Problem 95 - Chemistry: The Central Science in SI Units, Global Edition

Question

Identify the Lewis acid and Lewis base among the reactants in each of the following reactions: (a) \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}(s)+6 \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons\) $$ \left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}(a q)+3 \mathrm{ClO}_{4}^{-}(a q) $$ (b) \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}(g)+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(s)\) (d) \(\mathrm{HIO}(l q)+\mathrm{NH}_{2}^{-}(l q) \rightleftharpoons \mathrm{NH}_{3}(l q)+\mathrm{IO}^{-}(l q)\) (lg denotes liquid ammonia as solvent)

Step-by-Step Solution

Answer

In the given reactions: (a) \(\mathrm{Fe}^{3+}\) is the Lewis acid and \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis base. (b) \(\mathrm{H}_{2} \mathrm{O}\) is the Lewis acid and \(\mathrm{CN^{-}}\) is the Lewis base. (c) \(\mathrm{BF}_{3}\) is the Lewis acid and \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}\) is the Lewis base. (d) \(\mathrm{HIO}(l q)\) is the Lewis acid and \(\mathrm{NH}_{2}^{-}(l q)\) is the Lewis base.

Step 1: Reaction (a)

In this reaction \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}(s)+6 \mathrm{H}_{2}\mathrm{O}(l) \rightleftharpoons\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}(a q)+3\mathrm{ClO}_{4}^{-}(a q)\), we need to identify the Lewis acid and Lewis base. The six water molecules \((\mathrm{H}_{2} \mathrm{O})\) each donate an electron pair to \(\mathrm{Fe}^{3+}\). Thus, the water is the Lewis base, and \(\mathrm{Fe^{3+}}\) is the Lewis acid.

Step 2: Reaction (b)

In this reaction \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\), we need to identify the Lewis acid and Lewis base. In this case, the \(\mathrm{CN^{-}}\) ion donates an electron pair to the \(\mathrm{H}\) atom in water. So, the \(\mathrm{CN^{-}}\) ion is the Lewis base, and \(\mathrm{H}_{2} \mathrm{O}\) is the Lewis acid.

Step 3: Reaction (c)

In this reaction \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}(g)+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(s)\), we need to identify the Lewis acid and Lewis base. Trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}(g)\), donates an electron pair to the boron atom in \(\mathrm{BF}_{3}(g)\). Therefore, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}\) is the Lewis base, and \(\mathrm{BF}_{3}\) is the Lewis acid.

Step 4: Reaction (d)

In this reaction \(\mathrm{HIO}(l q)+\mathrm{NH}_{2}^{-}(l q) \rightleftharpoons \mathrm{NH}_{3}(l q)+\mathrm{IO}^{-}(l q)\), we need to identify the Lewis acid and Lewis base. In this case, \(\mathrm{NH}_{2}^{-}(l q)\) donates an electron pair to the \(\mathrm{H}\) atom in \(\mathrm{HIO}(l q)\). Thus, \(\mathrm{NH}_{2}^{-}(l q)\) is the Lewis base, and \(\mathrm{HIO}(l q)\) is the Lewis acid.

Key Concepts

Chemical Reactions

Understanding chemical reactions is crucial in chemistry. During a chemical reaction, substances transform to create new products. In the context of Lewis Acid-Base Theory, the focus is specifically on how molecules interact based on electron pair transfers.
A typical reaction involves one component acting as an acid, and the other as a base. The Lewis acid is typically an electron pair acceptor, while the Lewis base donates an electron pair. This interaction leads to the formation of a new compound or compounds as observed in the provided reactions.
To 'see' these reactions happening, consider the detailed participation of each molecule.

• For instance, in reaction (a), iron(III) perchlorate (\[\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}\]) reacts with water to form a hydrated iron cation complex.
• Identifying Lewis acids and bases in these transformations helps in understanding how components attract and share electrons during their interaction.

Electron Pair Donation

In the realm of Lewis Acid-Base Theory, electron pair donation is a fundamental concept. It describes the action where a molecule offers to share an electron pair with another atom or molecule.
This process is what defines a molecule as a Lewis base. A Lewis base must have at least one pair of non-bonding electrons available for donation.
Consider the following examples:

• In reaction (c), trimethylamine \( \left( \mathrm{CH}_{3} \right)_{3} \mathrm{N} \) serves as a Lewis base as it donates an electron pair to boron trifluoride, \( \mathrm{BF}_{3} \).
• This donation allows them to form a coordinate covalent bond, thus creating a new complex.

The significance of electron pair donation is in understanding how stability in chemical structures can be achieved through these interactions. It plays a major role in designing new reactions and in predicting reaction behavior.

Coordination Chemistry

Coordination chemistry revolves around the formation and characteristics of coordination compounds, where central atoms bond with surrounding ligands.
These are often Lewis acids, capable of accepting electron pairs from Lewis bases, the ligands. Reaction (a) is a perfect example of coordination chemistry in action.
Here, \[\mathrm{Fe}^{3+}\] ion acts as the central atom, while water molecules serve as ligands.

• Each water molecule donates an electron pair, resulting in the formation of a complex ion, \( \left[ \mathrm{Fe} \left( \mathrm{H}_{2} \mathrm{O} \right)_{6} \right]^{3+} \).
• These interactions are key to understanding both the structure and stability of coordination compounds.

Coordination chemistry is highly relevant in various applications, including the development of catalysts and materials with unique properties. It is a cornerstone of understanding complex formations across different fields of chemistry.