10.3 Water: Both an Acid and a Base

10.3 Water: Both an Acid and a Base

 Learning Objective

  1. Write chemical equations for water acting as an acid and as a base.

Water (H2O) is an interesting compound in many respects, including its ability to behave as an acid or a base.

In some circumstances, a water molecule will accept a proton and thus act as a Brønsted-Lowry base, which happens when HCl dissolves in H2O:

HCl + H2O(ℓ) → H3O+(aq) + Cl−(aq)

In other circumstances, a water molecule can donate a proton and thus act as a Brønsted-Lowry acid. For example, in the presence of the amide ion (see Example 4 in Section 10.2 “Brønsted-Lowry Definition of Acids and Bases”), a water molecule donates a proton, making ammonia as a product:

H2O(ℓ) + NH2−(aq) → OH−(aq) + NH3(aq)

So, depending on the circumstances, H2O can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base. Water is not the only substance that can react as an acid in some cases or a base in others, but it is certainly the most common example—and the most important one. A substance that can either donate or accept a proton, depending on the circumstances, is called an amphiprotic compound.

A water molecule can act as an acid or a base even in a sample of pure water. About 2 in every 555 million  water molecules undergo the following reaction:

H2O(ℓ) + H2O(ℓ) ⇆ H3O+(aq) + OH−(aq)

This process is called the autoionization of water (Figure 10.2 “Autoionization”) and occurs in every sample of water, whether it is pure or part of a solution.

 

image

Figure 10.2 Autoionization  Note: Each H+ion in the diagram above could be viewed as attaching to an H2O molecule, forming H3O+.  Also, it would require 555 million water molecules to have only one each of H3O+ and OH–, so the picture vastly over-represents the amount of autoionization of water.

Example 5

Identify water as either a Brønsted-Lowry acid or a Brønsted-Lowry base.

  1. H2O(ℓ) + NO2−(aq) → HNO2(aq) + OH−(aq)
  2. HC2H3O2(aq) + H2O(ℓ) ⇆ H3O+(aq) + C2H3O2−(aq)

Solution

  1. In this reaction, the water molecule donates a proton to the NO2− ion, making OH−(aq). As the proton donor, H2O acts as a Brønsted-Lowry acid.
  2. In this reaction, the water molecule accepts a proton from HC2H3O2, becoming H3O+(aq). As the proton acceptor, H2O is a Brønsted-Lowry base.

Skill-Building Exercise

  1. HCOOH(aq) + H2O(ℓ) ⇆ H3O+(aq) + HCOO−(aq)

  2. H2O(ℓ) + PO43−(aq) → OH−(aq) + HPO42−(aq)

Concept Review Exercises

  1. Explain how water can act as an acid.

  2. Explain how water can act as a base.

answers

  1. Under the right conditions, H2O can donate a proton, making it a Brønsted-Lowry acid.

  2. Under the right conditions, H2O can accept a proton, making it a Brønsted-Lowry base.

Key Takeaway

  • Water molecules can act as both an acid and a base, depending on the conditions.

Exercises

  1. Is H2O(ℓ) acting as an acid or a base?

    H2O(ℓ) + NH4+(aq) → H3O+(aq) + NH3(aq)

  2. Is H2O(ℓ) acting as an acid or a base?

    CH3−(aq) + H2O(ℓ) → CH4(aq) + OH−(aq)

  3. In the aqueous solutions of some salts, one of the ions from the salt can react with water molecules. In some C2H3O2− solutions, the following reaction can occur:

    C2H3O2−(aq) + H2O(ℓ) ⇆ HC2H3O2(aq) + OH−(aq)

    Is H2O acting as an acid or a base in this reaction?

  4. In the aqueous solutions of some salts, one of the ions from the salt can react with water molecules. In some NH4+ solutions, the following reaction can occur:

    NH4+(aq) + H2O ⇆ NH3(aq) + H3O+(aq)

    Is H2O acting as an acid or a base in this reaction?

Answers

1. base

2. acid

3. Al(OH)3 + H+ → HAl(OH)3+; Al(OH)3 + OH− → Al(OH)4−