Question

Describe this reaction according to the Lewis theory of acids and bases: $$\mathrm{AlCl}_{3}(s)+\mathrm{Cl}^{-}(a q) \longrightarrow \mathrm{AlCl}_{4}^{-}(a q)$$

Short answer

In this reaction, \(AlCl_3\) acts as the Lewis acid and \(Cl^-\) acts as the Lewis base. \(Cl^-\) donates an electron pair to \(AlCl_3\), forming a coordinate bond and resulting in the complex \(AlCl_4^-\).

Step-by-step solution

Identifying the Lewis Acid

A Lewis acid is a substance that can accept a pair of electrons. Looking at the reactants, the aluminum in \(AlCl_3\) has an empty orbital that can accept a pair of electrons. Therefore, \(AlCl_3\) is the Lewis acid in this reaction.

Identifying the Lewis Base

A Lewis base is a substance that can donate a pair of electrons. In the given reaction, the chloride ion (\(Cl^-\)) has a pair of electrons that it can donate. Therefore, \(Cl^-\) is the Lewis base.

Describing the Reaction

In this reaction, the Lewis base (\(Cl^-\)) donates an electron pair to the Lewis acid (\(AlCl_3\)), forming a coordinate bond and resulting in the tetrahydrate aluminum complex (\(AlCl_4^-\)).

Key concepts

Lewis Acid

In the realm of chemistry, a Lewis acid is essentially a molecule or ionic species eager to accept an electron pair from a donor molecule. Unlike the more restrictive Brønsted-Lowry definition, which stipulates a proton donor as an acid, the Lewis theory broadens the horizon by focusing on the electron pair acceptance.

For instance, in the reaction depicted in the textbook exercise where aluminum chloride (\(AlCl_3\) solid) reacts with a chloride ion (\(Cl^-\) in aqueous solution), the \(AlCl_3\) serves as the Lewis acid. This role is justified by the compound's intrinsic electronic arrangement: aluminum, with an empty p-orbital, is in a prime position to welcome additional electrons to form new bonds.

Lewis Base

Opposite to the Lewis acid, a Lewis base is typified by its readiness to donate an electron pair to any willing acceptor. These bases are defined by their electron-rich nature, housing lone pairs primed for pairing with an acid.

In our exercise, the chloride ion (\(Cl^-\) in aqueous solution) embodies the Lewis base. Sitting with a pair of electrons and no immediate use for them, it readily offers up its extra electrons to bond with a Lewis acid like \(AlCl_3\), which, as previously mentioned, is on the lookout for electrons to stabilize its structure and fulfill its octet.

Electron Pair Donation

Electron pair donation is the very crux of the Lewis theory, focusing on the transfer of electron pairs between substances. This exchange is fundamental to the formation of new chemical species in reactions involving Lewis acids and bases.

Using the provided reaction as our stage, the Lewis base (\(Cl^-\) ion) donates its electron pair to the Lewis acid (\(AlCl_3\) solid). This charitable act of electron sharing is what initiates the formation of the new product, indicating a successful chemical interaction and highlighting the collaborative nature of electrons in chemical bonding and reactions.

Coordinate Bond

The coordinate bond, also known as a dative covalent bond, is the powerful link that materializes when a Lewis base donates an electron pair to a Lewis acid. Unlike an ordinary covalent bond where each atom provides one electron, here only the donor (base) furnishes both electrons for the bond.

Resulting from our reaction, a coordinate bond is established between the aluminum atom of the \(AlCl_3\) and the incoming chloride ion \(Cl^-\) — a union that produces the \(AlCl_4^-\) complex. The genesis of this bond is pivotal, as it transforms the separate Lewis acid and base into one distinct compound, the tetrahedral \(AlCl_4^-\) ion in aqueous solution, hence demonstrating the transformative power of electron pair donation through the creation of coordinate bonds.