Problem 81 - Chemistry and Chemical Reactivity

Question

Decide whether each of the following substances should be classified as a Lewis acid or a Lewis base. (a) \(\mathrm{H}_{2} \mathrm{NOH}\) in the reaction \(\mathrm{H}_{2} \mathrm{NOH}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \longrightarrow\left[\mathrm{H}_{3} \mathrm{NOH}\right] \mathrm{Cl}(\mathrm{aq})\) (b) \(\mathrm{Fe}^{2+}\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (Hint: Draw the electron dot structure.

Step-by-Step Solution

Answer

(a) Lewis base, (b) Lewis acid, (c) Lewis base.

Step 1: Identify the Role of Each Component in Part (a)

In the reaction \(\mathrm{H}_{2}\mathrm{NOH} + \mathrm{HCl} \to \left[\mathrm{H}_{3}\mathrm{NOH}\right]\mathrm{Cl}\), \(\mathrm{H}_{2}\mathrm{NOH}\) is gaining \(\mathrm{H}^+\) ions to become \(\left[\mathrm{H}_{3}\mathrm{NOH}\right]^+\). Since it accepts a proton (or accepts an electron pair), it acts as a Lewis base.

Step 2: Determine the Role of Fe²⁺ in Part (b)

\(\mathrm{Fe}^{2+}\) has vacant orbitals that can accept electron pairs from donor molecules. Thus, it acts as a Lewis acid because it can accept an electron pair.

Step 3: Examine CH₃NH₂ in Part (c)

\(\mathrm{CH}_{3}\mathrm{NH}_{2}\) has a lone pair of electrons on nitrogen. This lone pair can be donated to accept protons or other electron pair acceptors, making \(\mathrm{CH}_{3}\mathrm{NH}_{2}\) a Lewis base.

Key Concepts

Lewis acid

A Lewis acid is any chemical species that can accept an electron pair. This idea comes from the Lewis theory, which expands on the traditional acid-base concept.

Lewis acids can include a variety of substances like metal cations, certain molecules, and more.

• In the Lewis theory, an acid does not necessarily need to contain protons.
• It focuses on the ability to accept electrons, rather than just donating protons as described in the classical definitions.

In part (b) of the exercise, \( \mathrm{Fe}^{2+} \) is an example of a Lewis acid. This ion has empty orbitals that allow it to accept electron pairs. Many metal cations, like \( \mathrm{Fe}^{2+} \), are good examples of Lewis acids because of their need to attain a stable electronic configuration by gaining electron pairs.

Lewis base

Understanding a Lewis base helps us fully comprehend how reactions occur and why certain substances behave as they do.

A Lewis base is a substance that donates an electron pair to form a chemical bond.

• The electron pair donor nature is central to the definition in Lewis theory.
• Unlike the Brownsted-Lowry definition, where a base must accept a proton, a Lewis base only needs to provide an electron pair.

In part (a) of the exercise, \( \mathrm{H}_{2}\mathrm{NOH} \) demonstrated this concept. It accepted a proton from \( \mathrm{HCl} \) by donating an electron pair. Likewise, in part (c),\( \mathrm{CH}_{3}\mathrm{NH}_{2} \) is identified as a Lewis base because of the lone pair of electrons on the nitrogen atom. These electrons are available for bonding with other species, showcasing the typical behavior of a Lewis base.

electron pair

Electron pairs are integral in the Lewis theory. They form the basis for understanding how Lewis acids and bases interact.

An electron pair is a pair of electrons occupying an orbital in an atom or molecule. Such pairs are significant as they define how atoms and molecules bond.

• In the context of Lewis acids and bases, "lone pairs" are highly important. These are electron pairs not engaged in bonding.
• Lone pairs can be donated in bonds. This characteristic is central to defining a Lewis base.

In the exercise, both part (a) with \( \mathrm{H}_{2}\mathrm{NOH} \) and part (c) \( \mathrm{CH}_{3}\mathrm{NH}_{2} \) employ electron pairs for bonding actions. The basic principle revolves around donating these lone pairs that they possess. In contrast, \( \mathrm{Fe}^{2+} \) as a Lewis acid, utilizes its capability to accept these electron pairs, revealing the complementary nature of acids and bases within Lewis theory.