Problem 93 - Chemistry and Chemical Reactivity

Question

What species (atoms, molecules, or ions) are present in an aqueous solution of each of the following compounds? (a) \(\mathrm{NH}_{3}\) (c) \(\mathrm{NaOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (d) HBr

Step-by-Step Solution

Answer

The species present are: (a) \(\mathrm{NH}_3\), \(\mathrm{NH}_4^+\), \(\mathrm{OH}^-\); (b) \(\mathrm{CH}_3\mathrm{CO}_2\mathrm{H}\), \(\mathrm{CH}_3\mathrm{CO}_2^-\), \(\mathrm{H}_3\mathrm{O}^+\); (c) \(\mathrm{Na}^+\), \(\mathrm{OH}^-\); (d) \(\mathrm{Br}^-\), \(\mathrm{H}_3\mathrm{O}^+\).

Step 1: Dissolution of Ammonia (\(\mathrm{NH}_3\))

When ammonia is dissolved in water, it partially ionizes. Ammonia accepts protons from water to produce ammonium ions and hydroxide ions: \[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\].Thus, the species present in an aqueous solution of ammonia are \(\mathrm{NH}_3\), \(\mathrm{NH}_4^+\), and \(\mathrm{OH}^-\).

Step 2: Dissolution of Sodium Hydroxide (\(\mathrm{NaOH}\))

Sodium hydroxide is a strong base and dissociates completely in water. It separates into sodium ions and hydroxide ions:\[\mathrm{NaOH} \rightarrow \mathrm{Na}^+ + \mathrm{OH}^-\].Thus, the species present in an aqueous solution of sodium hydroxide are \(\mathrm{Na}^+\) and \(\mathrm{OH}^-\).

Step 3: Dissolution of Acetic Acid (\(\mathrm{CH}_3\mathrm{CO}_2\mathrm{H}\))

Acetic acid is a weak acid, it only partially ionizes in water, donating a proton to form acetate ions and hydronium ions:\[\mathrm{CH}_3\mathrm{CO}_2\mathrm{H} + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{CH}_3\mathrm{CO}_2^- + \mathrm{H}_3\mathrm{O}^+\].Thus, the species present are \(\mathrm{CH}_3\mathrm{CO}_2\mathrm{H}\), \(\mathrm{CH}_3\mathrm{CO}_2^-\), and \(\mathrm{H}_3\mathrm{O}^+\).

Step 4: Dissolution of Hydrobromic Acid (HBr)

Hydrobromic acid is a strong acid and dissociates completely in water, producing bromide ions and hydronium ions:\[\mathrm{HBr} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Br}^- + \mathrm{H}_3\mathrm{O}^+\].Thus, the species present are \(\mathrm{Br}^-\) and \(\mathrm{H}_3\mathrm{O}^+\).

Key Concepts

Ionization

Ionization is a process where a molecule or compound reacts in a solution to produce ions. This involves breaking apart into charged particles, like either positively or negatively charged ions. When substances like acids or bases are dissolved in water, they often ionize to some extent, split into their respective ions. This is crucial because it determines how they behave in a solution. Understanding ionization helps us predict whether a compound will conduct electricity, react with other substances, or change the pH of a solution.

• Complete ionization: A substance fully breaks down into ions. This is often seen with strong acids and bases.
• Partial ionization: The substance only partially separates into ions. Seen in weak acids and bases.

Recognizing the degree of ionization is essential to understanding the behavior of an aqueous solution.

Strong Acids and Bases

Strong acids and bases are compounds that completely dissociate into their ions when placed in water. This means they ionize to a large extent, leaving no un-ionized molecules in the solution. For example, sodium hydroxide (NaOH) is a strong base. In water, it dissociates entirely into sodium ions (\( \mathrm{Na}^+ \)and hydroxide ions (\( \mathrm{OH}^- \)) without any remaining NaOH molecules. Similarly, hydrobromic acid (HBr) is a strong acid that splits fully into hydronium ions (\( \mathrm{H}_3 \mathrm{O}^+ \)) and bromide ions (\( \mathrm{Br}^- \)).

• Complete dissociation means stronger conductivity: more ions mean better electricity conduction.
• Strong acids/bases lead to rapid and often vigorous chemical reactions due to the abundance of available ions.

These characteristics make predicting the behavior of solutions with strong acids and bases quite straightforward.

Weak Acids and Bases

Weak acids and bases do not completely dissociate in water. They only partially ionize, which means a fraction of the molecules break into their constituent ions, while the rest remain intact in the aqueous solution.Acetic acid (\( \mathrm{CH}_3\mathrm{CO}_2\mathrm{H} \)) is a typical example of a weak acid. It only slightly ionizes in water, forming acetate ions (\( \mathrm{CH}_3\mathrm{CO}_2^- \)) and hydronium ions (\( \mathrm{H}_3\mathrm{O}^+ \)). Similarly, ammonia (NH₃) is a weak base that partially turns into ammonium ions (\( \mathrm{NH}_4^+ \)) and hydroxide ions (\( \mathrm{OH}^- \)).

• Partial ionization leads to fewer ions in the solution, so weak acids/bases conduct electricity less efficiently.
• Reactions involving weak acids and bases are typically slower due to the lower concentration of reactive ions.

Understanding the nature of weak acids and bases is essential for predicting their behavior and potential uses in various chemical processes.

Species in Solution

When substances are dissolved in water, they may form different species, depending on their chemical nature and their degree of ionization. These species can include atoms, molecules, and ions.For instance, in aqueous solutions carrying strong acids or bases such as NaOH or HBr, you will find a straightforward set of ions, like hydronium or bromide ions, due to complete ionization. In contrast, solutions with weak acids/bases like acetic acid or ammonia demonstrate a mixture of ions and un-ionized molecules.

• For strong bases like NaOH: \( \mathrm{Na}^+ \) and \( \mathrm{OH}^- \) ions only.
• For weak acids like acetic acid: \( \mathrm{CH}_3\mathrm{CO}_2\mathrm{H} \), \( \mathrm{CH}_3\mathrm{CO}_2^- \), and \( \mathrm{H}_3\mathrm{O}^+ \).
• The types and quantities of these species determine the solution's properties, such as pH and conductivity.

Knowing the species present in solution helps in predicting reactions and understanding the chemical environment in a given aqueous solution.