Problem 122 - General Chemistry: Principles and Modern Applications

Question

All of the following substances are bases except for (a) \(\mathrm{H}_{2} \mathrm{NNH}_{2} ;\) (b) \(\mathrm{NH}_{3} ;\) (c) \(\mathrm{HN}_{3} ;\) (d) \(\mathrm{NH}_{2} \mathrm{OH}\); (e) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\).

Step-by-Step Solution

Answer

Therefore, the only substance among the given alternatives that is not a base is (c) \(\mathrm{HN}_{3}\).

Step 1: Identify the nitrogen compounds

In each option, we are given compounds that have nitrogen (N) in them. They are (a) \(\mathrm{H}_{2}\mathrm{NNH}_{2}\), (b) \(\mathrm{NH}_{3}\), (c) \(\mathrm{HN}_{3}\), (d) \(\mathrm{NH}_{2}\mathrm{OH}\), and (e) \(\mathrm{CH}_{3}\mathrm{NH}_{2}\).

Step 2: Examine the nitrogen atom

Bases are substances that can donate an electron pair to an H+, a proton. The ability of these compounds to act as a base depends on the presence of a lone pair of electrons on the nitrogen atom. The nitrogen atom in options (a) \(\mathrm{H}_{2}\mathrm{NNH}_{2}\), (b) \(\mathrm{NH}_{3}\), (d) \(\mathrm{NH}_{2}\mathrm{OH}\), and (e) \(\mathrm{CH}_{3}\mathrm{NH}_{2}\) have a lone pair of electrons.

Step 3: Identify the compound with no lone pair on nitrogen

From our analysis, we see that only in (c) \(\mathrm{HN}_{3}\), the nitrogen doesn't have a lone pair of electrons. The three hydrogen atoms are bonded to the nitrogen, and there's no additional electron pair available for bonding with a proton.

Key Concepts

Lewis Bases

In chemistry, Lewis bases are substances that have the ability to donate an electron pair. This concept helps explain a wide range of chemical reactions, especially those involving acid-base interactions.
Lewis bases usually have a lone pair of electrons that they can offer to form a bond. This makes them key players in reacting with Lewis acids, which accept these electron pairs.
The nitrogen atom in molecules often serves as a site for these lone pairs, making nitrogen compounds common Lewis bases.

• An ammonia molecule \(\mathrm{NH_3}\) is a classic example of a Lewis base because the nitrogen atom has a lone pair of electrons.
• The lone pair is readily available to, for example, attach to a hydrogen ion (proton), forming a bond.

Understanding Lewis bases is paramount in fields such as organic chemistry, where reactions often hinge on the movement of electron pairs.

Nitrogen Compounds

Nitrogen is a critical element in many compounds due to its unique ability to form stable bonds with itself and other elements. In the context of bases, nitrogen often carries or is part of a group that has a lone pair of electrons.

• In substances like hydrazine \(\mathrm{H_2NNH_2}\), methylamine \(\mathrm{CH_3NH_2}\), and ammonia \(\mathrm{NH_3}\), the nitrogen atom can participate in reactions that involve electron transfer because of its lone pair.
• The versatility of nitrogen is highlighted in organic chemistry, where it forms the backbone of many amine groups, which are considered powerful bases.

However, not all nitrogen compounds behave as bases. The key lies in the presence or absence of a lone electron pair, which is crucial for the compound’s ability to donate electrons.

Lone Pairs

Lone pairs refer to valence electrons that do not partake in bonding, simply hanging around their atom, usually tightly bound. These electrons play a huge role in the chemistry of covalent compounds.

• They are essential for a molecule's ability to act as a donor in coordination or complexation reactions.
• In nitrogenous compounds, the presence of a lone pair enables the nitrogen atom to donate these electrons, acting as a base.

Lone pairs significantly affect the shape of molecules, their reactivity, and their physical properties like boiling and melting points. In the course of acid-base reactions, not all compounds with nitrogen act as bases. For instance, \(\mathrm{HN_3}\) lacks a lone pair, explaining why it doesn't act as a base even though it contains nitrogen.