Problem 79 - Chemistry and Chemical Reactivity

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. (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (c) NaOH (d) HBr

Step-by-Step Solution

Answer

(a) Weak base (\( \mathrm{NH}_3 \)); (b) Weak acid (\( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \)); (c) Strong base (NaOH); (d) Strong acid (HBr).

Step 1: Analyze NH3 in Water

When ammonia \( \mathrm{NH}_3 \) is dissolved in water, it acts as a weak base. The primary species present in the solution are \( \mathrm{NH}_3 \) molecules and water molecules, along with a small amount of \( \mathrm{NH}_4^+ \) ions and \( \mathrm{OH}^- \) ions due to partial ionization: \( \mathrm{NH}_3 + \mathrm{H}_2O \rightarrow \mathrm{NH}_4^+ + \mathrm{OH}^- \). \( \mathrm{NH}_3 \) is a weak Bronsted base because it accepts protons from water.

Step 2: Analyze \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) in Water

Acetic acid \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) is dissolved in water, acting as a weak acid. The primary species are \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) molecules, \( \mathrm{H}_2O \), \( \mathrm{CH}_3 \mathrm{CO}_2^- \) ions, and \( \mathrm{H}^+ \) ions. It partially ionizes according to \( \mathrm{CH}_3 \mathrm{CO}_{2} \mathrm{H} \rightleftharpoons \mathrm{CH}_3 \mathrm{CO}_2^- + \mathrm{H}^+ \). \( \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \) is a weak Bronsted acid.

Step 3: Analyze NaOH in Water

When sodium hydroxide \( \mathrm{NaOH} \) is dissolved in water, it dissociates completely into \( \mathrm{Na}^+ \) and \( \mathrm{OH}^- \) ions. The primary species present are \( \mathrm{Na}^+ \) and \( \mathrm{OH}^- \) ions in addition to water molecules. \( \mathrm{OH}^- \) is the Bronsted base, and NaOH is a strong base.

Step 4: Analyze HBr in Water

Hydrobromic acid \( \mathrm{HBr} \) dissociates fully when dissolved in water into \( \mathrm{H}^+ \) and \( \mathrm{Br}^- \) ions. Therefore, the primary species are \( \mathrm{H}^+ \), \( \mathrm{Br}^- \), and water molecules. \( \mathrm{HBr} \) is a strong Bronsted acid due to complete ionization.

Key Concepts

Weak Acid

In the world of chemistry, a weak acid is one that does not completely dissociate in water. This means when it is dissolved, only a small fraction of its molecules release hydrogen ions \( \text{H}^+ \) into the solution. An example of a weak acid is acetic acid \( \text{CH}_3\text{CO}_2\text{H} \), common in vinegar.

When dissolved, acetic acid partially ionizes as follows: \[ \text{CH}_3\text{CO}_2\text{H} \rightleftharpoons \text{CH}_3\text{CO}_2^- + \text{H}^+ \] Here, the equilibrium sign indicates that not all molecules break apart.

This means that both acetic acid molecules and its ions are present in the solution. Because of this partial ionization, weak acids like acetic acid do not conduct electricity as effectively as strong acids. They are useful in situations where a gentle acidic reaction is needed.

Strong Acid

A strong acid is one that completely ionizes in water, meaning all of its molecules break apart to release hydrogen ions \( \text{H}^+ \) when dissolved. This results in a high concentration of \( \text{H}^+ \) ions in the solution. One common example of a strong acid is hydrobromic acid \( \text{HBr} \).

When \( \text{HBr} \) dissolves in water, it undergoes full ionization as follows: \[ \text{HBr} \rightarrow \text{H}^+ + \text{Br}^- \]

This reaction highlights that in the solution, \( \text{H}^+ \) and bromide ions \( \text{Br}^- \) are the primary species present, leading to a stronger acidic solution, which is excellent for tasks requiring high reactivity and conductivity. However, this also means that strong acids can be more hazardous and require careful handling.

Weak Base

Like weak acids, weak bases are partially ionized in solution. This means that only a fraction of the base molecules accept protons from water to form hydroxide ions \( \text{OH}^- \). Ammonia \( \text{NH}_3 \) is a classic example of a weak base.

In water, the equilibrium reaction is: \[ \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]

This reaction does not go to completion, meaning \( \text{NH}_3 \) molecules remain in the solution along with some ammonium ions \( \text{NH}_4^+ \) and hydroxide ions. As a weak base, ammonia is less efficient at conducting electricity and reacting compared to stronger bases. These properties make weak bases ideal in roles requiring mild base effects.

Strong Base

Strong bases, in contrast to weak ones, dissociate completely in water to release hydroxide ions \( \text{OH}^- \). Sodium hydroxide \( \text{NaOH} \) is a well-known strong base often used in industry and laboratories.

Upon dissolving, it fully dissociates as follows: \[ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \]

This results in a high \( \text{OH}^- \) concentration making the solution very basic or alkaline. The sodium ions \( \text{Na}^+ \) also remain in the solution, not reacting further. Strong bases are highly effective in chemical reactions that require rapid pH changes or robust cleaning abilities. However, they are more dangerous to handle due to their high reactivity and potential to damage skin and other materials.