Acid-Base Properties of Salts

Salts that Produce Basic Solutions

When dissolved in water, a basic salt yields a solution with pH greater than 7.0.

Learning Objectives

Distinguish basic salts from non-basic salts

Key Takeaways

Key Points

  • In acid – base chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base.
  • Basic salts contain the conjugate base of a weak acid, so when they dissolve in water, they react with water to yield a solution with pH greater than 7.0.

Key Terms

  • basic salt: the product of the neutralization of a strong base and a weak acid; its anion is the conjugate base of the weak acid

In acid-base chemistry, a salt is defined as the ionic compound that results from a neutralization reaction between an acid and a base. As such, salts are composed of cations (positively charged ions ) and anions (negative ions), and in their unsolvated, solid forms, they are electrically neutral (without a net charge). The component ions in a salt can be inorganic; examples include chloride (Cl), the organic acetate (CH3COO), and monatomic fluoride (F), as well as polyatomic ions such as sulfate (SO42−).

The Reaction of a Basic Salt in Water

There are several varieties of salts, and in this section we will consider basic salts. What makes a basic salt basic? It is due to the fact that the anion in the salt is the conjugate base of a weak acid. For a generalized anion B, the net ionic reaction is:

[latex]\text{B}^-(\text{aq})+\text{H}_2\text{O}(\text{l})\rightleftharpoons \text{BH}(\text{aq})+\text{OH}^-(\text{aq})[/latex]

An example of a basic salt is sodium bicarbonate, NaHCO3. The bicarbonate ion is the conjugate base of carbonic acid, a weak acid. Therefore, it reacts with water in the following fashion:

[latex]\text{HCO}_3^-(\text{aq})+\text{H}_2\text{O}(\text{l})\rightleftharpoons \text{H}_2\text{CO}_3(\text{aq})+\text{OH}^-(\text{aq})[/latex]

Because it is capable of deprotonating water and yielding a basic solution, sodium bicarbonate is a basic salt.

Other examples of basic salts include:

  • Calcium carbonate (CaCO3)
  • Sodium acetate (NaOOCCH3)
  • Potassium cyanide (KCN)
  • Sodium sulfide (Na2S)

Notice that for all of these examples, the anion is the conjugate base of a weak acid (carbonic acid, bisulfate (second dissociation step of sulfuric acid), acetic acid, hydrocyanic acid, hydrogen sulfide).

Conjugate Bases of Weak vs. Strong Acids

Keep in mind that a salt will only be basic if it contains the conjugate base of a weak acid. Sodium chloride, for instance, contains chloride (Cl), which is the conjugate base of HCl. But because HCl is a strong acid, the Cl ion is not basic in solution, and it isn’t capable of deprotonating water.

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Sodium Bicarbonate: Because the bicarbonate ion is the conjugate base of carbonic acid, a weak acid, sodium bicarbonate will yield a basic solution in water.

Salts that Produce Acidic Solutions

When dissolved in water, acidic salts form solutions with pH less than 7.0.

Learning Objectives

Explain the formation of acid salts and their effects on a solution’s pH.

Key Takeaways

Key Points

  • Acid salts contain a hydrolyzable proton in the cation, anion, or both; for instance, the salt ammonium bisulfate (NH4HSO4) contains an acidic proton in both the cation and the anion.
  • To determine the acidity / alkalinity of a hydrolyzable anion, compare the Ka and Kb values for the ion; if Ka > Kb, the ion is acidic; if Kb > Ka, the ion is basic.

Key Terms

  • acid salt: a salt that yields a solution with pH less than 7.0
  • hydrolyzable: capable of dissociating in water

Salts With a Hydrolyzable Cation

When dissolved in water, acidic salts will yield solutions with pH less than 7.0. This is due either to the presence of a metal cation that acts as a Lewis acid (which will be discussed in a later concept), or, quite commonly, due to a hydrolyzable proton in the cation or the anion. Salts with acidic protons in the cation are most commonly ammonium salts, or organic compounds that contain a protonated amine group. Examples include:

  • ammonium (NH4+)
  • methyl ammonium (CH3NH3+)
  • ethyl ammonium (CH3CH2NH3+)
  • anilinium (C6H6NH2+)

An example of an acid salt is one containing any of these cations with a neutral base, such as ammonium chloride (NH4Cl).

Salts With Hydrolyzable Protons in the Anion

Acid salts can also contain an acidic proton in the anion. Examples of anions with an acidic proton include:

  • bisulfate (HSO4)
  • dihydrogen citrate (H2C6H5O7)
  • bioxalate (HO2C2O)

Each of these anions contains a proton that will weakly dissociate in water. Therefore, salts containing these anions—such as potassium bisulfate—will yield weakly acidic solutions in water.

Determining Acidity or Alkalinity of a Hydrolyzable Ion

From the previous concept, we know that salts containing the bicarbonate ion (HCO3) are basic, whereas salts containing bisulfate ion (HSO4) are acidic. We determine whether the hydrolyzable ion is acidic or basic by comparing the Ka and Kb values for the ion; if Ka > Kb, the ion will be acidic, whereas if Kb > Ka, the ion will be basic.

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Anilinium chloride: Anilinium chloride is an example of an acid salt. The NH3+ group contains an acidic proton capable of dissociating in solution; therefore, a solution of anilinium chloride in pure water will have a pH less than 7.

Overview of the Acid-Base Properties of Salt

Some salts, such as ammonium bicarbonate (NH4HCO3), contain cations and anions that can both undergo hydrolysis.

Learning Objectives

Predict the pH of a solution of a salt containing cations and anions, both of which participate in hydrolysis.

Key Takeaways

Key Points

  • Basic salts result from the neutralization of a strong base with a weak acid.
  • Acid salts result from the neutralization of a strong acid with a weak base.
  • For salts in which both cation and anion are capable of hydrolysis, compare Ka and Kb values to determine the solution ‘s resulting pH.

Key Terms

  • neutralization reaction: a reaction between an acid and a base in which water and a salt are formed
  • hydrolysis: a reaction with water in which chemical bonds break
  • salt: in acid-base chemistry, one of the products in a neutralization reaction

Summary of Acidic and Basic Salts

As we have discussed, salts can form acidic or basic solutions if their cations and/or anions are hydrolyzable (able to react in water). Basic salts form from the neutralization of a strong base and a weak acid; for instance, the reaction of sodium hydroxide (a strong base) with acetic acid (a weak acid) will yield water and sodium acetate. Sodium acetate is a basic salt; the acetate ion is capable of deprotonating water, thereby raising the solution’s pH.

Acid salts are the converse of basic salts; they are formed in the neutralization reaction between a strong acid and a weak base. The conjugate acid of the weak base makes the salt acidic. For instance, in the reaction of hydrochloric acid (a strong acid) with ammonia (a weak base), water is formed, along with ammonium chloride. The ammonium ion contains a hydrolyzable proton, which makes it an acid salt.

Salts in Which Both Ions Hydrolyze

The following is a more complicated scenario in which a salt contains a cation and an anion, both of which are capable of participating in hydrolysis. A good example of such a salt is ammonium bicarbonate, NH4HCO3; like all ammonium salts, it is highly soluble, and its dissociation reaction in water is as follows:

[latex]\text{NH}_4\text{CO}_3(\text{s})\rightarrow \text{NH}_4^+(\text{aq})+\text{HCO}_3^-(\text{aq})[/latex]

However, as we have already discussed, the ammonium ion acts as a weak acid in solution, while the bicarbonate ion acts as a weak base. The reactions are as follows:

[latex]\text{NH}_4^+(\text{aq})+\text{H}_2\text{O}(\text{l})\rightleftharpoons \text{H}_3\text{O}^+(\text{aq})+\text{NH}_3(\text{aq})\quad\quad \text{K}_\text{a}=5.6\times10^{-10}[/latex]

[latex]\text{HCO}_3^-(\text{aq})+\text{H}_2\text{O}(\text{l})\rightleftharpoons \text{H}_2\text{CO}_3(\text{aq})+\text{OH}^-(\text{aq})\quad\quad \text{K}_\text{b}=2.4\times 10^{-8}[/latex]

Because both ions can hydrolyze, will a solution of ammonium bicarbonate be acidic or basic? We can determine the answer by comparing Ka and Kb values for each ion. In this case, the value of Kb for bicarbonate is greater than the value of Ka for ammonium. Therefore, bicarbonate is a slightly more alkaline than ammonium is acidic, and a solution of ammonium bicarbonate in pure water will be slightly basic (pH > 7.0). In summary, when a salt contains two ions that hydrolyze, compare their Ka and Kb values:

  • If Ka > Kb, the solution will be slightly acidic.
  • If Kb > Ka, the solution will be slightly basic.

Hydrolysis of salts: This video examines the hydrolysis of an acid salt, a basic salt, and a salt in which both ions hydrolyze.