Question

Identify the weaker acid in each of the following pairs: a. \(\mathrm{HCl}\) or \(\mathrm{HSO}_{4}^{-}\) b. \(\mathrm{HNO}_{2}\) or \(\mathrm{HF}\) c. \(\mathrm{HCO}_{3}^{-}\) or \(\mathrm{NH}_{4}^{+}\)

Short answer

a. \(\text{HSO}_{4}^{-}\); b. \(\text{HF}\); c. \(\text{HCO}_{3}^{-}\)

Step-by-step solution

Compare Acid Strengths for Pair a

Examine the given acids \(\text{HCl}\) and \(\text{HSO}_{4}^{-}\). Hydrochloric acid \(\text{HCl}\) is a strong acid, almost completely dissociating in water. Hydrogen sulfate ion \(\text{HSO}_{4}^{-}\) is a weaker acid compared to \(\text{HCl}\) because it does not dissociate completely. Hence, the weaker acid is \(\text{HSO}_{4}^{-}\).

Compare Acid Strengths for Pair b

Evaluate the acid strengths of \(\text{HNO}_{2}\) and \(\text{HF}\). Nitrous acid \(\text{HNO}_{2}\) is a weak acid, as it partially dissociates in solution. Hydrofluoric acid \(\text{HF}\), on the other hand, is also a weak acid but is generally less strong than \(\text{HNO}_{2}\) due to its higher bond strength. Thus, \(\text{HF}\) is the weaker acid.

Compare Acid Strengths for Pair c

Assess the acid strength of \(\text{HCO}_{3}^{-}\) and \(\text{NH}_{4}^{+}\). Bicarbonate ion \(\text{HCO}_{3}^{-}\) is a weak acid, and ammonium ion \(\text{NH}_{4}^{+}\) is also a weak acid. Comparing their dissociation constants, \(\text{HCO}_{3}^{-}\) has a stronger conjugate base \(\text{CO}_{3}^{2-}\) compared to \(\text{NH}_{3}\), making \(\text{HCO}_{3}^{-}\) a weaker acid than \(\text{NH}_{4}^{+}\).

Key concepts

Weak Acids

A weak acid is an acid that only partially dissociates in water. This means only some of the acid molecules donate their hydrogen ions, or protons, to the solution. As a result, an equilibrium is established between the undissociated acid and its dissociated ions. Examples of weak acids include acetic acid \(\text{CH}_{3}\text{COOH}\), nitrous acid \(\text{HNO}_{2}\), and hydrofluoric acid \(\text{HF}\). Understanding weak acids is key to solving many chemistry problems involving pH and acid-base reactions.

Dissociation

Dissociation in chemistry refers to the process by which an acid, base, or salt separates into its ionic components when dissolved in water. For weak acids, this process is incomplete, meaning not all of the acid molecules dissociate. For example, nitrous acid \(\text{HNO}_{2}\) partially dissociates into \(\text{H}^+\) and \(\text{NO}_{2}^-\) ions in solution. The degree of dissociation is usually represented by the acid dissociation constant \(\text{K}_a\). A smaller \(\text{K}_a\) value indicates a weaker acid.

Hydrochloric Acid

Hydrochloric acid \(\text{HCl}\) is a strong acid, meaning it completely dissociates in water to form hydrogen ions \(\text{H}^+\) and chloride ions \(\text{Cl}^-\). This full dissociation makes hydrochloric acid one of the strongest acids commonly used in the laboratory. Because it fully dissociates, it has a very high acid dissociation constant \(\text{K}_a\). This property makes \(\text{HCl}\) a strong proton donor, significantly lowering the pH of its solution.

Nitrous Acid

Nitrous acid \(\text{HNO}_{2}\) is a weak acid, which means it does not fully dissociate in water. Only a small fraction of \(\text{HNO}_{2}\) molecules donate their hydrogen ions \(\text{H}^+\) to the solution, forming nitrite ions \(\text{NO}_{2}^-\). The weak dissociation is represented by a relatively low \(\text{K}_a\) value. Although it's weaker compared to strong acids like \(\text{HCl}\), it is generally stronger than hydrofluoric acid \(\text{HF}\).

Bicarbonate Ion

The bicarbonate ion \(\text{HCO}_{3}^{-}\) acts as an acid in aqueous solutions but is considered a weak acid. It partially dissociates to form carbonate ions \(\text{CO}_{3}^{2-}\) and hydrogen ions \(\text{H}^+\). This partial dissociation indicates a lower \(\text{K}_a\) compared to stronger acids. Because the bicarbonate ion forms a very stable conjugate base \(\text{CO}_{3}^{2-}\), it does not readily donate protons, making it a weaker acid when compared to something like the ammonium ion \(\text{NH}_{4}^{+}\).

Ammonium Ion

The ammonium ion \(\text{NH}_{4}^{+}\) is a weak acid derived from ammonia \(\text{NH}_3\). When \(\text{NH}_{4}^{+}\) donates a proton, it converts back to ammonia \(\text{NH}_3\), and this equilibrium process between \(\text{NH}_{4}^{+}\) and \(\text{NH}_3\) indicates its weak acidic nature. The ammonium ion is slightly stronger as an acid than bicarbonate \(\text{HCO}_{3}^{-}\) because it more readily donates its proton due to having a less stable conjugate base. This makes \(\text{NH}_{4}^{+}\) a relatively significant species in acid-base chemistry, especially in biological systems.