Hydrolysis and Conjugate Strength

Let's return to the reaction we've been using to exemplify weak acid processes and calculations: the dissociation of HCN.

HCN (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + CN^- (aq)

This reaction is reversible, which is what makes HCN a weak acid. HCN isn't very "good" at giving up protons to water, and CN^- is able to accept protons from hydronium, so it can proceed in reverse as well as forward. This suggests that the CN^- ion on its own might be a good enough proton acceptor to take a proton from water, as shown below.

CN^- (aq) + H₂O (l) ⇌ OH^- (aq) + HCN (aq)

Note that this reaction is not the reverse of the first one. Instead, it shows the cyanide ion acting as a base, accepting a proton, and generating hydroxide ion. Note that we still have double arrows: this reaction is reversible, making CN^- a weak base. This is similar to the reaction of acetate we observed in the previous section.

C₂H₃O₂^- (aq) + H₂O (l) ⇌ OH^- (aq) + HC₂H₃O₂ (aq)

Of course, CN^- is an ion, not a molecule. This means that it can never exist on its own (without a positive ion to balance it). We can prepare a solution of CN^- by dissolving an ionic compound like KCN or NaCN in water. The exact cation doesn't matter than much in this case, as it will be a spectator ion.

K⁺ (aq) + CN^- (aq) + H₂O (l) ⇌ OH^- (aq) + HCN (aq) + K⁺ (aq)

So, what we observe when the ionic compound KCN is added to water is that the cyanide ion acts as a weak base, forming hydroxide and HCN. If we wish, we can combine the ions on each side, as seen below.

KCN (aq) + H₂O (l) ⇌ KOH (aq) + HCN (aq)

This reaction is not fundamentally different than the reaction of ammonia, apart from the fact that in this case our base has an accompanying ion.

NH₃ (aq) + H₂O (l) ⇌ OH^- (aq) + NH₄⁺ (aq)

However, when an ionic compound like KCN (and many others we will soon encounter) acts as an acid or base, we give that reaction a special name: hydrolysis. For the rest of this section, we will be discussing how ions can act as acids and bases.

Acid/Base Strength in Conjugate Pairs

We have learned about two kinds of acids and bases: strong and weak. We will now add a third, which may seem a bit silly at first, but is important to our discussion of hydrolysis. This third category is the negligible acids and bases. A negligible acid/base is a substance that may be called the "conjugate acid" or "conjugate base" of some other species, does not actually donate/accept protons in water.

For example, the molecule CH₄ can reasonably called the "conjugate acid" of the ion CH₃^-. However, this does not mean that CH₄ actually exhibits any acidic reactivity. It does not donate protons in solution in water. Thus it is what we would call a "negligible acid."

Similarly, the ion Cl^- is the conjugate base of HCl; however, when dissolved in water, Cl^- does not accept any protons form water, so while we might call it a "base" in terms of its conjugate relationship, it is a "negligible base."

With this in mind, let's talk more about acid/base strength when it comes to conjugate pairs. As a reminder, a conjugate pair is a pair of species that differs by a single H⁺. Some examples would be HF/F^-, HCO₃^-/CO₃^2-, and HPO₄^2-/PO₄^3-. In every conjugate pair, there is an important relationship between the strength of the acid and the strength of the base.

For example, you know that HCl is a strong acid, meaning it is extremely "eager" to give up a proton and form Cl^-. Logically, then, it makes sense that Cl^- is not very "eager" to gain a proton, in fact, as I have just mentioned, it does not do so at all. It is a negligible base. This pattern holds for all strong acids: they have negligible conjugate bases. As a reminder, you have been asked to memorize which acids are strong; these are listed below.

HCl HBr HI HNO₃ HClO₄ H₂SO₄

Therefore, we can conclude that the ions below are negligible bases.

Cl^- Br^- I^- NO₃^- ClO₄^- HSO₄^-

On the other hand, if an acid is weak (meaning it is only so-so at giving up protons) its conjugate base will also be weak (so-so at accepting protons). There are many more weak acids than strong, but a few weak acids are listed below.

HF HNO₂ HC₂H₃O₂ HClO₂

Therefore, we can conclude that the ions below are weak bases.

F^- NO₂^- C₂H₃O₂^- ClO₂^-

Finally, while we have not learned many weak bases, the same principle can be applied to them. The conjugate acid of a weak base is a weak acid. The main example we will see of this is the conjugate pair NH₃/NH₄⁺. Since ammonia is a weak base, the ammonium ion is a weak acid.

You may have noticed that I have not mentioned the conjugate acids of strong bases. The reason is that, in this course, we do not encounter any strong bases other than hydroxides. However, some such bases exist; examples include CH₃O^-, NH₂^-, HCC^-, and H^-. The conjugate acids of these species are all negligible acids (as you might expect), however we will not encounter them and I mention them only for the sake of completeness.

Acid/Base Properties of Salt Solutions

At the beginning of this section, I described what happens when KCN, a salt (ionic compound) is dissolved in water. Of course, as an aqueous ionic compound, the ions separated in solution. Then, the CN^- ion behaved as a weak base, reacting with water to make OH^-, with K⁺ acting as a spectator ion. This should make more sense now in the context of what we just learned: because CN^- is the conjugate base of the weak acid HCN, it is a weak base.

Using this new information about conjugate pairs, we can now predict whether a given ionic compound, when dissolved in water, will produce an acidic, basic, or neutral solution. Let's examine how each of these outcomes might arise.

Acidic

Every ionic compound has a cation and an anion; if an ionic compound is going to generate an acidic solution (pH < 7) in water, it is usually (though not always) due to the cation (positive ion). There are two common ways for this to happen.

First, as mentioned above the ammonium ion (NH₄⁺) is a weak acid. It reacts with water to make hydronium. Therefore, solutions of compounds containing ammonium are almost always acidic. Common examples include NH₄Cl, NH₄NO₃, and (NH₄)₂SO₄.

We can write equations showing this kind of hydrolysis reaction. Let's see one for NH₄Cl, in a few different flavors.

NH₄Cl (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + NH₃ (aq) + Cl^- (aq) molecular

NH₄⁺ (aq) + Cl^- (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + NH₃ (aq) + Cl^- (aq) full ionic

NH₄⁺ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + NH₃ (aq) net ionic

You should be able to write these kinds of equations for hydrolysis reactions of ionic compounds.

Second, there are some cations, especially those outside Groups IA and IIA, that can cause aqueous solutions to be acidic. For example, if FeCl₃ is dissolved in water, the resulting solution can often be highly acidic. This should surprise you somewhat, since we usually think of acids as H⁺ donors, and there is no hydrogen at all in the formula of FeCl₃.

The explanation comes from the third definition of acids we learned in Lesson 5: Lewis acidity. As you hopefully recall, under the Lewis definition an acid is a lone pair acceptor. In the case of FeCl₃, what happens is that the Fe³⁺ ion acts by accepting a lone pair from water, bonding to it and forming a strange complex ion.

Fe³⁺ (aq) + H₂O (l) → [Fe-OH₂]³⁺ (aq)

Being bonded to the iron ion weakens the O-H bond in the water molecule, turning it into an H⁺ donor, which can react with another water molecule to form hydronium.

[Fe-OH₂]³⁺ (aq) + H₂O (l) → H₃O⁺ (aq) + [Fe-OH]²⁺ (aq)

The actual dynamics of these reactions can get quite complicated, often involving as many as four to six water molecules. However, it is not important that you be able to write equations for these reactions, just that you know that metal ions outside Group IA and IIA (so not Na⁺, K⁺, Ca²⁺, etc.) generally create acidic solutions. Examples might include W(NO₃)₃, CuSO₄, and NiBr₂.

Basic

If an ionic solution in water is going to be basic, it will almost always be due to the anion (negative ion). As we learned earlier in this section, many anions (the conjugate bases of weak acids) are weakly basic. So compounds of these ions will hydrolyze in water to form basic solutions (pH > 7). Note: this covers a lot of ions, so it is very common for ionic compounds to be basic in solution. Examples include the KCN we saw earlier, Na₂CO₃, CaF₂, and LiClO₂. We can write an equation showing the hydrolysis of this last compound, again in a few different ways.

LiClO₂ (aq) + H₂O (l) ⇌ HClO₂ (aq) + LiOH (aq) molecular

Li⁺ (aq) + ClO₂^- (aq) + H₂O (l) ⇌ HClO₂ (aq) + Li⁺ (aq) + OH^- (aq) full ionic

ClO₂^- (aq) + H₂O (l) ⇌ HClO₂ (aq) + OH^- (aq) net ionic

Neutral

For an ionic compound to produce a neutral solution when dissolved in water, it must avoid both these cases. So its cation cannot be NH₄⁺ or a transition metal, and its anion cannot be a weak base. This means the cation must be from Groups IA or IIA, and the anion must be one of the six negligible bases we have learned (chloride, bromide, iodide, nitrate, sulfate, and perchlorate). If both these conditions are met, then the compound will not undergo hydrolysis in water, and the solution will be neutral. Examples would include CaI₂, LiNO₃, NaBr, and Cs₂SO₄.

Practice Problems

1. Predict whether each of the following species would be a strong, weak, or negligible acid/base
   NO₃^-
   HNO₂
   CO₃^2-
   NH₄⁺
   I^-
   NH₃
   PO₄^3-

2. Predict whether each of the following salts would have an acidic, basic, or neutral pH when dissolved in water
   KF
   Ag₂SO₄
   SrBr₂
   Pb(NO₃)₄
   NH₄I
   NaNO₃

Practice Problem Answers

1. Predict whether each of the following species would be a strong, weak, or negligible acid/base
   NO₃^- negligible base (since its conjugate acid is strong)
   HNO₂ weak acid (not on list of strong acids)
   CO₃^2- weak base (since its conjugate acid is weak)
   NH₄⁺ weak acid (since its conjugate base is weak)
   I^- negligible base (since its conjugate acid is strong)
   NH₃ weak base (just one to know)
   PO₄^3- weak base (since its conjugate acid is weak)

2. Predict whether each of the following salts would have an acidic, basic, or neutral pH when dissolved in water
   KF basic (fluoride is a weak base)
   Ag₂SO₄ acidic (silver is not in Group IA or IIA)
   SrBr₂ neutral (strontium is in Group IIA, bromide is a negligible base)
   Pb(NO₃)₄ acidic (lead is not in Group IA or IIA)
   NH₄I acidic (ammonium is a weak acid)
   NaNO₃ neutral (sodium is in Group IA, nitrate is a negligible base)