Salt Hydrolysis

Prediction of whether a salt solution is

Acidic, Basic or Neutral

When a salt is dissolved in water, the ions making up that salt can hydrolyze. Hydrolysis is simply the reaction of a substance, in this case an ion or ions, with water. We have talked about hydrolysis reactions previously, although we have not called them that. The ionization of a weak acid or a weak base in water is a hydrolysis reaction. This is the case, since the formation of ions from the weak acid or weak base depends on that reaction. For example, the ionization of acetic acid is:

HC₂H₃O₂ (aq) + H₂O (l) <==> H₃O⁺ (aq) + C₂H₃O₂^- (aq)

This reaction is an hydrolysis, we called this reaction, the ionization of acetic acid, but in essence it is due to the reaction of the acid with water, which is acting as a base. Likewise, weak bases behave in the same way. Now the water is acting as the acid. For example:

NH₃ (g) + H₂O (l) <==> NH₄⁺ (aq) + OH^- (aq)

It is also a hydrolysis reaction.

Whether the components of a salt, i. e., the cation and the anion, will react with water when dissolved in it, will depend in whether they are the conjugate acid of a strong base or the cation present in a strong base, or the conjugate acid of a weak base or vice versa.

We know that for NaOH, the reaction in water can be simply written as:

NaOH (aq) ==> Na⁺ (aq) + OH^- (aq)

The fact that we can write this reaction with a single arrow pointing toward the products tells us that no backward reaction is taking place, i. e. Na⁺(aq) and OH^- (aq) do not combine in solution to form NaOH again. Likewise we can conclude that the reaction of Na⁺ (aq) with water itself is negligible, i. e, Na⁺ (aq) does not hydrolyzes in water. For the cation of any strong base, this is always the case, they will not hydrolyze, so Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ will not hydrolyze to any appreciable extent in water. The case is completely different if the cation is the conjugate acid of a weak base, or if the cation itself can form a complex ion with water. For example, NH₄⁺ is the conjugate acid of the weak base NH₃. When NH₄⁺ is dissolved in water, it undergoes hydrolysis according to the following reaction:

NH₄⁺ (aq) + H₂O (l) <==> NH₃ (aq) + H₃O⁺ (aq)

The cation is acting as a weak acid, its hydrolysis produces H₃O⁺ and therefore we should expect that the pH of the solution will become acidic as a consequence of this. A similar situation is encountered in Al³⁺ (aq). This ion can form a strong complex with water which has the formula [Al(H₂O)₆]³⁺, which can act as an acid according to:

[Al(H₂O)₆]³⁺ + H₂O (l) <==> [Al(H₂O)₅(OH)]²⁺ (aq) + H₃O⁺ (aq)

Just like in the case of NH₄⁺, we should expect the pH to be acidic when [Al(H₂O)₆]³⁺ is hydrolyzed, for the exact same reasons, H₃O⁺ is being produced as a result of the reaction of the complex ion with water. But the situation is not always that simple, the pH can also be altered by the hydrolysis of the anion. The same type of reasoning applies here, if the anion is the conjugate base of a strong acid, no hydrolysis will be observed, but if the anion is the conjugate base of a weak acid, hydrolysis will take place.

Therefore, CN^-, which is the conjugate base of the weak acid HCN, will hydrolyze in water according to the reaction:

CN^- (aq) + H₂O (l) <==> HCN (aq) + OH^- (aq)

We now should expect the pH to become basic due to the increase in concentration of OH^- because of the hydrolysis of the anion.

Therefore, we have four choices, the salt could be formed from the cation of a strong base and the anion of a strong acid, the cation of a strong base and the anion of a weak acid, the cation of a weak base and the anion of a strong acid or the cation of a weak base and the anion of a weak acid. How the pH is affected will depend then to the extent of the hydrolysis of cation and anion in water. The following table summarizes these observations:

Salt
Cation
Anion
Does
cation
Hydrolyzes?
Does
anion
Hydrolyzes?
K_a
K_b
pH
SB-SA
SB
SA
N
N
N/A
N/A
7.00
SB-WA
SB
WA
N
Y
N/A
Y
> 7.00
WB-SA
WB
SA
Y
N
Y
N/A
< 7.00
WB-WA
WB
WA
Y
Y
Y
Y
If K_a = K_b,pH = 7.00
WB-WA
WB
WA
Y
Y
Y
Y
If K_a > K_b, pH < 7.00
WB-WA
WB
WA
Y
Y
Y
Y
If K_a < K_b, pH > 7.00
SB stands for Strong Base, SA stands for Strong Acid, WB stands for Weak Base and WA stands for Weak Acid.

Let's do several examples to illustrate the concepts summarized in this table:

Let's take NaCl, when NaCl is dissolved in water the following reaction is observed:

NaCl (aq) ==> Na⁺ (aq) + Cl^- (aq)

The cation of NaCl, Na⁺, is the cation of a strong base, NaOH, therefore does not hydrolyzes, the anion, Cl^-, is the conjugate base of a strong acid, therefore its hydrolysis is also non-existent, since there is no hydrolysis present in this salt, the only source of H₃O⁺ and OH^- is the auto-ionization of water, so we should expect the pH to stay at 7.00.

Let's look now at the salt, NaC₂H₃O₂, the cation is still Na⁺, so there is no hydrolysis, but now the anion is C₂H₃O₂^-, the acetate ion, the conjugate base of a weak acid, which will hydrolyze in water in the following fashion:

C₂H₃O₂^- (aq) + H₂O (l) <==> HC₂H₃O₂ (aq) + OH^- (aq)

Since only the anion undergoes hydrolysis, producing OH^-, we should expect the pH of that solution to become basic.

In the case of NH₄Cl, now the Cl^- anion is not hydrolyzing, but NH₄⁺ is hydrolyzing according to the previously shown equation. Therefore, H₃O⁺ is being produced in this reaction and the pH should become acidic.

Finally, when we have salts in which both ions hydrolyze, the pH will depend on which one hydrolyzes the most. We know this by looking up their K_a and K_b values. If the anion hydrolyzes more, that is the value of K_b for that anion is bigger that the value of K_a for the cation, the solution will become basic, since more OH^- will be produced. If the opposite is true, K_a of the cation is bigger than K_b of the anion, the solution will be acidic, since more H₃O⁺ will be produced. If K_a of the cation happens to be equal to K_b of the anion, the pH will be neutral, since the same amounts of H₃O⁺ and OH^- are being produced, canceling each other out.

As an exercise, try to determine the resulting pH of a solution when the following salts are dissolved in water:

a. NaF

b. C₆H₅NH₃Cl

c. AlCl₃(Remember the true nature of Al³⁺ (aq) in water)

d. KNO₃

e. NH₄CN

If you need any help, do not hesitate to ask.

Salt	Cation	Anion	Does cation Hydrolyzes?	Does anion Hydrolyzes?	K_a	K_b	pH
SB-SA	SB	SA	N	N	N/A	N/A	7.00
SB-WA	SB	WA	N	Y	N/A	Y	> 7.00
WB-SA	WB	SA	Y	N	Y	N/A	< 7.00
WB-WA	WB	WA	Y	Y	Y	Y	If K_a = K_b,pH = 7.00
WB-WA	WB	WA	Y	Y	Y	Y	If K_a > K_b, pH < 7.00
WB-WA	WB	WA	Y	Y	Y	Y	If K_a < K_b, pH > 7.00