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) \(\mathrm{SO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{3}(a q)\)

Short answer

In the given reactions: (a) Lewis acid: \(\mathrm{HNO}_{2}\), Lewis base: \(\mathrm{OH}^{-}\) (b) Lewis acid: \(\mathrm{FeBr}_{3}\), Lewis base: \(\mathrm{Br}^{-}\) (c) Lewis acid: \(\mathrm{Zn}^{2+}\), Lewis base: \(\mathrm{NH}_{3}\) (d) Lewis acid: \(\mathrm{SO}_2\), Lewis base: \(\mathrm{H}_{2}\mathrm{O}\)

Step-by-step solution

Identify Lewis Acid in Reaction (a)

In this reaction, the \(\mathrm{HNO}_{2}\) molecule, when acting as an electron-pair acceptor, can form the bond with the \(\mathrm{OH}^{-}\) ion. Therefore, \(\mathrm{HNO}_{2}\) is the Lewis acid.

Identify Lewis Base in Reaction (a)

The hydroxide ion (\(\mathrm{OH}^{-}\)) has a lone pair of electrons which it can donate to form a bond with \(\mathrm{HNO}_{2}\). Thus, \(\mathrm{OH}^{-}\) is the Lewis base. (b) $\mathrm{FeBr}_{3}(s)+\mathrm{Br}^{-}(a q) \rightleftharpoons \mathrm{FeBr}_{4}^{-}(a q)$

Identify Lewis Acid in Reaction (b)

In this reaction, the \(\mathrm{FeBr}_{3}\) molecule can accept an electron pair from the \(\mathrm{Br}^{-}\) ion. Therefore, \(\mathrm{FeBr}_{3}\) is the Lewis acid.

Identify Lewis Base in Reaction (b)

The bromide ion (\(\mathrm{Br}^{-}\)) has a lone pair of electrons which it can donate to form a bond with \(\mathrm{FeBr}_{3}\). Thus, \(\mathrm{Br}^{-}\) is the Lewis base. (c) $\mathrm{Zn}^{2+}(a q)+4 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}(a q)$

Identify Lewis Acid in Reaction (c)

In this reaction, the \(\mathrm{Zn}^{2+}\) ion can accept electron pairs from the four ammonia (\(\mathrm{NH}_{3}\)) molecules. Therefore, \(\mathrm{Zn}^{2+}\) is the Lewis acid.

Identify Lewis Base in Reaction (c)

Ammonia (\(\mathrm{NH}_{3}\)) has a lone pair of electrons it can donate to form bonds with the \(\mathrm{Zn}^{2+}\) ion. Thus, \(\mathrm{NH}_{3}\) is the Lewis base. (d) $\mathrm{SO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{3}(a q)$

Identify Lewis Acid in Reaction (d)

In this reaction, the \(\mathrm{SO}_{2}\) molecule can accept a lone pair of electrons from the water (\(\mathrm{H}_{2}\mathrm{O}\)) molecule to form \(\mathrm{H}_{2}\mathrm{SO}_{3}\). Therefore, \(\mathrm{SO}_{2}\) is the Lewis acid.

Identify Lewis Base in Reaction (d)

Water (\(\mathrm{H}_{2}\mathrm{O}\)) can donate a lone pair of electrons to form a bond with the \(\mathrm{SO}_{2}\) molecule. Thus, \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis base.

Key concepts

Lewis Acid

In chemistry, a Lewis acid is defined as a substance that can accept an electron pair from another substance during a chemical reaction. Lewis acids are not limited to protons. They can include metal cations, molecules with partial positive charges, or any atom which has an empty orbital ready to accommodate an electron pair.

• Examples of Lewis acids include \( ext{HNO}_2\), \( ext{FeBr}_3\), and \( ext{Zn}^{2+}\) as demonstrated in the exercise reactions.
• In reaction (a), \( ext{HNO}_2\) acts as a Lewis acid because it accepts an electron pair from \( ext{OH}^-\). In reaction (b), \( ext{FeBr}_3\) accepts an electron pair from \( ext{Br}^-\).
• Being positively charged or having a positively polarized region often signifies the ability to act as a Lewis acid, such as the \( ext{Zn}^{2+}\) ion in reaction (c).

Lewis Base

A Lewis base is a substance capable of donating an electron pair to a Lewis acid to form a chemical bond. This relation works hand in hand with the concept of Lewis acids.

• A common characteristic of Lewis bases is the presence of lone pair electrons ready to be shared with an electron-pair acceptor.
• For example, in the given reactions, \( ext{OH}^-\), \( ext{Br}^-\), and \( ext{NH}_3\) exhibit Lewis base behavior because they donate electron pairs to their respective Lewis acids.

Lewis bases can be molecules or ions, often possessing lone pair electrons on elements like nitrogen or oxygen. This property enables them to form coordinate bonds with Lewis acids.

Electron Pair Donor

An electron pair donor is a central characteristic of any Lewis base. During reactions, the electron pair donor holds a pair of electrons not used in bonding, often referred to as a lone pair.

• For instance, the hydroxide ion \( ext{OH}^-\) in reaction (a) donates a pair of electrons to \( ext{HNO}_2\), while ammonia \( ext{NH}_3\) in reaction (c) donates a lone pair to \( ext{Zn}^{2+}\).
• This act of donation forms a coordinate covalent bond between the donor and acceptor, with the formation of such bonds being a hallmark of Lewis acid-base reactions.

Understanding electron pair donors is crucial in predicting how substances will interact in solution, especially in complexation reactions or biosystems.

Electron Pair Acceptor

The role of an electron pair acceptor is integral to the function of a Lewis acid in chemical reactions. These substances are adept at taking in the electron pairs offered by Lewis bases.

• In our example reactions, \( ext{SO}_2\) interacts with water \( ext{H}_2 ext{O}\) by accepting an electron pair to form \( ext{H}_2 ext{SO}_3\). Similarly, \( ext{FeBr}_3\) accepts electrons from \( ext{Br}^-\).
• Compounds which act as electron pair acceptors typically have incomplete electron shells or positive regions that allow electrons to be accepted readily.

Recognizing an electron pair acceptor helps in identifying potential reactions between molecules, especially in the fields of inorganic and coordination chemistry.