Question

Classify each of the following as a Lewis acid or a Lewis base.

Drag the appropriate items to their respective bins.

1) CO2

2)P(CH3)3

3)H2O

4)B(CH3)3

5)FE3+

6)CN-

7)OH-

8)H+

Answer 1

Concepts and reason

Any chemical substance can be classified as an acid or base by using Lewis acid base theory. Further,the acidic and basic nature of the substance can be analyzed through the ability of molecules to accept and donate electrons.

Fundamentals

According to Lewis acid -base theory, An acid is a substance that accepts a pair of electrons. A base is a substance that donates a pair of electrons.

 

The given molecules are \(\mathrm{CO}_{2}, \mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}, \mathrm{H}_{2} \mathrm{O},\) and \(\mathrm{B}\left(\mathrm{CH}_{3}\right)_{3}\)

\(\mathrm{CO}_{2}\) is the electron pair acceptor. \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\) is the electron pair donor. \(\mathrm{H}_{2} \mathrm{O}\) can act as both electron pair acceptor and donor. \(\mathrm{B}\left(\mathrm{CH}_{3}\right)_{3}\) is the electron pair acceptor

 

In \(\mathrm{CO}_{2}\) the central atom Carbon is electron deficient since two oxygen atoms (more electronegative than carbon) pull the bonding pair electron towards itself. Thus, the carbon dioxide acts as electron pair acceptor and it is a Lewis acid. In \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\) the central atom has one unpaired electron and it can act as an electron pair donor. So it is a Lewis base. When \(\mathrm{H}_{2} \mathrm{O}\) ionizes, it forms both \(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-}\). The proton \(\left(\mathrm{H}^{+}\right)\) can accept a pair of electrons and so it is a Lewis acid. The hydroxide ion \(\left(\mathrm{OH}^{-}\right)\) can donate a pair of electrons and so it is a Lewis base. Hence \(\mathrm{H}_{2} \mathrm{O}\) molecule can acts as both Lewis acid and base.

 

In \(\mathrm{B}\left(\mathrm{CH}_{3}\right)_{3}\) the central atom boron (B) has vacant d orbital to accept a pair of electrons and so it can act as a Lewis base.

 

The given molecules are \(\mathrm{Fe}^{3+}, \mathrm{CN}^{-}, \mathrm{OH}^{-}, \mathrm{H}^{+}\)

\(\mathrm{Fe}^{3+}\) is the electron pair acceptor. \(\mathrm{CN}^{-}\) is the electron pair donor. \(\mathrm{OH}^{-}\) is the electron pair donor. \(\mathrm{H}^{+}\) is the electron pair acceptor.

The molecules that are Lewis acids are \(\mathrm{CO}_{2}, \mathrm{H}^{+}, \mathrm{H}_{2} \mathrm{O}, \mathrm{Fe}^{3+}\) and \(\mathrm{B}\left(\mathrm{CH}_{3}\right)_{3}\)

The molecules that are Lewis bases are \(\mathrm{OH}^{-}, \mathrm{CN}^{-} \mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\)

 

\(\mathrm{Fe}^{3+}\) is a positively charged species (cation). It accepts a pair of electrons to become neutral (Fe) atom. So it is a Lewis acid. \(\mathrm{CN}^{-}\) is a negatively charged species (anion). It can donate a pair of electrons. So it is a Lewis base. The hydroxide ion \(\left(\mathrm{OH}^{-}\right)\) can donate a pair of electrons and so it is a Lewis base. The proton \(\left(\mathrm{H}^{+}\right)\) can accept a pair of electron and so it is a Lewis acid.