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, ${\mathrm{CH}}_3\mathrm{CN},$ (c) ${\mathrm{NaClO}}_4,$ (d) $\mathrm{Ba}(\mathrm{OH}{)}_2$.

Short answer

(a) HF is an acid and exists as a mixture of molecules and ions, (b) ${\mathrm{CH}}_3\mathrm{CN}$ is none of the above and exists entirely in molecular form, (c) ${\mathrm{NaClO}}_4$ is a salt and exists entirely as ions, and (d) $\mathrm{Ba}(\mathrm{OH}{)}_2$ is a base and exists as a mixture of molecules and ions in aqueous solution.

Step-by-step solution

a) Classify HF and determine its form in aqueous solution.

Hydrogen fluoride (HF) is a compound containing hydrogen and one halogen, which makes it a binary acid. When placed in water, it forms the hydronium ion ${\mathrm{H}}_3{\mathrm{O}}^{+}$ and the fluoride ion $F^{-}$. So, HF is an acid and it exists as a mixture of molecules and ions in aqueous solution.

b) Classify CH3CN and determine its form in aqueous solution.

Acetonitrile (${\mathrm{CH}}_3\mathrm{CN}$) is an organic compound that does not have any acidic or basic functional group. Therefore, it can be classified as none of the above. In aqueous solution, acetonitrile does not ionize; hence it exists entirely in molecular form.

c) Classify NaClO4 and determine its form in aqueous solution.

Sodium perchlorate (${\mathrm{NaClO}}_4$) is a compound composed of the sodium ion (${\mathrm{Na}}^{+}$) and the perchlorate ion (${\mathrm{ClO}}_4^{-}$). Since it is formed from a reaction between a base (sodium hydroxide) and an acid (perchloric acid), it is classified as a salt. In an aqueous solution, sodium perchlorate completely dissociates into its constituent ions, existing entirely as ions.

d) Classify Ba(OH)2 and determine its form in aqueous solution.

Barium hydroxide ($\mathrm{Ba}(\mathrm{OH}{)}_2$) is a metallic hydroxide with a metal ion (Ba2+) and hydroxide ions (OH-). It is considered a base. In an aqueous solution, barium hydroxide dissociates, forming barium ions (${\mathrm{Ba}}^{2+}$) and hydroxide ions (${\mathrm{OH}}^{-}$). Ba(OH)2 exists as a mixture of molecules and ions in its aqueous solution. In summary, (a) HF is an acid and exists as a mixture of molecules and ions, (b) ${\mathrm{CH}}_3\mathrm{CN}$ is none of the above and exists entirely in molecular form, (c) ${\mathrm{NaClO}}_4$ is a salt and exists entirely as ions, and (d) $\mathrm{Ba}(\mathrm{OH}{)}_2$ is a base and exists as a mixture of molecules and ions in aqueous solution.

Key concepts

Dissociation in Aqueous Solution

When compounds dissolve in water, they may split into smaller particles such as ions. This process is called dissociation. Understanding this concept helps us predict how a compound behaves when it interacts with water.
Dissociation mainly depends on the nature of the compound:

• Acids: They tend to release hydrogen ions. For instance, hydrogen fluoride (HF) partially dissociates to produce hydrogen ( H^+ ) and fluoride ( F^- ) ions.
• Bases: Like barium hydroxide ( Ba(OH)_2 ), bases release hydroxide ions ( OH^- ) in water.
• Salts: Such as sodium perchlorate ( NaClO_4 ), usually dissociate completely into their constituent ions.

Not all substances dissociate fully. Some remain partially dissociated, as a mixture of molecules and ions. Understanding the extent of dissociation is crucial for predicting the chemical behavior of a solution.

Classification of Compounds

Classifying compounds helps us predict their behavior in reactions, especially in solutions. Here’s a quick rundown:

• Acids: These are often identified by containing hydrogen that can be released as ions in water. HF is an example of a binary acid.
• Bases: They commonly have metals bonded to hydroxide ions, like barium hydroxide ( Ba(OH)_2 ). Bases tend to increase the concentration of hydroxide ions in solution.
• Salts: These are formed from the neutralization reaction of an acid and a base. Sodium perchlorate ( NaClO_4 ) is one example, derived from the base sodium hydroxide and the acid perchloric acid.
• None of the above: These are compounds like acetonitrile ( CH_3CN ) which do not exhibit acidic or basic behavior in water.

This classification allows chemists to effectively understand and predict chemical interactions in different environments.

Molecular and Ionic Forms

Substances can exist in aqueous solutions in different forms. This depends on their ionic nature and ability to dissociate.
Different compounds dissolve in water as:

• Entirely molecular: Compounds like acetonitrile ( CH_3CN ) do not ionize in water, so they stay in their original form.
• Entirely ionic: Salts such as sodium perchlorate ( NaClO_4 ), which dissociate completely into sodium and perchlorate ions.
• Mixtures of molecules and ions: Some compounds like hydrogen fluoride ( HF ) and barium hydroxide ( Ba(OH)_2 ) partially ionize in water. This means they exist as both molecules and ions simultaneously.

Recognizing the form in which a compound exists in an aqueous solution helps predict properties such as conductivity and reactivity.