Problem 77 - Chemistry The Molecular Science

Question

Which of these is a Lewis acid? A Lewis base? (a) \(\mathrm{Al}^{3+}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{SCN}^{-}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Al}^{3+}\) is a Lewis acid; (b) \(\mathrm{H}_{2} \mathrm{O}\) and (c) \(\mathrm{SCN}^{-}\) are Lewis bases.

Step 1: Define Lewis Acid and Base

A Lewis acid is a species that can accept a pair of electrons, while a Lewis base is one that can donate a pair of electrons.

Step 2: Analyze Option (a) - \(\mathrm{Al}^{3+}\)

\(\mathrm{Al}^{3+}\) is a positively charged ion, indicating it has a deficiency of electrons and can accept electron pairs. Therefore, it acts as a Lewis acid.

Step 3: Analyze Option (b) - \(\mathrm{H}_{2} \mathrm{O}\)

Water, \(\mathrm{H}_{2} \mathrm{O}\), has lone pairs of electrons on the oxygen atom that can be donated, categorizing it as a Lewis base.

Step 4: Analyze Option (c) - \(\mathrm{SCN}^{-}\)

The thiocyanate ion, \(\mathrm{SCN}^{-}\), carries a negative charge and has lone pairs that can donate electrons, classifying it as a Lewis base.

Key Concepts

Lewis Acid Definition

A Lewis acid is a chemical species that plays a key role in many chemical reactions. It is defined by its ability to accept an electron pair from another molecule. Think of a Lewis acid as having a kind of 'electron hunger'. It doesn't have enough electrons to fulfill its energy needs, so it seeks out other molecules willing to share their electrons.

When looking for Lewis acids, remember these essential features:

• They often have a positive charge, like the \( \mathrm{Al}^{3+} \) ion.
• They may also have an empty orbital that can accommodate an electron pair, even if they are not charged.

These characteristics make them attractive to molecules with lone pairs of electrons to donate.

Lewis Base Definition

In contrast to a Lewis acid, a Lewis base is a substance that can donate an electron pair. Imagine a Lewis base as someone who is willing to share what they have to form a bond with another. This type of molecule often has lone pairs of electrons that are not involved in bonding and are available for sharing.

Key features of Lewis bases include:

• Possession of lone electron pairs that are ready for donation, like the oxygen in water, \( \mathrm{H}_{2} \mathrm{O} \).
• Usually, they have a negative charge or neutral molecules with lone pairs, as seen with the thiocyanate ion, \( \mathrm{SCN}^{-} \).

These bases are essential in forming new bonds, collaborating with Lewis acids to create more complex structures.

Electron Pairs

Electron pairs are fundamental players in the world of Lewis acids and bases. They consist of two electrons that are paired together, typically occupying the same orbital. Electrons naturally repel each other because they have the same negative charge, but within a pair, they stick together due to opposite spins.

Here’s what you should remember about electron pairs:

• They can be shared between atoms to form covalent bonds, a critical process in chemical bonding.
• In Lewis theory, they are either donated by a Lewis base or received by a Lewis acid to form a new bond.
• Electron pair availability often determines whether a molecule will act as a Lewis acid or base in a reaction.

Understanding electron pairs helps in figuring out the bonding interactions and the role a molecule might play as a Lewis acid or base in a given chemical context.