Question

Lewis Acids and Bases Classify each species as a Lewis acid or a Lewis base. $$\begin{array}{l}{\text { a. } \mathrm{BeCl}_{2}} \\ {\text { b. } \mathrm{OH}^{-}} \\ {\text { c. } \mathrm{B}(\mathrm{OH})_{3}} \\ {\text { d. } \mathrm{CN}^{-}}\end{array}$$

Short answer

a. BeCl2 is a Lewis acid, b. OH- is a Lewis base, c. B(OH)3 is a Lewis acid, d. CN- is a Lewis base.

Step-by-step solution

Understanding Lewis Acid-Base Theory

In the Lewis theory of acid-base reactions, a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. This definition does not require the acid to have a hydrogen ion (H+), so it's broader than the Brønsted-Lowry definition.

Identifying the Lewis Acid/Base for BeCl2

BeCl2 can accept an electron pair because the beryllium (Be) atom only has four electrons in its valence shell, meaning it is electron-deficient and can accept a pair to complete its octet. This trait qualifies BeCl2 as a Lewis acid.

Identifying the Lewis Acid/Base for OH-

The hydroxide ion (OH-) has a lone pair of electrons on the oxygen atom that it can donate. Since it can donate this pair of electrons, OH- is classified as a Lewis base.

Identifying the Lewis Acid/Base for B(OH)3

Boron in B(OH)3, similar to beryllium in BeCl2, is electron-deficient with only six valence electrons. Thus, it can accept an electron pair and is classified as a Lewis acid.

Identifying the Lewis Acid/Base for CN-

The cyanide ion (CN-) has a lone pair of electrons on the carbon atom, which it can donate. Therefore, CN- is a Lewis base.

Key concepts

Lewis acid-base theory

The Lewis acid-base theory is a fundamental concept in chemistry that enhances our understanding of how acids and bases react. Unlike other theories, such as the Brønsted-Lowry theory, which is centered around proton donors and acceptors, the Lewis theory focuses on the transfer of electron pairs. It provides a broader insight into acid-base reactions by considering any species that can accept or donate an electron pair, not just those that involve hydrogen ions.

This theory was proposed by Gilbert N. Lewis in 1923 and has since been invaluable for predicting the outcomes of a wide range of chemical reactions. Understanding this theory allows for the identification of reactants in complex reactions where traditional definitions of acids and bases might not be applicable.

Electron pair acceptor

In the realm of the Lewis acid-base theory, an electron pair acceptor is simply known as a Lewis acid. These are species that have an incomplete valence shell and thus possess the capability to accept an electron pair to achieve a stable electronic arrangement, typically an octet for main-group elements.

Common characteristics of Lewis acids include central atoms with a positive charge or those that are electron-deficient. Examples include metallic ions such as Fe³⁺ and molecules like BF₃ or BeCl₂ from our exercise, where the central atom can accept electron pairs to form covalent bonds.

Electron pair donor

Opposite to the electron pair acceptor, a Lewis base is a species capable of donating an electron pair. These donors are pivotal in the formation of coordinate covalent bonds where the donated electron pair is shared with a Lewis acid. Lewis bases usually have at least one lone pair of valence electrons that is not involved in bonding and can therefore be shared.

An example, as indicated in the solution to our exercise, is the hydroxide ion (OH^-), which can donate its lone electron pair to form a bond. Other typical Lewis bases may include molecules with nitrogen, oxygen, or sulfur atoms that tend to harbor lone pairs, such as NH₃ or H₂O.

Valence shell electrons

Valence shell electrons are the outermost electrons of an atom and are of prime importance as they determine an atom's chemical properties and its ability to bond with other atoms. They are involved in forming both ionic and covalent bonds – the two main types of chemical bonding.

In the context of Lewis acid-base theory, valence electrons play a critical role as the electron pair donor or acceptor interaction is based primarily on the atoms' valence electrons. For instance, the Be in BeCl₂ seeks two more electrons to complete its valence shell, signifying its role as a Lewis acid in our exercise. In contrast, species like OH^- and CN^- with extra pairs of valence electrons act as Lewis bases, ready to donate those pairs to form bonds.