Problem 13 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

Identify the Lewis acid and Lewis base in the following reactions. (Section 7.8) (a) \(\quad I_{2}+I^{-} \rightarrow I_{3}^{-}\) (b) \(\mathrm{NH}_{3}+\mathrm{HBr} \rightarrow \mathrm{NH}_{4} \mathrm{Br}\) (c) \(\operatorname{SiF}_{4}+2 p y \rightarrow \operatorname{SiF}_{4}(p y)_{2}\) (d) \(\mathrm{CO}_{2}+\mathrm{OH}^{-} \rightarrow \mathrm{HCO}_{3}^{-}\) Which, if any, of the reactions are also Bronsted-Lowry acidbase reactions.

Step-by-Step Solution

Answer

Lewis acids: (a) \( I_2 \), (b) \( H^+ \), (c) \( SiF_4 \), (d) \( CO_2 \). Bronsted-Lowry reactions: (b) and (d).

Step 1: Identify the Species in Reaction (a)

For the reaction \( I_2 + I^- \rightarrow I_3^- \), \( I^- \) donates an electron pair to \( I_2 \), forming \( I_3^- \). Therefore, \( I_2 \) is the Lewis acid because it accepts an electron pair, and \( I^- \) is the Lewis base since it donates an electron pair.

Step 2: Determine Lewis Acid/Base for Reaction (b)

In the reaction \( NH_3 + HBr \rightarrow NH_4Br \), \( NH_3 \) donates an electron pair to \( H^+ \) from \( HBr \), forming \( NH_4^+ \). Here, \( NH_3 \) is the Lewis base (electron pair donor), and \( H^+ \) coming from \( HBr \) is the Lewis acid (electron pair acceptor). This is also a Brønsted-Lowry acid-base reaction because \( HBr \) donates a proton to \( NH_3 \).

Step 3: Analyze Reaction (c)

In \( SiF_4 + 2 py \rightarrow SiF_4(py)_2 \), the \( SiF_4 \) accepts electron pairs from the two pyridine (#2 py) molecules. Thus, \( SiF_4 \) acts as the Lewis acid and pyridine (py) acts as the Lewis base.

Step 4: Examine Reaction (d)

For \( CO_2 + OH^- \rightarrow HCO_3^- \), \( CO_2 \) accepts an electron pair from \( OH^- \) to form \( HCO_3^- \). Thus, \( CO_2 \) is the Lewis acid and \( OH^- \) is the Lewis base. This reaction is also Brønsted-Lowry because \( OH^- \) acts as a base by accepting a proton to form \( HCO_3^- \).

Step 5: Summary of Brønsted-Lowry Reactions

The reactions \( NH_3 + HBr \rightarrow NH_4Br \) and \( CO_2 + OH^- \rightarrow HCO_3^- \) are also Brønsted-Lowry acid-base reactions due to proton transfer processes.

Key Concepts

Lewis acid

A Lewis acid is a species that can accept an electron pair from a Lewis base. It typically has an electron-deficient structure, which makes it eager to accept electrons. In the given reactions, we can see examples of Lewis acids:

• In reaction (a), \( I_2 \) acts as the Lewis acid because it lacks sufficient electrons and accepts an electron pair from \( I^- \) to form \( I_3^- \).
• In reaction (b), \( H^+ \) from \( HBr \) acts as the Lewis acid, eager to accept the electron pair from \( NH_3 \).
• Reaction (c) demonstrates \( SiF_4 \) as the Lewis acid by accepting electron pairs from pyridine molecules.
• Finally, in reaction (d), \( CO_2 \) serves as the Lewis acid by accepting electron pairs from \( OH^- \).

In each case, the species accepting electron pairs earn the title of Lewis acid.

Lewis base

A Lewis base is any species that is capable of donating an electron pair to a Lewis acid. This quality makes Lewis bases rich in electrons or electron-rich regions:

• Looking at reaction (a), \( I^- \) serves as the Lewis base due to its ability to donate an electron pair to \( I_2 \).
• In reaction (b), \( NH_3 \) is the Lewis base, lending an electron pair to \( H^+ \).
• Reaction (c) showcases pyridine (\( py \)) as the Lewis base, supplying electron pairs to \( SiF_4 \).
• Lastly, in reaction (d), \( OH^- \) takes on the role of Lewis base, providing an electron pair to \( CO_2 \).

In these reactions, Lewis bases are typically substances that possess lone pairs or electrons that readily interact with the electron-deficient Lewis acids.

Brønsted-Lowry acid-base reactions

The Brønsted-Lowry theory expands on the idea of acids and bases as proton donors and accepters:

• In reaction (b), \( HBr \) donates a proton (\( H^+ \)) to \( NH_3 \), forming \( NH_4Br \), and thus represents a Brønsted-Lowry acid-base reaction.
• Similarly, in reaction (d), \( OH^- \) accepts a proton to become \( HCO_3^- \), characterizing a Brønsted-Lowry interaction as well.

Unlike the Lewis concept, which is centered on electron pair transfer, Brønsted-Lowry reactions are all about the movement of protons (\( H^+ \)). Identifying these reactions help in understanding both electron pair donation and proton transfer actions.

electron pair donor

An electron pair donor is essentially a Lewis base. It is a species that donates an electron pair to another atom or molecule. In essence, while operating as a donor, the following reactions depict electron pair transactions:

• In reaction (a), \( I^- \) donates an electron pair to \( I_2 \) to form \( I_3^- \).
• \( NH_3 \) donates a pair of electrons to \( H^+ \) from \( HBr \) in reaction (b).
• For reaction (c), pyridine donates electron pairs to \( SiF_4 \) .
• Finally, in reaction (d), \( OH^- \) acts as an electron pair donor to \( CO_2 \).

In chemistry, mastering the concept of electron pair donation is fundamental to understanding reactions that involve the formation of new chemical bonds.

proton transfer

Proton transfer is a hallmark of Brønsted-Lowry reactions. A proton, essentially a hydrogen ion (\( H^+ \)), is transferred from an acid to a base, forming a conjugate base-acid pair.

• In reaction (b), \( HBr \) transfers a proton to \( NH_3 \), creating \( NH_4^+ \) and \( Br^- \).
• Similarly, in reaction (d), the \( OH^- \) ion transforms by receiving a proton to become \( HCO_3^- \) .

Proton transfer reactions are crucial for understanding the concept of acidity and basicity in environments influenced by the Brønsted-Lowry theory.These transformations highlight the flow of protons and how they drive chemical changes in various substances.