Lesson 5: Hydrolysis of Salts

Part a: Predicting and Explaining the Acidity Level of Salts

Part a: Predicting and Explaining the Acidity Level of Salts
Part b: K_a and K_b Values of Conjugate Acid-Base Pairs

 
 

The Big Idea

Dissolved salts don’t always form neutral solutions. Some form acidic or basic solutions through hydrolysis. The key is understanding how their ions interact with water. This lesson explains the principles and rationale and uses clear examples to help students master the hydrolysis of salts.

 

What Do Dissolved Salts Do?

In Chemistry, a salt is simply an ionic compound. We readily associate NaCl with salts; but there are many other examples. Sodium acetate (NaC₂H₃O₂), ammonium chloride (NH₄Cl), calcium iodide (CaI₂), potassium carbonate (K₂CO₃), and countless other substances are salts. A salt consists of a cation and an anion held together by ionic bonding.
 
As we discussed in the Solutions Chapter of this Chemistry Tutorial, a soluble salt dissociates or splits apart to form solvated ions. For instance, sodium acetate (NaC₂H₃O₂) dissociates as follows:

NaC₂H₃O₂(s)  →  Na⁺(aq)  +  C₂H₃O₂^-(aq) 

 
And ammonium chloride, NH₄Cl, dissociates as follows:

NH₄Cl(s)  →  NH₄⁺(aq)  +  Cl^-(aq) 

 
The ions of a dissolved salt can do one of three things:

• The ion can donate a proton to water to form an acidic solution.
• The ion can accept a proton from water to form a basic solution.
• The ion can do nothing, in which case we could refer to it as a spectator ion.

 
In Lesson 5a, we want to inspect the interaction of these dissolved ions with water in order to predict whether the ions would form an acidic, basic, or neutral solution.
 
 
 

Hydrolysis of a Salt

In acid-base chemistry, hydrolysis of a salt refers to the reaction of one or more its ions with water. The reactions involve proton transfer. The ammonium ion, NH₄⁺, is a relatively good proton donor. In aqueous solution, it wishes to donate an H⁺ ion to a base. If there is no base present, then water would act as a base to receive the proton. The reaction is referred to as a hydrolysis reaction and described by the equation:
   
Since water becomes H₃O⁺ when it receives the proton, we categorize the ammonium ion as having acidic properties. We would describe NH₄⁺ as being a weak acid.
 
The acetate ion, C₂H₃O₂^-, is a relatively good proton acceptor. In aqueous solution, it wishes to accept an H⁺ ion from an acid. If there is no acid present, then water would act as the acid to donate a proton to the acetate ion. The hydrolysis reaction is described by the equation:
   
Since water becomes OH^- after donating a proton, we categorize the acetate ion as having basic properties. We would describe C₂H₃O₂^- as being a weak base.
 
Hydrolysis of the dissolved ions of a salt explain why some salts are acidic and others are basic. The fact that some ions do not hydrolyze explains why some salt solutions are neutral.
   
 

What Ions are Neutral in Aqueous Solutions?

There are some ions which do not hydrolyze in solution. These include the anions of the strong, monoprotic acids. The strong, monoprotic acids (discussed in Lesson 3a) are HCl, HBr, HI, HNO₃, HClO₄, and HClO₃. The anions that are paired with H⁺ in these acids do not hydrolyze in water. In fact, when we introduced strong acids, we wrote the dissociation equation with the irreversible reaction arrow:
   
The reverse reaction of the above dissociation does not occur because Cl^- is such a weak proton acceptor that it doesn’t even accept a proton from the very-willing-to-donate hydronium ion. In general, the anions of the strong, monoprotic acids are spectator ions in solution. They do not hydrolyze and do not contribute to the acidity or basicity of the solution.
 
The cations of strong bases do not hydrolyze either. The strong bases (discussed in Lesson 3a) are LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂. The cation paired with these OH^- ions do not accept or donate protons. They have neither acidic or basic qualities. In solution, they do not hydrolyze and act as spectator ions.
 
 
 
 

What Ions are Acidic in Aqueous Solutions?

In Lesson 3a, we used the Brønsted-Lowry model to write chemical equations for the dissociation of weak bases in aqueous solutions. Weak bases are proton acceptors. They accept a proton (H⁺ ion) from water and form their conjugate acid. Here are three weak base dissociation equations:
   
The conjugate acid of the base is recognized as having a similar-looking formula with the exception that there is an extra H⁺ in the conjugate acid. The table at the right lists several weak bases and their conjugate acids.
 
A conjugate acid is an acid. And the conjugate acid of a weak base is a strong enough acid to interact with water and hydrolyze. Being an acid, it would donate a proton to water. Water, having received an H⁺ ion, would become H₃O⁺.
   
Since hydronium ions are produced by the hydrolysis of the weak acid, the pH is decreased. The conclusion can be made that any ion that is the conjugate acid of a weak base will have acidic properties and lower the pH of the solution.
 
 
 

What Ions are Basic in Aqueous Solutions?

We can apply the same Brønsted-Lowry reasoning to the conjugate bases of weak, monoprotic acids. For instance, the acetate ion, C₂H₃O₂^-, is the conjugate base of the weak acid, acetic acid (HC₂H₃O₂). It is formed by the acid’s dissociation in water:
   
The conjugate base of a weak acid is recognized as having a similar-looking formula with the exception that there is one less H⁺ in the conjugate base. The table at the right lists several weak, monoprotic acids and their conjugate bases.
 
A conjugate base is a base. And the conjugate base of a weak acid is a strong enough base to interact with water and hydrolyze. Being a base, it would accept a proton from water. Water, having donated an H⁺ ion, would become OH^-.
   
Since hydroxide ions are produced by the hydrolysis of the weak base, the pH is increased. The conclusion can be made that any ion that is the conjugate base of a weak, monoprotic acid will have basic properties and increase the pH of the solution.
   
 

How to Predict the Acidity Level of Salts

A salt consists of a cation and an anion. These ions will fall into one of three categories:

1. Spectator Ions (Neutral)
   The anions of strong, monoprotic acids and the cations of strong bases fall into this category.
2. Hydrolyze with H₂O to form acidic solutions (Acidic)
   The conjugate acids of weak bases fall into this category.
3. Hydrolyze with H₂O to form basic solutions (Basic)
   The conjugate bases of weak, monoprotic acids fall into this category.

   

We can use the above conceptual understanding with the following step-by-step procedure to predict the acidity level of salts:

1. Determine the cation and the anion present in the salt.
2. Determine if the cation falls into category A, B, or C above. That is, is it neutral, acidic, or basic.
3. Determine if the anion falls into category A, B, or C above. That is, is it neutral, acidic, or basic.
4. From your findings in Steps 2 and 3, categorize the salt as being acidic or basic. If the cation is acidic and the anion is basic (or vice versa), then additional reasoning must be done. This is discussed in the next section.

 
 
 

What if a Salt Contains an Acidic and a Basic Ion?

There will be occasions in which both the cation and the anion of a salt have opposing properties. For instance, the cation may be acidic and the anion may be basic. There are a few principles that can be considered to break the tie in situations in which the cation and anion have opposing acid-base properties:
 

• The stronger that the weak acid is (i.e., the larger its K_a value), the weaker that its conjugate base will be.
• Conversely, the weaker that the weak acid is (i.e., the smaller its K_a value), the stronger that its conjugate base will be.
• The stronger that the weak base is (i.e., the larger its K_b value), the weaker that its conjugate acid will be.
• Conversely, the weaker that the weak base is (i.e., the smaller its K_b value), the stronger that its conjugate acid will be.

 
With these principles, comparing the strength of the weak acid (from which the basic anion is derived) to the strength of the weak base (from which the acidic cation is derived), you can reason towards whether the salt solution is acidic or basic.
 

Here are three situations to illustrate the reasoning.

Situation 1: Ammonium cyanide - NH₄CN

• The cation is NH₄⁺. It is the conjugate acid of the weak base NH₃ (K_b = 1.8x10^-5).
• The anion is CN^-. It is the conjugate base of the weak acid HCN (K_a = 6.2x10^-10).
• HCN is a weaker acid than NH₃ is a base. Therefore, the strength of the conjugate base CN^- is greater than the strength of the conjugate acid NH₄⁺.
• Therefore, the greater strength of CN^- as a base means that this salt solution will be slightly basic.

 
 

Situation 2: Ammonium chlorite - NH₄ClO₂

• The cation is NH₄⁺. It is the conjugate acid of the weak base NH₃ (K_b = 1.8x10^-5).
• The anion is ClO₂^-. It is the conjugate base of the weak acid HClO₂ (K_a = 1.1x10^-2).
• HClO₂ is a stronger acid than NH₃ is a base. Therefore, the strength of the conjugate base ClO₂^- is less than the strength of the conjugate acid NH₄⁺.
• Therefore, the greater strength of NH₄⁺ as an acid means that this salt solution will be slightly acidic.

 
 

Situation 3: Ammonium acetate - NH₄C₂H₃O₂

• The cation is NH₄⁺. It is the conjugate acid of the weak base NH₃ (K_b = 1.8x10^-5).
• The anion is C₂H₃O₂^-. It is the conjugate base of the weak acid HC₂H₃O₂ (K_a = 1.8x10^-5).
• HC₂H₃O₂ is equally strong as an acid as NH₃ is as a base. Therefore, the strength of the conjugate base C₂H₃O₂^- is equal to the strength of the conjugate acid NH₄⁺.
• Because these two ions are of equal strength, the solution will be neutral.

 
 

Practice Questions with Answers - Predicting the Acidity Level of Salts

Determine if the aqueous solutions of the following salts are acidic, basic, or neutral. Think through your rationale for your conclusion. Then tap Check Answer to see how you did. (If necessary, use the listing of strong and weak acids and bases found in our Reference section.)

a. KBr


b. NaC₂H₃O₂
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c. NH₄Cl
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d. CaCl₂
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e. CaF₂
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f. C₂H₅NH₃Cl
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g. C₅H₁₁NHF
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Acid-Base Neutralization Revisited

In Lesson 4b, we discussed pH curves for acid-base neutralization reactions. The equivalence point for such reactions was the point at which the acid-to-base mole ratio matched the acid-to-base coefficient ratio. The pH value at the equivalence point depended upon the products of the reaction. The products were described as “a salt and water.” Predicting the pH (or acidity level) at the equivalence point required an inspection of these products. Now that we have a better understanding of the hydrolysis of salts, let’s give a deeper explanation of the pH at these equivalence points.
 
 
 

Reaction Between a Strong Acid and a Strong Base

We learned that the neutralization of a strong acid (like HCl) and strong base (like NaOH) results in an equivalence point with a pH of 7. Why? Since both acids and bases are strong, they exist as ions in solution. The H⁺ ion is more appropriately described as the hydronium ion, H₃O⁺. The complete ionic equation for the neutralization reaction is represented by the particle diagram below.
   
The products are a salt, NaCl, and water. The salt is composed of Na⁺ and Cl^- ions. We have described Na⁺ and Cl^- ions as neutral ions. In solution, they are spectator ions. And in this neutralization reaction, they are also spectator ions. Hydrolysis does not occur. Altogether, the products are pH-neutral, thus explaining the pH of 7 at the equivalence point for this strong acid-strong base neutralization.
 
 
 

Reaction Between a Weak Acid and a Strong Base

We learned that the neutralization of a weak acid (like HF) and strong base (like NaOH) results in an equivalence point with a pH greater than 7. Why? Being a weak acid, the HF exists primarily as undissociated molecules. The NaOH exists as dissociated ions, Na⁺ and OH^-. The complete ionic equation for the neutralization reaction is represented by the particle diagram below.
   
The products are a salt, NaF, and water. The salt is composed of Na⁺ and F^- ions. The Na⁺ ions are the spectator ions, consistent with being a cation of a strong base. The F^- is the conjugate base of the weak acid HF. As such, it is basic in solution. It would undergo hydrolysis, resulting in a pH greater than 7.0.
 
 
 

Reaction Between a Strong Acid and a weak Base

We learned that the neutralization of a strong acid (like HCl) and weak base (like NH₃) results in an equivalence point with a pH less than 7. Why? Being a strong acid, the HCl exists in solution as hydronium ions (H₃O⁺) and chloride ions (Cl^-). Being a weak base, the NH₃ primarily as undissociated molecules. The complete ionic equation for the neutralization reaction is represented by the particle diagram below.
   
The products are a salt, NH₄Cl, and water. The salt is composed of NH₄⁺ and Cl^- ions. The Cl^- ions are the spectator ions, consistent with being the anion of a strong acid. The NH₄⁺ is the conjugate acid of the weak base NH₃. As such, it is acidic in solution. It would undergo hydrolysis, resulting in a pH less than 7.0.
 
 
Using what we have learned on this page regarding the acidity levels of the ions of a dissolved salt, we can quickly analyze, explain, and predict the acidity level of a solution containing a salt.
 
 
 


 

 

Before You Leave - Practice and Reinforcement

Now that you've done the reading, take some time to strengthen your understanding and to put the ideas into practice. Here's some suggestions.

• Download our Study Card on Hydrolysis of Salts. Save it to a safe location and use it as a review tool. (Coming Soon.)
• The Check Your Understanding section below includes questions with answers and explanations. It provides a great chance to self-assess your understanding.

 
 

Check Your Understanding

Use the following questions to assess your understanding. Tap the Check Answer buttons when ready.
 
1. In your own words, explain why some salts are neutral, others are acidic, and still others are basic. Think it through; then craft your explanation ... maybe even practice it on a friend (or enemy).

 
 

2. In Chemistry, a salt is ____.

1. an option that you have for making food taste better.
2. a physical or verbal or chemical attack launched against an adversary.
3. a food additive that increases blood pressure.
4. an ionic compound with a cation and an anion held together by ionic bonding.

 
 
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3. Write the chemical equation for the hydrolysis of the following ions:
a. F^- ion:




b. OCl^- ion:


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c. CH₃NH₃⁺ ion:


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d. CO₃^2- ion:


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4. Identify the conjugate base of the following species:
a. HNO₂:


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b. HOCN:


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c. HIO₃:


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d. HC₃H₅O₃:


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5. Identify the conjugate acid of the following species:
a. C₉H₇N:


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b. C₆H₅NH₂:


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c. HCO₃^-:
 

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6. Identify solutions of the following salts as being either acidic, basic, or neutral. Provide reasoning for your answer.
a. KC₂H₃O₂


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b. NH₄Br


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c. CH₃NH₃Br


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d. Ca(CN)₂


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e. RbCl


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