Question

Label each of the following substances as an acid, base, salt, or none of the above. Indicate whether the substance exists in aqueous solution entirely in molecular form, entirely as ions, or as a mixture of molecules and ions. (a) HF, (b) acetonitrile,\(\mathrm{CH}_{3} \mathrm{CN},(\mathbf{c}) \mathrm{NaClO}_{4},(\mathbf{d}) \mathrm{Ba}(\mathrm{OH})_{2} .\)

Short answer

(a) HF is an acid, existing as a mixture of molecules and ions in aqueous solution. (b) Acetonitrile (CH₃CN) is none of the above and exists almost entirely in molecular form in an aqueous solution. (c) NaClO₄ is a salt, existing as ions in an aqueous solution. (d) Ba(OH)₂ is a strong base, existing as ions in an aqueous solution.

Step-by-step solution

(a) HF

Hydrogen Fluoride (HF) is a binary compound of hydrogen and a halogen (fluorine). HF is an acid, specifically a hydrohalic acid. In an aqueous solution, it forms hydrogen ions (H+) and fluoride ions (F−). Therefore, HF exists in aqueous solution as a mixture of molecules and ions.

(b) Acetonitrile (CH₃CN)

Acetonitrile (CH₃CN) is an organic compound which does not contain any ionizable hydrogen or hydroxide ions. It is not an acid, a base, or a salt. In an aqueous solution, it exists almost entirely in molecular form, as the interaction with water molecules is weak.

(c) NaClO₄

Sodium perchlorate (NaClO₄) is a compound consisting of a metal (sodium) and a nonmetal polyatomic ion (perchlorate). Thus, it is classified as a salt. In an aqueous solution, NaClO₄ dissociates completely into sodium ions (Na⁺) and perchlorate ions (ClO₄⁻), making it exist as ions in solution.

(d) Ba(OH)₂

Barium hydroxide (Ba(OH)₂) is a compound composed of a metal (barium) and hydroxide ions (OH⁻). It is a strong base. In an aqueous solution, it dissociates completely into barium ions (Ba²⁺) and hydroxide ions (OH⁻), so it exists as ions in aqueous solution.

Key concepts

Aqueous Solution

When we talk about an aqueous solution, we are referring to a situation where a substance is dissolved in water. Water, a universal solvent, has the remarkable ability to dissolve a wide variety of molecules, making it the medium for many chemical reactions.

In an aqueous solution, the dissolved substance can exist in different forms, depending on its chemical nature:

• Molecular form: Some compounds, like acetonitrile (\(\mathrm{CH}_{3}\mathrm{CN}\)), do not ionize significantly in water. They remain largely as molecules because they lack ionizable groups.
• Ionized form: Substances such as sodium perchlorate (\(\mathrm{NaClO}_{4}\)) dissociate entirely into ions. This behavior is common for salts and strong acids or bases.
• Mixed form: Compounds like hydrogen fluoride (HF) exist as a mix of molecules and ions. Partially ionized solutions are typical for weak acids and bases.

Understanding how a substance behaves in an aqueous solution is crucial for predicting its reactivity and the type of chemical reactions it might undergo.

Molecular Ions

Molecular ions or polyatomic ions are charged species composed of two or more atoms covalently bonded. These ions play an important role in both organic and inorganic chemistry.

For instance, in the compound sodium perchlorate (\(\mathrm{NaClO}_{4}\)), the perchlorate ion (\(\mathrm{ClO}_{4}^{-}\)) is a polyatomic ion and carries a negative charge. Sodium ions (\(\mathrm{Na}^{+}\)) balance this negative charge in an aqueous solution.

To understand the behavior of molecular ions:

• Dissolution in water: When salts containing polyatomic ions dissolve in water, the ions separate and disperse evenly, making them reactive.
• Examples in acids and bases: Strong acids like hydrochloric acid (HCl) also ionize to release simple ions, while weaker acids, like HF, only partially release hydrogen ions.

Recognizing molecular ions and their interactions helps predict chemical behavior and reaction pathways.

Chemical Classification

Chemical classification involves categorizing substances based on their chemical characteristics and interactions. This classification helps in understanding the behavior of compounds in chemical reactions.

The primary classifications include acids, bases, and salts.

• Acids: Acidic compounds like hydrogen fluoride (HF) release hydrogen ions (\(\mathrm{H}^{+}\)) when dissolved in water. They typically have low pH values (<7).
• Bases: Compounds such as barium hydroxide (\(\mathrm{Ba(OH)}_{2}\)) release hydroxide ions (\(\mathrm{OH}^{-}\)) in aqueous solutions and have high pH values (>7).
• Salts: Ionic compounds, like sodium perchlorate (\(\mathrm{NaClO}_{4}\)), result from the neutralization reaction of acids and bases. They dissolve in water to yield free cations and anions.
• Neutral compounds: These don't fit into the acid, base, or salt categories, like acetonitrile (\(\mathrm{CH}_{3}\mathrm{CN}\)), and tend to remain unaltered when dissolved in water.

This systematic classification aids in understanding a compound's reactivity and potential chemical interactions.