Question

Lewis Acids and Bases Identify the Lewis acid and Lewis base among the reactants in each equation. $$\begin{array}{l}{\text { a. } \mathrm{Ag}^{+}(a q)+2 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}(a q)} \\\\{\text { b. } \mathrm{AlBr}_{3}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{H}_{3} \mathrm{NAlBr}_{3}} \\ {\text { c. } \mathrm{F}^{-}(a q)+\mathrm{BF}_{3}(a q) \rightleftharpoons \mathrm{BF}_{4}^{-}(a q)}\end{array}$$

Short answer

a. Lewis acid: \(\mathrm{Ag}^{+}\), Lewis base: \(\mathrm{NH}_{3}\). b. Lewis acid: \(\mathrm{AlBr}_{3}\), Lewis base: \(\mathrm{NH}_{3}\). c. Lewis acid: \(\mathrm{BF}_{3}\), Lewis base: \(\mathrm{F}^{-}\).

Step-by-step solution

Understanding Lewis Acids and Bases

Recall that a Lewis acid is a chemical species that accepts an electron pair, and a Lewis base is a species that donates an electron pair. In these reactions, identify the species that donates an electron pair (Lewis base) and the one that accepts it (Lewis acid).

Analyzing Reaction a

In the reaction \(\mathrm{Ag}^{+}(aq)+2\mathrm{NH}_{3}(aq) \rightleftharpoons \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}(aq)\), \(\mathrm{Ag}^{+}\) is the Lewis acid because it accepts an electron pair and \(\mathrm{NH}_{3}\) is the Lewis base because it donates an electron pair to form a complex ion.

Analyzing Reaction b

In the reaction \(\mathrm{AlBr}_{3} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{H}_{3}\mathrm{NAlBr}_{3}\), \(\mathrm{AlBr}_{3}\) is the Lewis acid as it accepts an electron pair from \(\mathrm{NH}_{3}\), which acts as a Lewis base.

Analyzing Reaction c

In the reaction \(\mathrm{F}^{-}(aq)+\mathrm{BF}_{3}(aq) \rightleftharpoons \mathrm{BF}_{4}^{-}(aq)\), \(\mathrm{BF}_{3}\) is the Lewis acid because it accepts an electron pair and \(\mathrm{F}^{-}\) is the Lewis base as it donates an electron pair.

Key concepts

Lewis Acid and Base Identification

Lewis Acid and Base identification is integral in understanding chemical interactions involving electron transfer. In this context, a Lewis acid is a molecule or ion that can accept a pair of electrons, while a Lewis base has a pair of electrons it is ready to donate.

For example, in the first given reaction, \(\mathrm{Ag}^{+}(aq)+2\mathrm{NH}_{3}(aq) \rightleftharpoons \mathrm{Ag}(\mathrm{NH}_{3})_{2}^{+}(aq)\), the silver ion, \(\mathrm{Ag}^{+}\), lacking a complete valence electron shell, acts as a Lewis acid. Conversely, the ammonia molecule, \(\mathrm{NH}_{3}\), with its lone pair of electrons, fulfills the role of a Lewis base. This method of identification is crucial when predicting the products of a reaction or understanding the reactivity of different compounds.

Chemical Reaction Analysis

Chemical Reaction analysis involves examining the changes and interactions between the reactants to predict the products. Looking deeper into our first reaction, we can see that each \(\mathrm{NH}_{3}\) molecule donates a pair of electrons to the \(\mathrm{Ag}^{+}\) ion, forming a coordination complex. Through such analysis, one can infer the stoichiometry and mechanism of the reaction.

Similarly, in reaction b, \(\mathrm{AlBr}_{3} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{H}_{3}\mathrm{NAlBr}_{3}\), the \(\mathrm{AlBr}_{3}\) accepts an electron pair to form a three-dimensional structure with \(\mathrm{NH}_{3}\), which is a classic example of Lewis acid-base interaction leading to complex formation.

Electron Pair Donation and Acceptance

Understanding the process of electron pair donation and acceptance is key to grasping how Lewis bases and acids interact. A Lewis base donates its available pair of electrons, while a Lewis acid accepts this pair to achieve a more stable electronic configuration.

In our third reaction, \(\mathrm{F}^{-}(aq)+\mathrm{BF}_{3}(aq) \rightleftharpoons \mathrm{BF}_{4}^{-}(aq)\), fluoride ion, \(\mathrm{F}^{-}\), donates its extra pair of electrons to the boron trifluoride, \(\mathrm{BF}_{3}\), which doesn't have a full octet, making \(\mathrm{BF}_{3}\) the Lewis acid and \(\mathrm{F}^{-}\) the Lewis base. This transaction results in the formation of the \(\mathrm{BF}_{4}^{-}\) ion, showcasing the concept of electron pair acceptance and donation.

Complex Ion Formation

Complex ion formation often involves a central metal ion acting as a Lewis acid and coordinating with several Lewis bases. This coordination results in the formation of a complex ion, which is a central metal atom or ion bonded to one or more molecules or ions through coordinate covalent bonds.

Taking another look at the first reaction, the \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}\) ion is an example of such a complex. In this reaction, the silver ion coordinates with two ammonia molecules, which donate their electron pairs, forming a complex ion with a charge of +1. Complex ions are common in many areas of chemistry, including biochemistry, where they are involved in critical functions such as oxygen transport and enzyme activity.