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 entirely as ions in aqueous solution (H+ and F-) (b) ${\mathrm{CH}}_3\mathrm{CN}$ is none of the above and exists in molecular form in aqueous solution (c) ${\mathrm{NaClO}}_4$ is a salt and exists entirely as ions in aqueous solution (Na+ and ClO${}_4^{-}$) (d) $\mathrm{Ba}(\mathrm{OH}{)}_2$ is a base and exists entirely as ions in aqueous solution (Ba${}^{2+}$ and OH${}^{-}$)

Step-by-step solution

Classify HF

The chemical formula is HF, which is Hydrofluoric acid. Since it is an acid and also a strong acid, when dissolved in water, it will completely ionize into hydrogen ions, H+, and fluoride ions, F-.

Classify ${\mathrm{CH}}_3\mathrm{CN}$

This chemical is acetonitrile with the chemical formula of ${\mathrm{CH}}_3\mathrm{CN}$. It is a polar organic solvent and it's not an acid, base, or salt. In aqueous solutions, it mostly remains in the molecular form, hence there are no ions produced.

Classify ${\mathrm{NaClO}}_4$

The chemical formula is ${\mathrm{NaClO}}_4$ which is sodium perchlorate, a salt. In aqueous solutions, it will be completely dissociated into sodium ions, Na+, and perchlorate ions, ClO${}_4^{-}$.

Classify $\mathrm{Ba}(\mathrm{OH}{)}_2$

The chemical formula is $\mathrm{Ba}(\mathrm{OH}{)}_2$ which is barium hydroxide, a base. It's a strong base and thus when dissolved in water, it will ionize completely into barium ions, Ba${}^{2+}$, and hydroxide ions, OH${}^{-}$. To summarize, we have: (a) HF - Acid, exists entirely as ions in aqueous solution (H+ and F-) (b) ${\mathrm{CH}}_3\mathrm{CN}$ - None of the above, exists in molecular form in aqueous solution (c) ${\mathrm{NaClO}}_4$ - Salt, exists entirely as ions in aqueous solution (Na+ and ClO${}_4^{-}$) (d) $\mathrm{Ba}(\mathrm{OH}{)}_2$ - Base, exists entirely as ions in aqueous solution (Ba${}^{2+}$ and OH${}^{-}$)

Key concepts

Chemical Ionization

Chemical ionization is a critical concept that explains how molecules break apart into charged particles, or ions, particularly when dissolved in a liquid like water. This phenomenon is crucial for understanding acids, bases, and salts in chemistry.

For instance, when hydrofluoric acid (HF) is dissolved in water, it undergoes chemical ionization to form hydrogen ions (H+) and fluoride ions (F-). This complete ionization is characteristic of strong acids in water. However, not all substances ionize entirely. For example, acetonitrile (CH₃CN) doesn't ionize when dissolved in water and remains in its molecular form. Understanding chemical ionization helps us predict the behavior of different substances in aqueous solutions and is a fundamental aspect of the study of acid-base reactions.

Aqueous Solutions

Aqueous solutions are a staple in chemical reactions, especially when discussing acid-base chemistry. An aqueous solution is formed when a substance is dissolved in water. The nature of the dissolved substance determines whether it will remain in molecular form or ionize.

For the substances mentioned in our exercise, sodium perchlorate (NaClO₄) and barium hydroxide (Ba(OH)₂) both disassociate entirely into their respective ions when in aqueous solution, indicating their ionic character. On the other hand, acetonitrile (CH₃CN), being a non-ionic compound, doesn't split into ions but remains in the solution as complete molecules. Solubility and degree of ionization in water are essential factors in predicting the behavior of substances in aqueous solutions, and this knowledge is incredibly beneficial in fields like biochemistry and environmental science.

Molecular Form in Chemistry

The molecular form in chemistry refers to the existence of a substance as discrete, neutral molecules rather than ions. Molecular forms are particularly interesting because they often maintain the structure they have before dissolving in a solvent.

In contrast to substances that ionize in solution, such as salts and strong acids or bases, substances like acetonitrile (CH₃CN) typically remain intact and do not release ions into the solution. The ability to exist in molecular form can affect how a substance interacts with its environment. For instance, molecular substances may not conduct electricity in solution or may have different reactivity patterns compared to their ionic counterparts. Understanding when and why a compound will remain in its molecular form is beneficial for students studying physical chemistry and for applications across pharmaceutical and materials sciences.