) \end{align}

Since sodium fluoride is soluble, the sodium ion is a spectator ion in the neutralization reaction. The fluoride ion is capable of reacting, to a small extent, with water, accepting a proton.

$\ce{F^-} \left( aq \right) + \ce{H_2O} \left( l \right) \rightleftharpoons \ce{HF} \left( aq \right) + \ce{OH^-} \left( aq \right)$

The fluoride ion is acting as a weak Brønsted-Lowry base. The hydroxide ion that is produced as a result of the above reaction makes the solution slightly basic. Salt hydrolysis is a reaction in which one of the ions from a salt reacts with water, forming either an acidic or basic solution.

The ability of a cation or anion in a salt to undergo hydrolysis, and affect the pH, depends on its strength as an acid or a base, respectively. Only cations that are the conjugate acid of a weak base are strong enough to undergo hydrolysis. This is because a weak base (low Kb value) will generate a strong conjugate acid (high Ka value) (remember that KaKb=Kw for a conjugate acid-base pair).

Examples for cations:

• Cation generated from a strong base: example Na+ from NaOH

NaOH(aq)         →      Na+(aq)                          +       HO-(aq)

strong base (high Kb)       weak conjugate acid (low Ka)

Therefore, Na+ present in a salt cannot react with H2O. Na+ is a neutral cation:

Na+(aq)   +  H2O(l)     →   N.R. (no reaction)

• Cation generated from a strong base: example NH4+ from NH3

NH3(aq)   + H2O(l)      ⇔      NH4+(aq)                          +       HO-(aq)

weak base (low Kb)            stronger conjugate acid (high Ka)

Therefore, NH4+ present in a salt can react with H2O in a hydrolysis reaction. NH4+ is an acidic cation:

NH4+(aq)   + H2O(l)      ⇔      NH3(aq)   +    H3O+(aq)

Examples for anions:

• Anion generated from a strong acid: example Cl-from HCl

HCl(aq)                 →           Cl-(aq)                          +       H+(aq)

strong acid (high Ka)       weak conjugate base (low Kb)

Therefore, Cl- present in a salt cannot react with H2O. Cl- is a neutral anion:

Cl-(aq)    +  H2O(l)     →   N.R. (no reaction)

• Anion generated from a weak acid: example F- from HF

HF(aq)                          F-(aq)                          +       H+(aq)

weak acid (low Ka)       stronger conjugate base (high Kb)

Therefore, F- present in a salt can react with H2O. F- is a basic anion:

F-(aq)    +  H2O(l)   ⇔   HF(aq) +   OH-(aq)

Estimating the pH of salts

To be able to estimate the pH for a salt, we need to identify the acid-base strength of the cation and the anion in solution, and their ability to undergo hydrolysis with H2O. Remember: only relatively strong conjugate acid and bases can react with water.

Salts That Form Basic Solutions

When solid sodium fluoride is dissolved into water, it completely dissociates into sodium ions and fluoride ions. The sodium ions do not have any capability of hydrolyzing (NaOH is a strong base), but the fluoride ions hydrolyze to produce a small amount of hydrofluoric acid and hydroxide ion. That is because F- is the conjugate base of a weak acid (HF), therefore F- can undergo hydrolysis in water:

$\ce{F^-} \left( aq \right) + \ce{H_2O} \left( l \right) \rightleftharpoons \ce{HF} \left( aq \right) + \ce{OH^-} \left( aq \right)$

Salts that are derived from the neutralization of a weak acid $$\left( \ce{HF} \right)$$ by a strong base $$\left( \ce{NaOH} \right)$$ will always produce salt solutions that are basic.

Salts That Form Acidic Solutions

Ammonium chloride $$\left( \ce{NH_4Cl} \right)$$ is a salt that is formed when the strong acid $$\ce{HCl}$$ is neutralized by the weak base $$\ce{NH_3}$$. Ammonium chloride is soluble in water. The chloride ion produced is incapable of hydrolyzing because it is the conjugate base of the strong acid $$\ce{HCl}$$. In other words, the $$\ce{Cl^-}$$ ion cannot accept a proton from water to form $$\ce{HCl}$$ and $$\ce{OH^-}$$, as the fluoride ion did in the previous section. However, the ammonium ion is capable of reacting slightly with water, donating a proton and so acting as an acid. That is because NH3 is a weak base.

$\ce{NH_4^+} \left( aq \right) + \ce{H_2O} \left( l \right) \rightleftharpoons \ce{H_3O^+} \left( aq \right) + \ce{NH_3} \left( aq \right)$

Salts That Form Neutral Solutions

A salt that is derived from the reaction of a strong acid with a strong base forms a solution that has a pH of 7. An example is sodium chloride, formed from the neutralization of $$\ce{HCl}$$ by $$\ce{NaOH}$$. A solution of $$\ce{NaCl}$$ in water has no acidic or basic properties, since neither ion (Na+ or Cl-) is capable of hydrolyzing with H2O. Other salts that form neutral solutions include potassium nitrate $$\left( \ce{KNO_3} \right)$$ and lithium bromide $$\left( \ce{LiBr} \right)$$. The table below summarizes how to determine the acidity or basicity of a salt solution.

 Table $$\PageIndex{1}$$ Salt formed from: Salt Solution Example Strong acid $$+$$ Strong base Neutral NaCl Strong acid $$+$$ Weak base Acidic NH4Cl Weak acid $$+$$ Strong base Basic NaF

Salts formed from the reaction of a weak acid and a weak base are more difficult to analyze because competing hydrolysis reactions between the cation and the anion. These salts will not be considered in this concept.

Summary

• Salt hydrolysis is defined.
• Salt hydrolysis reactions resulting in acidic, basic, or neutral solutions are described.

Contributors and Attributions

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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