Question

Explain why the addition of a proton to a water molecule to give \(\mathrm{H}_{3} \mathrm{O}^{+}\) can be considered a Lewis acid-base reaction.

Short answer

The addition of a proton (H+) to a water molecule (H2O) to form hydronium ion (H3O+) is a Lewis acid-base reaction because H+ (acid) accepts an electron pair from H2O (base) to form a coordinate covalent bond.

Step-by-step solution

Understanding Lewis Acid-Base Theory

According to Lewis acid-base theory, a Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. The reaction involves a Lewis acid accepting an electron pair from a Lewis base to form a coordinate covalent bond.

Identifying the Lewis Acid in the Reaction

In the reaction where a water molecule (H_2O) adds a proton (H+), the proton is the Lewis acid. This is because the proton, having no electrons, can accept an electron pair.

Identifying the Lewis Base in the Reaction

The water molecule, with its lone pair of electrons, acts as the Lewis base. It donates an electron pair to the proton.

Describing the Formation of Hydronium Ion

When the proton (H+) from the acid comes into contact with the lone pair on the oxygen atom of the water molecule, it forms a coordinate covalent bond, resulting in the formation of the hydronium ion (H_3O^+).

Key concepts

Lewis Acid

In the realm of chemistry, the term Lewis acid refers to a substance that has the propensity to accept an electron pair. Unlike the classical definition of acids, which are species that donate protons (H+), Lewis acids can include a vast array of chemical entities, not limited to hydrogen ions.

For instance, a metal ion like Fe³⁺ can serve as a Lewis acid because it can accept electron pairs, often from water molecules or other Lewis bases, to form a complex ion. In the context of the exercise, the proton (H+) is a quintessential example of a Lewis acid, as it eagerly accepts a lone pair of electrons from a water molecule within the reaction.

Lewis Base

Contrasting with Lewis acids, a Lewis base is characterized by its ability to donate an electron pair. These are often molecules or ions that possess lone pairs of electrons which are not used in bonding but are available for sharing.

Common examples include ammonia (NH₃), which has a lone pair on the nitrogen atom, and hydroxide ions (OH⁻). In our exercise, the water molecule (H₂O) exemplifies a Lewis base, as it donates its electron pair to bond with the incoming proton, epitomizing the very essence of a Lewis acid-base reaction.

Coordinate Covalent Bond

The coordinate covalent bond (also known as a dative bond) is a type of chemical bond where both electrons in the bonding pair come from the same atom. This is an extension to the traditional covalent bond, where each atom typically contributes one electron to the bond's electron pair.

When a Lewis base donates its lone pair to a Lewis acid, a coordinate bond is formed. For example, when ammonia (NH₃) reacts with boron trifluoride (BF₃), the nitrogen atom in ammonia donates its lone pair to boron atom in BF₃, forming a bond without BF₃ contributing electrons. Similarly, in the exercise, the oxygen atom from the water molecule donates a pair of electrons to form a bond with the proton, resulting in a new molecule - hydronium.

Hydronium Ion

The hydronium ion (H₃O⁺) is a positively charged ion created when a water molecule accepts an extra proton. This ion is of paramount significance in the chemistry of acids and bases.

Neutral water molecules often engage in autoprotolysis, where a proton is transferred between two water molecules, resulting in a hydronium ion and a hydroxide ion (OH⁻). The formation of the hydronium ion in the exercise illustrates the culmination of a Lewis acid-base reaction, with the water acting as the Lewis base and the proton as the Lewis acid, to form a stable ion that plays a central role in acid-base chemistry in aqueous solutions.