Problem 35 - Chemistry: The Central Science in SI Units, Global Edition

Question

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

Step-by-Step Solution

Answer

(a) HF is an acid and exists as a mixture of molecules and ions in aqueous solution. (b) Acetonitrile (CH3CN) is none of the above and exists predominantly in its molecular form in aqueous solution. (c) NaClO4 is a salt and exists entirely as ions in aqueous solution. (d) Ba(OH)2 is a base and dissociates completely into ions in aqueous solution.

Step 1: (a) Analyzing HF

Hydrofluoric acid (HF) is a compound of hydrogen and fluorine. When dissolved in water, it donates protons (H+) to form the hydronium ion (H3O+). Therefore, HF is classified as an acid. In an aqueous solution, HF exists as a mixture of molecules and ions, as not all HF molecules ionize in water.

Step 2: (b) Analyzing Acetonitrile (CH3CN)

Acetonitrile is a neutral organic compound and does not donate protons (H+) or accept protons (like a base). Thus, it's neither an acid nor a base nor a salt. In an aqueous solution, acetonitrile exists predominantly in its molecular form.

Step 3: (c) Analyzing NaClO4

Sodium perchlorate (NaClO4) is composed of a positive sodium ion (Na+) and a negative perchlorate ion (ClO4-). Since it is made up of a metal cation and a non-metal anion, it is classified as a salt. In an aqueous solution, NaClO4 exists entirely as ions, as it dissociates fully into Na+ and ClO4-.

Step 4: (d) Analyzing Ba(OH)2

Barium hydroxide (Ba(OH)2) is a compound formed by the reaction of the Ba2+ cation with two hydroxide (OH-) anions. When dissolved in water, it donates hydroxide ions (OH-) to form a basic (alkaline) solution. Therefore, Ba(OH)2 is classified as a base. In an aqueous solution, Ba(OH)2 dissociates completely into ions (Ba2+ and OH-).

Key Concepts

Molecular Form

Understanding the molecular form of substances is crucial in acid-base chemistry. Molecular forms refer to substances composed entirely of molecules in a solution.

This means these molecules haven't dissociated into ions in a significant or complete manner. For instance, acetonitrile (\(\mathrm{CH}_3\mathrm{CN}\)) exists predominantly in molecular form when dissolved in water. It neither donates nor accepts protons, classifying it as neither an acid nor a base.

When thinking about molecular forms:

• These substances retain their molecular integrity.
• They do not ionize substantially in solution.
• They are often neutral organic compounds like acetonitrile.

Recognizing substances in their molecular form can help understand their chemical behavior in solutions.

Ions in Solution

Ions in solution are charged particles that result from the dissociation of compounds in water. When compounds fully dissociate, they produce ions, which consequently influence the solution's properties.

Sodium perchlorate (\(\mathrm{NaClO}_4\)) is an ideal example. It fully dissociates into the sodium ion (\(\mathrm{Na}^+\)) and the perchlorate ion (\(\mathrm{ClO}_4^-\)), classifying it as a salt. Similarly, barium hydroxide (\(\mathrm{Ba(OH)}_2\)) dissociates completely to give \(\mathrm{Ba^{2+}}\) and hydroxide ions (\(\mathrm{OH^-}\)).

Important points about ions in solution include:

• Complete dissociation leads to strong electrolytes.
• These ions can conduct electricity in solution.
• Substances like salts and bases typically dissociate into ions.

Understanding ions in solution helps predict and manipulate a solution's conductivity and reactivity.

Aqueous Solution Behavior

Aqueous solution behavior describes how substances act when dissolved in water. Studying how they behave can reveal significant chemical properties.

For example, hydrofluoric acid (\(\mathrm{HF}\)) behaves uniquely, as it exists as a mix of molecules and ions in water. This partial ionization makes \(\mathrm{HF}\) a weak acid, unlike barium hydroxide, which fully dissociates into ions, showcasing strong basic behavior.

Key takeaways:

• The behavior of a solute in water reveals its strength as an acid or base.
• Substances may exist as a mixture, only molecules, or only ions.
• Understanding solution behavior is essential for predicting reactions.

Analyzing the behavior of substances in aqueous solutions is vital for grasping their overall chemical properties and potential interactions.