Problem 35 - Chemistry: The Central Science

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) \(\mathrm{Ba}(\mathrm{OH})_{2}\).

Step-by-Step Solution

Answer

(a) HF is an Acid and exists as ions in aqueous solution. (b) CH3CN is None of the above and exists in molecular form in aqueous solution. (c) NaClO4 is a Salt and exists as ions in aqueous solution. (d) Ba(OH)2 is a Base and exists as ions in aqueous solution.

Step 1: (a) Identifying HF as Acid, Base, Salt, or None and its state in Aqueous Solution.

HF is a compound of hydrogen and fluorine. Hydrogen has the tendency to donate a proton in a solution, making it an acid. HF is known as Hydrofluoric Acid. In an aqueous solution, it will dissociate into its ions: \[HF \rightarrow H^{+} + F^{-} \] Hence, it exists as ions in aqueous solution.

Step 2: (b) Identifying CH3CN as Acid, Base, Salt, or None and its state in Aqueous Solution.

CH3CN, also known as Acetonitrile, is an organic compound that consists of a carbon and triple-bonded nitrogen. It does not exhibit a characteristic to donate or accept protons, so it is not an acid or a base. It is also not an ionic compound, and hence not a salt. Therefore, we classify it as None of the above. In an aqueous solution, acetonitrile doesn't ionize, so it exists in molecular form.

Step 3: (c) Identifying NaClO4 as Acid, Base, Salt, or None and its state in Aqueous Solution.

NaClO4, Sodium Perchlorate, is an ionic compound formed by the reaction of a strong acid HClO4 (Perchloric Acid) and a strong base NaOH (Sodium Hydroxide). It is a salt. In an aqueous solution, it dissociates into its ions: \[NaClO_{4} \rightarrow Na^{+} + ClO_{4}^{-}\] Hence, it exists as ions in an aqueous solution.

Step 4: (d) Identifying Ba(OH)2 as Acid, Base, Salt, or None and its state in Aqueous Solution.

Ba(OH)2, Barium Hydroxide, is a compound formed with Barium ions and Hydroxide ions. Hydroxide ions can accept protons in a solution, making it a base. In an aqueous solution, it dissociates into its ions: \[Ba(OH)_{2} \rightarrow Ba^{2+} + 2OH^{-}\] Hence, it exists as ions in aqueous solution.

Key Concepts

Chemical Dissociation in Aqueous Solutions

When substances dissolve in water, they often undergo a process called chemical dissociation. This is where a compound separates into its constituent ions. For example, when hydrofluoric acid (HF) is added to water, it dissociates into hydrogen ions (\(H^+\)) and fluoride ions (\(F^-\)). Chemical dissociation is crucial for many chemical reactions, especially in acid-base chemistry.

Ions in aqueous solutions can conduct electricity, making the solution electrolytic. Substances that dissociate completely are known as strong electrolytes, such as sodium perchlorate (\(NaClO_4\)), which dissociates completely into sodium (\(Na^+\)) and perchlorate (\(ClO_4^-\)) ions. Substances that do not dissociate or do so only partially, like acetonitrile (\(CH_3CN\)), are called non-electrolytes or weak electrolytes, respectively, and do not produce a solution that conducts electricity well. Dissociation is essential for understanding the behavior of substances in solutions and their reactions.

Properties of Acids and Bases

Acids and bases have distinct properties that allow us to classify substances and predict their behavior in reactions. Acids, like hydrofluoric acid (HF), are substances that donate protons (\(H^+\)) in aqueous solutions. They often taste sour and can corrode metals.

Bases, on the other hand, are substances that accept protons. Barium hydroxide (\(Ba(OH)_2\)) is a base and in an aqueous solution, it dissociates to produce hydroxide ions (\(OH^-\)), which can react with acids to form water. Bases are typically bitter and have a slippery feel.

Acids turn blue litmus paper red, while bases turn red litmus paper blue. These properties are used in laboratory settings to test the acidity or basicity of solutions. Understanding the properties of acids and bases is important for chemical reactions, including neutralization, where an acid and a base react to form water and a salt.

Ionic Compounds in Chemistry

Ionic compounds are substances composed of positively charged ions (cations) and negatively charged ions (anions) held together by electrostatic forces known as ionic bonds. An example from the exercise is sodium perchlorate (\(NaClO_4\)), where sodium (\(Na^+\)) is the cation and perchlorate (\(ClO_4^-\)) is the anion. These compounds are often formed from the reaction between metals and nonmetals, or from a reaction between a strong acid and a strong base.

Characteristic features of ionic compounds include high melting and boiling points, and their solubility in water. When dissolved in water, they dissociate into their respective ions, conducting electricity and forming electrolytic solutions. In solid form, ionic compounds form a lattice structure, which explains their high melting points. Understanding ionic compounds is fundamental in chemistry, as they make up a significant proportion of the substances we encounter in everyday life and are involved in various industrial processes.