Which agent would be considered a Lewis acid?

A Lewis acid is defined as a substance that can accept a pair of electrons to form a covalent bond. In this context, Aluminum chloride acts as a Lewis acid because it has an incomplete octet and can accept electrons due to the presence of an empty p-orbital on the aluminum atom. When aluminum chloride interacts with a Lewis base, it forms a coordinate covalent bond, effectively completing its octet.

In contrast, water is a polar molecule that can donate electrons but does not function as an electron pair acceptor in the context of Lewis acid-base theory. Hydrochloric acid, while a strong acid, primarily behaves as a Brønsted-Lowry acid that donates protons rather than accepting electron pairs. Ammonia acts as a Lewis base because it has a lone pair of electrons that it can donate but does not have the capacity to accept electron pairs effectively, which is the defining characteristic of a Lewis acid.