Question

Identify the primary species (atoms, molecules, or ions) present in an aqueous solution of each of the following compounds. Decide which species are Bronsted acids or bases and whether they are strong or weak. (b) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (a) \(\mathrm{NH}_{3}\) (c) \(\mathrm{NaOH}\) (d) HBr

Short answer

(b) Weak acid, (a) Weak base, (c) Strong base, (d) Strong acid.

Step-by-step solution

Identify Species for \(\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\)

\(\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\) is acetic acid. In an aqueous solution, it partially ionizes to form \(\mathrm{CH}_3\mathrm{CO}_2^-\) and \(\mathrm{H}^+\). Therefore, the primary species are \(\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\), \(\mathrm{CH}_3\mathrm{CO}_2^-\), and \(\mathrm{H}^+\). Acetic acid is a weak Bronsted acid since it does not completely dissociate in water.

Identify Species for \(\mathrm{NH}_3\)

\(\mathrm{NH}_3\) is ammonia. In water, it partially ionizes to form \(\mathrm{NH}_4^+\) and \(\mathrm{OH}^-\). Thus, the primary species are \(\mathrm{NH}_3\), \(\mathrm{NH}_4^+\), and \(\mathrm{OH}^-\). Ammonia is a weak Bronsted base as it does not completely ionize.

Identify Species for \(\mathrm{NaOH}\)

\(\mathrm{NaOH}\) is sodium hydroxide, a strong base. In water, it completely dissociates into \(\mathrm{Na}^+\) and \(\mathrm{OH}^-\). The primary species in solution are \(\mathrm{Na}^+\) and \(\mathrm{OH}^-\). Since \(\mathrm{NaOH}\) completely dissociates, it is a strong Bronsted base.

Identify Species for \(\mathrm{HBr}\)

\(\mathrm{HBr}\) is hydrobromic acid. It completely dissociates in water to form \(\mathrm{H}^+\) and \(\mathrm{Br}^-\). Thus, the primary species are \(\mathrm{H}^+\) and \(\mathrm{Br}^-\). \(\mathrm{HBr}\) is a strong Bronsted acid because it fully ionizes in water.

Key concepts

Acetic Acid

Acetic acid, also known by its formula \(\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\), is a common weak acid found in vinegar. It is a well-studied example of a Bronsted acid, meaning it can donate a proton (\(\mathrm{H}^+\)) to a base. However, acetic acid is characterized as a weak acid because it does not completely dissociate into ions in aqueous solution. This means that in a solution, you will find both acetic acid molecules \(\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H}\) and the ions \(\mathrm{CH}_3 \mathrm{CO}_2^-\) and \(\mathrm{H}^+\). Understanding acetic acid's behavior in water helps in predicting reactions in organic and inorganic chemistry. The equilibrium can be described by:\[\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H} \rightleftharpoons \mathrm{CH}_3\mathrm{CO}_2^- + \mathrm{H}^+\]The equilibrium is important because it dictates the concentration of ions in the solution, directly influencing the solution's acidity.

Ionization

Ionization refers to the process by which a molecule gains or loses electrons and thus forms ions. For instance, in the case of acetic acid, ionization takes place when acetic acid donates a proton to water, forming acetate \(\mathrm{CH}_3 \mathrm{CO}_2^-\) and hydronium ions \(\mathrm{H}^+\). This partial ionization is indicative of a weak acid, as it does not completely convert into ions. The same concept applies broadly to bases. For example, ammonia \(\mathrm{NH}_3\) undergoes partial ionization in water to create ammonium \(\mathrm{NH}_4^+\) and hydroxide \(\mathrm{OH}^-\) ions. Key points about ionization include:

• It impacts the electrical conductivity of a solution.
• Complete ionization often indicates a strong acid/base, while partial ionization suggests weakness.

Understanding ionization is essential for predicting the behavior of acids and bases in chemical reactions.

Aqueous Solution

An aqueous solution is a solution where water is the solvent. In chemistry, water is often referred to as the 'universal solvent' because it can dissolve a vast array of substances, including acids and bases. When an acid like acetic acid or a base such as sodium hydroxide dissolves in water, they form an aqueous solution.In this environment, acids can donate \(\mathrm{H}^+\) ions while bases can accept them. This behavior is the basis of the Bronsted-Lowry acid-base theory. For example, when acetic acid is dissolved in water, it interacts with the solvent to form the ionized species \(\mathrm{CH}_3 \mathrm{CO}_2^-\) and \(\mathrm{H}^+\). Such interactions are fundamental for conducting acid-base reactions, analyzing pH, and understanding reaction mechanisms.

Dissociation

Dissociation is the process by which molecules split into smaller particles, such as atoms, ions, or radicals, and it plays a crucial role in determining an acid or base’s strength. When an acid or base dissociates in water, it breaks down into its component ions. For example, when acetic acid dissociates, it forms acetate \(\mathrm{CH}_3 \mathrm{CO}_2^-\) and \(\mathrm{H}^+\).This contrasts with strong acids or bases, like hydrochloric acid \(\mathrm{HCl}\) or sodium hydroxide \(\mathrm{NaOH}\), which fully dissociate in aqueous solutions, releasing all their ions.It's important to note:

• A weak dissociation represents a weak acid/base, as not all molecules convert into ions.
• Dissociation equilibria can be shifted by changing concentrations, temperatures, or pressures.

Understanding dissociation helps predict how a solution will behave, especially in terms of acidity or basicity, and informs how you might use that solution in practical applications.