Question

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

Short answer

\( \mathrm{AlCl}_3 \) is the Lewis acid, \( \mathrm{Cl}^- \) is the Lewis base, forming \( \mathrm{AlCl}_4^- \).

Step-by-step solution

Identify the Lewis Acid

In the given reaction, we have \( \mathrm{AlCl}_3 \) combining with \( \mathrm{Cl}^- \) to form \( \mathrm{AlCl}_4^- \). According to Lewis theory, a Lewis acid is a substance that can accept a pair of electrons. Here, \( \mathrm{AlCl}_3 \) is the electron pair acceptor, making it the Lewis acid.

Identify the Lewis Base

Next, identify the Lewis base in the reaction. A Lewis base is a substance that donates a pair of electrons. In the reaction, \( \mathrm{Cl}^- \) donates its pair of electrons to \( \mathrm{AlCl}_3 \), so \( \mathrm{Cl}^- \) is the Lewis base.

Describe the Formation of the Product

During the reaction, \( \mathrm{Cl}^- \), the Lewis base, donates an electron pair to \( \mathrm{AlCl}_3 \), the Lewis acid. This electron pair transfer results in the formation of \( \mathrm{AlCl}_4^- \), where the Lewis base has successfully donated its electron pair to the Lewis acid.

Summarize the Reaction

The reaction can be summarized as \( \mathrm{AlCl}_3 \) acting as the Lewis acid and \( \mathrm{Cl}^- \) acting as the Lewis base. Through the donation of an electron pair from \( \mathrm{Cl}^- \) to \( \mathrm{AlCl}_3 \), the compound \( \mathrm{AlCl}_4^- \) is formed, illustrating the behavior of Lewis acids and bases.

Key concepts

Lewis theory

The Lewis theory of acids and bases offers a broader definition than the more traditional proton-based theories. Instead of focusing on protons, the Lewis theory examines the transfer of electron pairs.
According to this theory, a Lewis acid is any substance that can accept a pair of electrons, whereas a Lewis base is any substance that can donate a pair of electrons.
This approach helps explain reactions where no protons are exchanged, allowing us to understand and predict chemical interactions that involve electron pair sharing or transfer.

electron pair donor

In the context of a Lewis acid-base reaction, the electron pair donor is known as the Lewis base. This entity has lone pairs of electrons ready to be shared with an electron-pair acceptor.
In the example given, chloride ion (\( \mathrm{Cl}^- \)) acts as the Lewis base. It donates its lone pair of electrons to the aluminum chloride (\( \mathrm{AlCl}_3 \)), thereby participating in the formation of a new chemical species.

• Lewis bases usually possess lone pairs of electrons.
• They are often negatively charged or neutral molecules with lone pairs.

electron pair acceptor

An electron pair acceptor is termed a Lewis acid. In a chemical reaction driven by the Lewis theory's principles, the Lewis acid is the participant that receives electrons.
In the provided reaction, aluminum chloride (\( \mathrm{AlCl}_3 \)) plays the role of the Lewis acid because it accepts an electron pair from the chloride ion (\( \mathrm{Cl}^- \)).

• Lewis acids are typically positively charged or neutral molecules with vacant orbitals.
• They seek to complete their electron configuration by gaining electron pairs.

product formation

Product formation in a Lewis acid-base reaction depends on the successful electron pair transfer from the Lewis base to the Lewis acid. In the reaction discussed, the chloride ion transfers an electron pair to aluminum chloride to form a new compound called tetrachloroaluminate ion (\( \mathrm{AlCl}_4^- \)).
This process highlights the essential role of electron movement in creating new products in chemical reactions. Through this interaction:

• A new chemical bond is formed by sharing electrons.
• The resulting product exhibits properties from both original substances.

Understanding product formation in Lewis reactions is crucial for predicting the outcomes of such chemical processes.