Lesson 7: Hydrolysis

In pure water at room temperature, only about one out of every 2 x 10^-9 water molecule is separated into H⁺ and OH^- ions at any one time.  This corresponds to a concentration of H⁺ (and OH^-) of about 1.0 x 10^-7 M.  The negative log of this number, 7, is the pH of neutral solution.

The solution is neutral not because the pH is seven, but because the concentrations of H⁺ and OH^- are the same.  At different temperatures, pure, neutral water has slightly different pH’s.

A fluoride ion in aqueous solution.

The fluoride has a strong attraction for hydrogen ions.

After the hydrolysis reaction, the solution contains free hydroxide ion and HF molecules.

The solution is basic.

Calcium ion in solution.

The calcium ion has a strong attraction for hydroxide ions.

After the hydrolysis reaction, there is free hydrogen ion and CaOH⁺.

The solution is acidic.

For this to happen requires a molecule that has a strongly electronegative atom and also a strongly electropositive atom, or, as in this example, an ionic compound whose negative ion binds to the H⁺ and whose positive ion binds to OH^-.

This is a bit of an oversimplification.  It implies that the two hydrogen atoms in water are somehow different and, of course, they are not.  What is true is that the hydogen-oxygen bonds are quite polar, so that the two hydrogens have a partial positive charge and the oxygen has a partial negative charge.  It is therefore relatively easy to split one of the hydrogens off of the water molecule leaving the shared electrons behind, resulting in a H⁺ and an OH^- ion.

And they are bases by the Arrhenius definition as well beause they cause hydroxide ions to form in water.
Here are the reactions by which two of these strongly basic ions hydrolyze water.

CH₃O^- + H₂O CH₃OH + OH^-

NH₂^- + H₂O NH₃ + OH^-

But a negative ion does not have to have a stronger attraction for H⁺ ions than OH^- ions do in order to cause a significant number of water molecules to hydrolyze.  The conjugate base of any weak acid will cause enough water molecules to hydrolyze to turn a solution basic.

Most negative ions fit this description.  In fact, our list of strong acids includes only six members: HCl, HBr, HI, HNO₃, H₂SO₄, and HClO₄.  Their conjugate bases are Cl^-, Br^-, NO₃^-, HSO₄^-, and ClO₄^-. 

These very weak conjugate bases do not cause hydrolysis – their attraction for H⁺ ions is simply not great enough.  All the other negative ions we will encounter form solutions of varying degrees of base strength when they are dissolved in water.

These ions can be added to water as part of a soluble ionic compound, usually a sodium or potassium salt.  For example, when sodium fluoride is dissolved in water, the solution becomes basic because the fluoride ion causes a hydrolysis reaction.

The second reaction occurs because HF is a weak acid.  It is also necessary, of course, for the NaF to be soluble, or the second reaction never gets a chance to occur.  However, virtually all sodium and potassium salts are soluble in water.

To determine whether an ionic compound will cause basic hydrolysis, identify the negative ion in the compound.  Next write the formula of the acid that ion forms when a H⁺ ion bonds to it.  If this acid is not on the list of strong acids, it’s a weak acid, hydrolysis will occur, and the solution will become basic.

Select the compounds that you think will cause basic hydrolysis.

Once you have completed this exercise, see if you can write the hydrolysis reactions that will occur.  Here’s an example: KBrO₃ will cause hydrolysis because HBrO₃ is a weak acid.  The hydrolysis reaction that will occur is

BrO₃^- + H₂O HBrO₃ + OH^-

KBrO₃

  The hydrolysis reaction that will occur is

BrO₃^- + H₂O HBrO₃ + OH^-

K₂SO₃

SO₃^2- + H₂O HSO_3^- + OH^-

We have shown each positive ion bonding to only one hydroxide ion, even though some, such as the Al³⁺ and Ca²⁺ could bond to two or three.  In the same way, a negative ion like SO₄^2- could attract two H⁺ ions, but in the hydrolysis reaction we wrote, we showed it bonded to only one.  To predict what actually happens requires some calculations.  It will be sufficient for our purposes to show only a single H⁺ or OH^- bonded to each ion.

Just as the negative ions that cause basic hydrolysis come from weak acids, the positive ions that cause acid hydrolysis come from weak bases.

You may recall that the list of strong bases included NaOH and KOH.  RbOH and CsOH are also strong bases.  LiOH is the only Group IA metal hydroxide that is not.

The metal hydroxides in this list are weak bases because they do not dissolve well in water.  They are either insoluble or only slightly soluble.

In general, only the Group IA metals (except Li) form highly soluble hydroxides and thus, strong bases.  Ba(OH)₂, Sr(OH)₂, and AuOH are slightly soluble.

NH₄OH is also a weak base.  However, it can dissolve in water and so is very soluble.  It is the one weak base we will encounter that is.

The ammonium ion forms a weakly basic hydroxide because, like most positive metal ions, it has a relatively strong attraction for hydroxide ions.  This is the property that results in acid hydrolysis.  Unlike the metals, its undissociated hydroxide dissolves in aqueous solution because it can form many hydrogen bonds with water.

All of the positive ions that come from weak bases have a strong attraction for OH^-.  Therefore, when they dissolve in water, they induce hydrolysis by binding to the OH^- ion in water and releasing the H⁺ ions into solution, turning the solution acidic.

The stronger the attraction the positive ion has for hydroxide, the more acidic the resulting solution will be.  The acidity of the solution also depends on the concentration of the ions.  We will only be interested in being able to say whether the solution becones acidic or not.

These, of course, are the positive ions associated with NaOH, KOH, RbOH, and CsOH.

Since Ba(OH)₂, Sr(OH)₂, and AuOH are slightly soluble, the Ba²⁺, Sr²⁺, and Au⁺ ions do not cause appreciable hydrolysis as long as their concentration is low.

Ions that cause acidic hydrolysis can be added to a solution in the form of ionic compounds, usually as nitrates or chlorates, but sometimes as chlordies, bromides, iodides, or sulfates, depending on the solubility of these compounds.

All nitrate, acetate, and chlorate salts are soluble.  Chlorides, bromides, iodides, and sulfates can be soluble or insoluble, depending on which positive ion they are combined with.

Choose the compounds that you think will cause an acidic hydrolysis reaction to occur when they are dissolved in water.

Once you have completed this exercise, see if you can write the hydrolysis reactions that will occur.  Here is an example.  The positive ion in AgNO₃ is Ag⁺.  This forms AgOH, an insoluble hydroxide, thus Ag⁺ will cause an acidic hydrolysis reaction:

Ag⁺ + H₂O AgOH + H⁺

Here are the answers you should have determined:

AgNO₃

Ag⁺ + H₂O AgOH + H⁺

NH₄NO₃

NH₄⁺ + H₂O NH₄OH + H⁺

Fe(NO₃)₃

Fe³⁺ + H₂O FeOH²⁺ + H⁺

There are actually not too many examples of common ionic compounds that are both soluble and consist of a weak acid combined with a weak base.  The ammonium compounds are the most plentiful.  Most of the rest are only slightly soluble if they are soluble at all.

When this happens, you must know the relative strengths of the weak acid and the weak base to say whether the solution will be acidic or basic.  For our purposes it will be sufficient just to say that the solution will be approximately neutral.