Question

Identify the Lewis acid and the Lewis base in each of the following reactions. a. \(\mathrm{Fe}^{3+}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}(a q)\) b. \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CN}^{-}(a q) \Longrightarrow \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) c. \(\mathrm{HgI}_{2}(s)+2 \mathrm{I}^{-}(a q) \rightleftharpoons \mathrm{Hgl}_{4}^{2-}(a q)\)

Short answer

For 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. \(\mathrm{HgI}_{2}\) is the Lewis acid and \(\mathrm{I}^{-}\) is the Lewis base.

Step-by-step solution

Identifying Lewis acid and Lewis base in reaction (a)#

Given the reaction: \(\mathrm{Fe}^{3+}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}(a q)\) In this reaction, the \(\mathrm{Fe}^{3+}\) ion can accept a pair of electrons from the water molecules. Therefore, the \(\mathrm{Fe}^{3+}\) ion acts as the Lewis acid, while the \(\mathrm{H}_{2} \mathrm{O}\) molecules act as the Lewis base.

Identifying Lewis acid and Lewis base in reaction (b)#

Given the reaction: \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CN}^{-}(a q) \Longrightarrow \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) In this reaction, the \(\mathrm{CN}^{-}\) ion donates a pair of electrons to the \(\mathrm{H}_{2} \mathrm{O}\) molecule. Therefore, the \(\mathrm{CN}^{-}\) ion acts as the Lewis base, while the \(\mathrm{H}_{2} \mathrm{O}\) molecule acts as the Lewis acid.

Identifying Lewis acid and Lewis base in reaction (c)#

Given the reaction: \(\mathrm{HgI}_{2}(s)+2 \mathrm{I}^{-}(a q) \rightleftharpoons \mathrm{Hgl}_{4}^{2-}(a q)\) In this reaction, the \(\mathrm{I}^{-}\) ion donates a pair of electrons to the \(\mathrm{HgI}_{2}\) molecule. Therefore, the \(\mathrm{I}^{-}\) ion acts as the Lewis base, while the \(\mathrm{HgI}_{2}\) molecule acts as the Lewis acid.

Key concepts

Lewis acid

In the world of chemistry, a Lewis acid is a substance that can accept a pair of electrons. Think of it as an electron "hungry" molecule or ion that wants to form a bond by taking electrons from another compound. This electron acceptance is a key feature in Lewis acid-base reactions.
Some everyday examples of Lewis acids include:

• Metal ions like \(\mathrm{Fe}^{3+}\) or \(\mathrm{HgI}_{2}\).
• Non-metal compounds like boron trifluoride (BF₃).

When these substances come into contact with electron-rich molecules (Lewis bases), they form a new compound through electron pair sharing.
This interaction emphasizes the central role of electron density in chemical bonding, illustrating how a substance with an electron deficit pairs up with one having excess electrons.

Lewis base

A Lewis base is essentially the opposite of a Lewis acid. It is a substance that can donate a pair of electrons. Think of it as an electron "rich" molecule or ion willing to share its electrons with a Lewis acid to create a new bond. This donation of an electron pair is what characterizes a molecule or ion as a Lewis base.
Some common examples of Lewis bases include:

• Water \(\mathrm{H}_2\mathrm{O}\)
• Cyanide ion \(\mathrm{CN}^{-}\)

These molecules or ions have lone pairs of electrons that they can "donate" to a Lewis acid, facilitating the formation of a bond and stabilizing the resulting structure.
In a chemical reaction, identifying the Lewis base involves pinpointing which substance is giving up an electron pair to form a connection with the Lewis acid.

Electron pair donor

An electron pair donor is a substance that has an extra pair of electrons that it can give away to form a bond. This term is essentially synonymous with a Lewis base, as it describes the molecule or ion's readiness to share its electron pair.
The action of donating an electron pair plays a crucial role in initiating chemical reactions, as it allows the formation of stable chemical bonds.
When identifying an electron pair donor in a chemical reaction:

• Look for molecules or ions with lone pairs of electrons.
• Identify which substance is transferring these electrons to another compound.

In the reactions we've discussed, substances like \(\mathrm{CN}^{-}\) and water act as electron pair donors.
Recognizing the electron pair donor helps us understand how molecules interact and bond with each other.

Electron pair acceptor

An electron pair acceptor is a substance characterized by its ability to receive an electron pair from another compound. This trait makes it akin to a Lewis acid, as both terms describe a substance that wants to form a bond by accepting electron pairs.
In the broad scope of chemistry, electron pair acceptors are crucial for forming compounds since they often bind with electron pair donors to form stable structures.
When you want to spot an electron pair acceptor in a reaction:

• Look for molecules or ions with an electron deficiency or empty orbits ready to accept electrons.
• Identify which substance is forming a bond by taking electrons from another.

In the examples previously mentioned, the \(\mathrm{Fe}^{3+}\) ion acts as an electron pair acceptor by bonding with water molecules, showing its role in electron pair transfer crucial in many reactions.