Problem 61 - Chemistry The Molecular Nature of Matter

Question

Use Lewis symbols to diagram the reaction $$ \mathrm{NH}_{2}^{-}+\mathrm{H}^{+} \longrightarrow \mathrm{NH}_{3} $$ Identify the Lewis acid and Lewis base in the reaction.

Step-by-Step Solution

Answer

In the reaction, (NH2-) is the Lewis base because it donates a pair of electrons, and H+ is the Lewis acid because it accepts a pair of electrons.

Step 1: Draw the Lewis Structures

Start by drawing the Lewis structure for each reactant. The Lewis structure for (NH2-) shows a nitrogen atom with two hydrogen atoms and one lone pair of electrons, making it a total of eight valence electrons due to the negative charge. The Lewis symbol for H+ is simply a hydrogen nucleus without any electrons as it has lost its one valence electron.

Step 2: Identify the Reaction

In the reaction, the (NH2-) ion is capable of donating a pair of electrons, while the H+ ion is capable of accepting a pair of electrons.

Step 3: Determine the Lewis Acid and Base

According to Lewis theory, a base is an electron pair donor and an acid is an electron pair acceptor. Here, (NH2-) donates a pair of electrons to H+, so (NH2-) is the Lewis base and H+ is the Lewis acid.

Step 4: Depict the Reaction

Draw the Lewis structures showing the lone pair from the (NH2-) ion bonding with the proton (H+) to form ammonia (NH3). The resulting Lewis structure of NH3 has the nitrogen atom with three single bonds to hydrogen atoms and one lone pair of electrons.

Key Concepts

Lewis Structures

Understanding how atoms connect in a compound is fundamental in chemistry, and this is where Lewis structures come to play. They are diagrams that represent the valence electrons of atoms within a molecule. These valence electrons are shown as dots around the elements' symbols, and they participate in the chemical bonding.

The Lewis structure aids in visualizing the lone pairs of electrons, which are electrons not involved in bonding, and the bonding pairs of electrons, which are shared between atoms to form a bond. In the exercise with the reaction \(\mathrm{NH}_2^- + \mathrm{H}^+ \longrightarrow \mathrm{NH}_3\), the Lewis structure reveals that the nitrogen in \(\mathrm{NH}_2^-\) has one lone pair of electrons, positioning it to react with \(\mathrm{H}^+\), which has no electrons after losing its single valence electron.

Electron Pair Donors and Acceptors

Digging deeper into chemical reactivity, the terms electron pair donor and electron pair acceptor classify substances based on how they interact with other substances through their electrons. In Lewis theory, an electron pair donor is synonymous with a Lewis base, whereas an electron pair acceptor is a Lewis acid.

In the given exercise, the \(\mathrm{NH}_2^-\) ion, with its pair of lonely, non-bonding electrons, willingly donates this pair to the electron-deficient \(\mathrm{H}^+\). Hence, \(\mathrm{NH}_2^-\) serves as the Lewis base (electron pair donor), and \(\mathrm{H}^+\), craving a pair of electrons to achieve stability, acts as the Lewis acid (electron pair acceptor). This interaction is the driving force behind the formation of ammonia (\(\mathrm{NH}_3\)).

• Lewis base (Electron pair donor): \(\mathrm{NH}_2^-\)
• Lewis acid (Electron pair acceptor): \(\mathrm{H}^+\)

Chemical Bonding

Chemical bonding is the force of attraction that holds atoms together in compounds. This bonding enables atoms to gain, lose, or share electrons, achieving stable electron configurations. There are different types of chemical bonds, including ionic, covalent, and metallic bonds.

In the reaction \(\mathrm{NH}_2^- + \mathrm{H}^+ \longrightarrow \mathrm{NH}_3\), the bond formed is covalent, meaning the nitrogen atom is sharing the lone pair of its electrons with hydrogen's empty orbital, leading to a bond that holds the ammonia molecule together. This covalent bond formation epitomizes the Lewis acid-base interaction, demonstrating chemical bonding at a fundamental level. The lewis structure of the product, ammonia, shows that nitrogen now shares three pairs of electrons with three hydrogens and retains one lone pair, securing a stable octet configuration.