Problem 96 - Chemistry: The Central Science

Question

Identify the Lewis acid and Lewis base in each of the following reactions: (a) \(\mathrm{HNO}_{2}(a q)+\mathrm{OH}^{-}(a q) \rightleftharpoons \mathrm{NO}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l)\) (b) \(\mathrm{FeBr}_{3}(s)+\mathrm{Br}^{-}(a q) \rightleftharpoons \mathrm{FeBr}_{4}^{-}(a q)\) (c) \(\mathrm{Zn}^{2+}(a q)+4 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}{\underline{\phantom{xx}}}^{2+}(a q)\) (d) \(\mathrm{SO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{3}(a q)\)

Step-by-Step Solution

Answer

(a) Lewis acid: \(\mathrm{HNO}_{2}(a q)\), Lewis base: \(\mathrm{OH}^{-}(a q)\) (b) Lewis acid: \(\mathrm{FeBr}_{3}(s)\), Lewis base: \(\mathrm{Br}^{-}(a q)\) (c) Lewis acid: \(\mathrm{Zn}^{2+}(a q)\), Lewis base: \(\mathrm{NH}_{3}(a q)\) (d) Lewis acid: \(\mathrm{SO}_{2}(g)\), Lewis base: \(\mathrm{H}_{2} \mathrm{O}(l)\)

Step 1: Reaction (a)

In this reaction, \(\mathrm{HNO}_{2}(a q)\) reacts with \(\mathrm{OH}^{-}(a q)\) to produce \(\mathrm{NO}_{2}^{-}(a q)\) and \(\mathrm{H}_{2} \mathrm{O}(l)\). The \(\mathrm{OH}^{-}(a q)\) ion donates a pair of electrons to form a bond with \(\mathrm{HNO}_{2}\) and transforms into water, while \(\mathrm{HNO}_{2}\) accepts this pair of electrons to create \(\mathrm{NO}_{2}^{-}(a q)\). Thus, the Lewis acid is \(\mathrm{HNO}_{2}(a q)\), and the Lewis base is \(\mathrm{OH}^{-}(a q)\).

Step 2: Reaction (b)

In this reaction, \(\mathrm{FeBr}_{3}(s)\) reacts with \(\mathrm{Br}^{-}(a q)\) to produce \(\mathrm{FeBr}_{4}^{-}(a q)\). The \(\mathrm{Br}^{-}(a q)\) ion donates a pair of electrons to form a bond with \(\mathrm{FeBr}_{3}\) and creates \(\mathrm{FeBr}_{4}^{-}(a q)\). \(\mathrm{FeBr}_{3}\) accepts this pair of electrons, so the Lewis acid is \(\mathrm{FeBr}_{3}(s)\), and the Lewis base is \(\mathrm{Br}^{-}(a q)\).

Step 3: Reaction (c)

In this reaction, \(\mathrm{Zn}^{2+}(a q)\) reacts with \(4 \mathrm{NH}_{3}(a q)\) to produce \(\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}{\underline{\phantom{xx}}}^{2+}(a q)\). Each \(\mathrm{NH}_{3}\) molecule donates a pair of electrons to form a bond with \(\mathrm{Zn}^{2+}\) and creates the complex ion \(\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}{\underline{\phantom{xx}}}^{2+}(a q)\). \(\mathrm{Zn}^{2+}\) accepts these pairs of electrons, so the Lewis acid is \(\mathrm{Zn}^{2+}(a q)\), and the Lewis base is \(\mathrm{NH}_{3}(a q)\).

Step 4: Reaction (d)

In this reaction, \(\mathrm{SO}_{2}(g)\) reacts with \(\mathrm{H}_{2} \mathrm{O}(l)\) to produce \(\mathrm{H}_{2} \mathrm{SO}_{3}(a q)\). The water molecule \(\mathrm{H}_{2} \mathrm{O}(l)\) donates a pair of electrons to form a bond with \(\mathrm{SO}_{2}\) and creates \(\mathrm{H}_{2} \mathrm{SO}_{3}(a q)\). \(\mathrm{SO}_{2}\) accepts this pair of electrons, so the Lewis acid is \(\mathrm{SO}_{2}(g)\), and the Lewis base is \(\mathrm{H}_{2} \mathrm{O}(l)\).

Key Concepts

Lewis Acid Definition

In chemistry, understanding Lewis acids is crucial for grasping various reactions. A Lewis acid is a chemical species that accepts an electron pair to form a covalent bond. Unlike the traditional definition of an acid (proton donor), a Lewis acid does not need to possess a hydrogen atom.

Examples of Lewis acids include metal ions like Fe³⁺ or molecules with polar double bonds, such as carbon dioxide (CO₂). In essence, any species that is electron-deficient and ready to receive an electron pair can be classified as a Lewis acid.

Characteristics of Lewis Acids:

• Electron-deficient
• Can accept an electron pair
• Does not necessarily release a proton

Understanding the concept of Lewis acids not only helps in identification during reactions but also is pivotal in predicting the products and mechanisms of chemical processes.

Lewis Base Definition

Complementing the Lewis acids, a Lewis base is a species that donates an electron pair to form a covalent bond. These bases are electron-rich and have lone pairs of electrons readily available for donation.

An example of a Lewis base is the hydroxide ion (OH⁻), which can donate its lone pair to an acid. The capability of donating an electron pair makes Lewis bases an integral part of numerous chemical reactions, such as complexation and neutralization.

Properties of Lewis Bases:

• Electron-rich species
• Donates an electron pair
• Involved in bond formation

When studying reactions, it's vital to identify Lewis bases as they often dictate the course of chemical events and the stability of resulting compounds.

Chemical Reactions Identification

In order to understand the behavior of substances during a chemical reaction, one must excel at reaction identification. This involves recognizing reactants and predicting products. Notably, in reactions involving Lewis acids and bases, the transfer of electron pairs is a key feature to watch out for.

When identifying these reactions, observe the movement of electrons and how new bonds are formed. It's significant to distinguish the Lewis acid, the recipient of an electron pair, and the Lewis base, the donor of the electron pair. By pinpointing these roles, one can deduce the mechanism and the possible products in the reaction.

Steps for Reaction Identification:

• Observe electron pair transfer
• Identify the Lewis acid and base
• Predict the direction of the reaction

By mastering these steps, students can approach complex chemistry problems with ease and develop a keen analytical mind for substances in reactions.

Electron Pair Donors and Acceptors

Understanding electron pair donors and acceptors is fundamental to the Lewis theory of acids and bases. Electron pair donors are Lewis bases, as they provide electron pairs for bonding. Acceptors, on the other hand, are Lewis acids and they accept electron pairs to complete bonding.

The interaction of donors and acceptors can lead to the formation of a coordinate covalent bond, where one atom provides both electrons for the bond. This concept is vital for the prediction and explanation of complex formations, reactivity, and the properties of the products formed.

Implications of Electron Pair Donation and Acceptance:

• Formation of coordinate covalent bonds
• Determine the strength and stability of complexes
• Influence the structure and geometry of molecules

By thoroughly understanding donors and acceptors, students can better visualize and approach complex chemical reactions, enabling them to succeed in their chemistry education and beyond.