Problem 41 - Chemistry The Molecular Science

Question

Classify each of these as an acid or a base. Which are strong and which are weak? What ions are produced when each is dissolved in water? (a) \(\mathrm{HNO}_{3}\) (b) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (e) \(\mathrm{KOH}\) (f) \(\mathrm{CH}_{3} \mathrm{COOH}\)

Step-by-Step Solution

Answer

(a) Strong acid; (b) Strong base; (c) Weak base; (d) Weak acid; (e) Strong base; (f) Weak acid. Ions vary as per dissociation.

Step 1: Identify Acid or Base

To classify a substance as an acid or base, remember that acids typically start with hydrogen (H) and bases often contain hydroxide (OH). Other bases, like ammonia (NH3), do not contain OH but can accept protons.

Step 2: Analyze Compound (a) \( \mathrm{HNO}_3 \)

\( \mathrm{HNO}_3 \) is nitric acid. It is an acid because it releases \( \mathrm{H}^+ \) ions in water. This compound is a strong acid because it completely dissociates in solution into \( \mathrm{H}^+ \) and \( \mathrm{NO}_3^- \) ions.

Step 3: Analyze Compound (b) \( \mathrm{Ca(OH)}_2 \)

\( \mathrm{Ca(OH)}_2 \) is calcium hydroxide. It is a base because it contains hydroxide ions, \( \mathrm{OH}^- \). It is a strong base as it dissociates completely in water into \( \mathrm{Ca}^{2+} \) and \( \mathrm{OH}^- \) ions.

Step 4: Analyze Compound (c) \( \mathrm{NH}_3 \)

\( \mathrm{NH}_3 \) is ammonia. It is a weak base because it does not contain \( \mathrm{OH}^- \), but in solution, it accepts \( \mathrm{H}^+ \) to form ammonium ions \( \mathrm{NH}_4^+ \) while leaving \( \mathrm{OH}^- \) ions in solution.

Step 5: Analyze Compound (d) \( \mathrm{H}_3 \mathrm{PO}_4 \)

\( \mathrm{H}_3 \mathrm{PO}_4 \) is phosphoric acid. It is an acid as it can release \( \mathrm{H}^+ \) ions. It is a weak acid because it does not completely dissociate, producing ions like \( \mathrm{H}_2 \mathrm{PO}_4^- \), \( \mathrm{HPO}_4^{2-} \), and \( \mathrm{PO}_4^{3-} \) in solution.

Step 6: Analyze Compound (e) \( \mathrm{KOH} \)

\( \mathrm{KOH} \) is potassium hydroxide. It is a base as it contains the \( \mathrm{OH}^- \) ion. It is a strong base that dissociates completely in water into \( \mathrm{K}^+ \) and \( \mathrm{OH}^- \) ions.

Step 7: Analyze Compound (f) \( \mathrm{CH}_3 \mathrm{COOH} \)

\( \mathrm{CH}_3 \mathrm{COOH} \) is acetic acid. It is an acid because it can release \( \mathrm{H}^+ \) ions. It is considered a weak acid as it does not dissociate completely, producing \( \mathrm{CH}_3 \mathrm{COO}^- \) and \( \mathrm{H}^+ \) ions in solution.

Key Concepts

Strong Acids and Bases

Strong acids and bases are chemical compounds that completely dissociate into their respective ions when dissolved in water. This means every molecule of a strong acid or base splits apart to produce ions. Strong acids release hydrogen ions (H\(^+\)), while strong bases release hydroxide ions (OH\(^-\)).
For example:

• Strong Acids: Nitric acid (\( \mathrm{HNO}_3 \)) is a strong acid because it completely ionizes in water to produce \( \mathrm{H}^+ \) and \( \mathrm{NO}_3^- \) ions.
• Strong Bases: Calcium hydroxide (\( \mathrm{Ca(OH)}_2 \)) and potassium hydroxide (\( \mathrm{KOH} \)) are strong bases as they fully dissociate to give \( \mathrm{Ca}^{2+} \), \( \mathrm{K}^+ \), and \( \mathrm{OH}^- \) ions.

Complete dissociation is what defines the strength of these acids and bases, making them very effective in chemical reactions.
This high degree of ionization makes strong acids and bases typically more hazardous and reactive than their weaker counterparts.

Weak Acids and Bases

Weak acids and bases are compounds that partially dissociate in water. This means only a fraction of the molecules split into ions, while the rest remain intact in solution. Consequently, weak acids and bases are not as effective in producing ions as their strong counterparts.
Examples include:

• Weak Acids: Acetic acid (\( \mathrm{CH}_3 \mathrm{COOH} \)) and phosphoric acid (\( \mathrm{H}_3 \mathrm{PO}_4 \)) are weak acids. They only partially dissociate, producing few \( \mathrm{H}^+ \) ions along with \( \mathrm{CH}_3 \mathrm{COO}^- \) and the various forms of phosphate ions (\( \mathrm{H}_2 \mathrm{PO}_4^- \), \( \mathrm{HPO}_4^{2-} \), \( \mathrm{PO}_4^{3-} \)).
• Weak Bases: Ammonia (\( \mathrm{NH}_3 \)) is a typical weak base. It does not contain \( \mathrm{OH}^- \) directly but can accept \( \mathrm{H}^+ \) ions to form \( \mathrm{NH}_4^+ \), contributing to the equilibrium with \( \mathrm{OH}^- \) in water.

Weak acids and bases are important in many biological systems where reaction control and pH stability are crucial.

Ion Dissociation in Water

When acids and bases dissolve in water, they undergo ion dissociation. This process involves the breaking apart of a compound into its positive and negative ions.
For acids:

• The dissociation typically involves the release of \( \mathrm{H}^+ \) ions into the solution.

For bases:

• The dissociation often involves the release of \( \mathrm{OH}^- \) ions.

Understanding ion dissociation is important for predicting the behavior of substances in water.
For instance, with nitric acid (\( \mathrm{HNO}_3 \)), the dissociation releases \( \mathrm{H}^+ \) and \( \mathrm{NO}_3^- \) ions, which greatly influences the acidity of the solution.
With calcium hydroxide (\( \mathrm{Ca(OH)}_2 \)), the process releases \( \mathrm{Ca}^{2+} \) and \( \mathrm{OH}^- \) ions, increasing the solution's basicity.
This process is fundamental for various scientific applications, including titration methods and pH calculations.