Question

In A water molecule, \(\mathrm{H}_{2} \mathrm{O}\), becomes strongly held to a dissolved aluminum ion, \(\mathrm{Al}^{3+}\). According to the Lewis definition of acids and bases, which is behaving as an acid: the watcr or the aluminum ion? Which is behaving as a base?

Short answer

\( \mathrm{Al}^{3+} \) acts as the Lewis acid; water acts as the Lewis base.

Step-by-step solution

Define Lewis Acids and Bases

Lewis acids are chemical species that can accept a pair of electrons, whereas Lewis bases are species that can donate a pair of electrons.

Identify Electron Acceptor and Donor

In the interaction between a water molecule, \( \mathrm{H}_{2}\mathrm{O} \), and a dissolved aluminum ion, \( \mathrm{Al}^{3+} \), the aluminum ion is positively charged, indicating that it can accept electron pairs. The water molecule, being neutral and containing lone pairs on oxygen, can donate electrons.

Determine the Role of Water

As water molecules donate their unshared pairs of electrons to the \( \mathrm{Al}^{3+} \) ion, they act as electron pair donors. Hence, by the Lewis definition, water is behaving as a Lewis base.

Determine the Role of Aluminum Ion

The \( \mathrm{Al}^{3+} \) ion accepts the electron pairs from water, thus it is acting as a Lewis acid, as it fulfills the role of an electron pair acceptor.

Key concepts

Water Molecule

Water, a simple molecule composed of two hydrogen atoms and one oxygen atom ( H_2O ), plays various roles in chemistry beyond just being a solvent. Remarkably, in aqueous solutions, water's polarity and lone pairs of electrons on the oxygen atom allow it to interact significantly with other ions or molecules.

• Oxygen is more electronegative than hydrogen, leading to a partial negative charge on the oxygen and a partial positive charge on the hydrogen atoms.
• This polarity makes water molecules capable of forming hydrogen bonds with each other and other solutes.
• The oxygen in water holds two lone pairs of electrons, making it able to form coordinate covalent bonds with Lewis acids.

These properties make water an essential participant in chemical reactions, particularly as a donor of electron pairs, which characterizes it as a Lewis base in its interactions, such as with aluminum ions.

Electron Pair Acceptor

In the context of Lewis acids and bases, an electron pair acceptor is a chemical species that can accept an electron pair to form a bond. This attribute defines a Lewis acid. When a water molecule interacts with an aluminum ion, the aluminum ion, with a charge of + 3, shows a readiness to accept electron pairs, fulfilling the role of a Lewis acid.

• Al^{3+} ions are highly charged and have vacant orbitals that can accommodate electron pairs from the lone pairs of donor molecules like water.
• Its positive charge makes it electrostatically attracted to electron-rich areas.
• By accepting electron pairs, Al^{3+} forms stable coordinate covalent bonds, enhancing its stability in the solution.

Through this process, the aluminum ion exhibits typical behavior of an electron pair acceptor by effectively drawing in electrons from the donor, which in our scenario, is the water molecule.

Electron Pair Donor

An electron pair donor, according to Lewis theory, is a substance that can donate a pair of electrons to another substance, forming a new coordinate covalent bond. In the scenario provided, the water molecule acts as the donor due to its lone pairs of electrons on the oxygen atom.

• The oxygen atom in water has two lone pairs available for donation.
• When water acts as a Lewis base, it gives one of its lone pairs to form a new bond with an electron-deficient species like Al^{3+} .
• Raising the hybridization state of the Al^{3+} molecule helps stabilize the new bond.

By donating its lone pairs to an aluminum ion, water fulfills the role of a Lewis base, showcasing the flexibility and essential reactivity water can have in various chemical reactions.