In A Single-Molecule Layer, Water Can Act Like Neither Solid Nor Liquid

By | 16/09/2022

This is “Water: Both an Acid and a Base”, section 10.three from the book Introduction to Chemistry: General, Organic, and Biological (five. 1.0). For details on it (including licensing), click hither.

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ten.3
Water: Both an Acid and a Base

Learning Objective

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

H2o (HiiO) is an interesting compound in many respects. Here, we volition consider its ability to behave as an acid or a base.

In some circumstances, a water molecule volition accept a proton and thus act as a Brønsted-Lowry base of operations. We saw an instance in the dissolving of HCl in H2O:


HCl + H2O(ℓ) → HiiiO+(aq) + Cl(aq)

In other circumstances, a h2o molecule tin can donate a proton and thus deed equally a Brønsted-Lowry acrid. For example, in the presence of the amide ion (see Example 4 in Department ten.two “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)

In this case, NH2

is a Brønsted-Lowry base (the proton acceptor).

So, depending on the circumstances, H2O can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base. H2o is not the only substance that can react as an acrid in some cases or a base in others, but it is certainly the about common case—and the well-nigh important ane. A substance that can either donate or accept a proton, depending on the circumstances, is called an
amphiproticA substance that tin can either donate or accept a proton, depending on the circumstances.

compound.

A water molecule tin can act every bit an acid or a base even in a sample of pure water. Nigh 6 in every 100 million (6 in 10eight) h2o molecules undergo the post-obit reaction:


HiiO(ℓ) + H2O(ℓ) → HthreeO+(aq) + OH(aq)

This procedure is called the
autoionization of waterThe procedure by which water ionizes into hydronium ions and hydroxide ions as it acts as an acid and a base.

(Figure x.two “Autoionization”) and occurs in every sample of water, whether it is pure or role of a solution. Autoionization occurs to some extent in any amphiprotic liquid. (For comparison, liquid ammonia undergoes autoionization as well, but only near ane molecule in a one thousand thousand billion (1 in 1015) reacts with some other ammonia molecule.)

Figure x.2
Autoionization

A modest fraction of water molecules—approximately 6 in 100 million—ionize spontaneously into hydronium ions and hydroxide ions. This picture necessarily overrepresents the amount of autoionization that really occurs in pure water.

Instance v

Place water every bit either a Brønsted-Lowry acrid or a Brønsted-Lowry base of operations.

  1. H2O(ℓ) + NOtwo
    (aq) → HNO2(aq) + OH(aq)
  2. HC2H3O2(aq) + HiiO(ℓ) → HthreeO+(aq) + CtwoH3Oii
    (aq)

Solution

  1. In this reaction, the water molecule donates a proton to the NOii

    ion, making OH(aq). As the proton donor, H2O acts as a Brønsted-Lowry acid.
  2. In this reaction, the h2o molecule accepts a proton from HCiiHiiiO2, becoming H3O+(aq). As the proton acceptor, H2O is a Brønsted-Lowry base.

Skill-Building Exercise

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

  1. HCOOH(aq) + H2O(ℓ) → H3O+(aq) + HCOO(aq)

  2. HtwoO(ℓ) + POfour
    three−(aq) → OH(aq) + HPO4
    2−(aq)

Concept Review Exercises

  1. Explain how h2o can deed as an acid.

  2. Explain how h2o can act every bit a base.

Answers

  1. Under the right conditions, H2O tin 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.

Central Takeaway

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

Exercises

  1. Is H2O(ℓ) acting as an acrid or a base of operations?


    H2O(ℓ) + NH4
    +(aq) → HiiiO+(aq) + NHthree(aq)

  2. Is H2O(ℓ) acting every bit an acrid or a base?


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

  3. In the aqueous solutions of some salts, i of the ions from the salt can react with h2o molecules. In some CtwoHthreeOtwo

    solutions, the post-obit reaction can occur:


    CiiHiiiO2
    (aq) + HiiO(ℓ) → HC2HiiiOii(aq) + OH(aq)

    Is H2O acting every bit an acrid or a base of operations in this reaction?

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


    NHiv
    +(aq) + H2O → NHthree(aq) + HiiiO+(aq)

    Is HtwoO acting every bit an acid or a base in this reaction?

  5. Aluminum hydroxide [Al(OH)3] is
    amphoteric; it reacts with both acids and bases. Propose the chemical equations for the reactions of Al(OH)3
    with H+
    and with OH.

  6. Based on the information in this department, does ammonia (NH3) autoionize more or less than water? Write the chemical equation for the autoionization of ammonia.

Answers

  1. base

  2. acid

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

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