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) \(\mathrm{Zn}^{2+}(a q)+4 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}(a q)\) (d) \(\mathrm{SO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{O}_{3}(a q)\)

Step-by-Step Solution

Answer

In the given reactions: (a) Lewis base: OH⁻; Lewis acid: HNO₂ (b) Lewis base: Br⁻; Lewis acid: FeBr₃ (c) Lewis base: NH₃; Lewis acid: Zn²⁺ (d) Lewis base: H₂O; Lewis acid: SO₂

Step 1: Reaction (a)

In this reaction, the OH⁻ ion donates a pair of electrons to the hydrogen in HNO₂, forming a bond and creating H₂O. The HNO₂ molecule accepts the electron pair. Thus, OH⁻ is the Lewis base, and HNO₂ is the Lewis acid: Lewis base: OH⁻ Lewis acid: HNO₂

Step 2: Reaction (b)

In this reaction, the Br⁻ ion donates a pair of electrons to the FeBr₃ molecule, creating a bond and forming FeBr₄⁻. The FeBr₃ molecule accepts the electron pair. Thus, Br⁻ is the Lewis base, and FeBr₃ is the Lewis acid: Lewis base: Br⁻ Lewis acid: FeBr₃

Step 3: Reaction (c)

In this reaction, the NH₃ molecules each donate a pair of electrons to the Zn²⁺ ion, creating bonds and forming Zn(NH₃)₄²⁺. The Zn²⁺ ion accepts the electron pairs. Thus, NH₃ is the Lewis base, and Zn²⁺ is the Lewis acid: Lewis base: NH₃ Lewis acid: Zn²⁺

Step 4: Reaction (d)

In this reaction, the H₂O molecule donates a pair of electrons to the sulfur atom in the SO₂ molecule, creating a bond and forming H₂O₃. The SO₂ molecule accepts the electron pair. Thus, H₂O is the Lewis base, and SO₂ is the Lewis acid: Lewis base: H₂O Lewis acid: SO₂

Key Concepts

Electron Pair Donor

In Lewis acid-base theory, one important player is the electron pair donor, known as the Lewis base. A Lewis base is any species that donates an electron pair to form a chemical bond. This donation occurs during chemical reactions and helps create new compounds.
In reaction (a), the hydroxide ion (\(\text{OH}^-\)) donates an electron pair to \(\text{HNO}_2\), forming water (\(\text{H}_2\text{O}\)).
This makes \(\text{OH}^-\) the Lewis base here. In reaction (b), bromide ions (\(\text{Br}^-\)) act as the electron pair donors when they interact with \(\text{FeBr}_3\) to form \(\text{FeBr}_4^-\), establishing \(\text{Br}^-\) as the Lewis base.
Similarly, in reaction (c), ammonia molecules (\(\text{NH}_3\)) provide electron pairs to the zinc ion (\(\text{Zn}^{2+}\)) to form \(\text{Zn(NH}_3)_4^{2+}\), making \(\text{NH}_3\) the electron pair donor.
Finally, in reaction (d), water (\(\text{H}_2\text{O}\)) provides electron pairs to \(\text{SO}_2\), becoming the Lewis base as it transforms into \(\text{H}_2\text{O}_3\).
In essence, whenever you identify a Lewis base, look for the species that gives away electron pairs to form new bonds.

Electron Pair Acceptor

The counterpart to the electron pair donor in Lewis acid-base theory is the electron pair acceptor, or the Lewis acid. A Lewis acid is a species that accepts an electron pair during a chemical reaction. This acceptance allows the formation of new chemical bonds and compounds.
In a close examination of these reactions, you will notice the trend where the Lewis acid generally becomes more stable through the acceptance of electron pairs.
For instance:

• In reaction (a), \(\text{HNO}_2\) accepts an electron pair from \(\text{OH}^-\). Thus, \(\text{HNO}_2\) is the Lewis acid.
• In reaction (b), \(\text{FeBr}_3\) is the one accepting the electron pair from \(\text{Br}^-\), making \(\text{FeBr}_3\) the Lewis acid.
• For reaction (c), \(\text{Zn}^{2+}\) ion is the electron pair acceptor from \(\text{NH}_3\), classifying it as the Lewis acid.
• Lastly, in reaction (d), \(\text{SO}_2\) accepts electron pairs from water (\(\text{H}_2\text{O}\)), reinforcing \(\text{SO}_2\) as the Lewis acid in this scenario.

The general role of Lewis acids is crucial because they facilitate the formation of chemical bonds by accepting electron pairs.

Chemical Reactions Analysis

To truly understand Lewis acid-base interactions, one must analyze the chemical reactions involved. This involves recognizing which species in a reaction donates or accepts electron pairs, thus identifying the Lewis base and acid.
Each reaction symbolizes a dynamic exchange of electrons, forming new chemical bonds. In a detailed analysis, spotting the changes in how electrons are shared or transferred is key.

• For instance, in reaction (a) between \(\text{HNO}_2\) and \(\text{OH}^-\), tracking electron movement reveals the transformation into water, emphasizing the \(\text{OH}^-\) as a donor, and \(\text{HNO}_2\) as an acceptor.
• In reaction (b) involving \(\text{FeBr}_3\) and \(\text{Br}^-\), the shift to form \(\text{FeBr}_4^-\) showcases a clear pairing of donator and acceptor roles.
• Similarly, in reaction (c), the electron exchange between \(\text{Zn}^{2+}\) and \(\text{NH}_3\) enlarges the molecular understanding by forming a stable complex ion, \(\text{Zn(NH}_3)_4^{2+}\).
• Lastly, reaction (d) displays how molecular interactions transform \(\text{SO}_2\) and water into a new compound, \(\text{H}_2\text{O}_3\), marking another classic example of electron pair movement.

Studying these reactions helps break down complex interactions into more manageable steps, solidifying your grasp of how compounds engage during reactions.