Question

State whether each of the following statements is true or false. Justify your answer in each case. (a) \(\mathrm{NH}_{3}\) contains no \(\mathrm{OH}^{-}\) ions, and yet its aqueous solutions are basic. (b) HF is a strong acid. (c) Although sulfuric acid is a strong electrolyte, an aqueous solution of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) contains more \(\mathrm{HSO}_{4}^{-}\) -ions than \(\mathrm{SO}_{4}^{2-}\) ions.

Short answer

Statement (a) is True. Although NH3 does not contain OH- ions, it reacts with water in an aqueous solution to form hydroxide ions, making the solution basic. Statement (b) is False. HF, hydrofluoric acid, is a weak acid, not a strong acid, as it does not completely dissociate in water. Statement (c) is True. Sulfuric acid (H2SO4) is a diprotic acid and dissociates in two steps, resulting in more HSO4- ions than SO4^2- ions in the aqueous solution.

Step-by-step solution

Statement (a)

In order to determine if the statement "NH3 contains no OH- ions, and yet its aqueous solutions are basic" is true or false, one should know that NH3 is an ammonia molecule. While NH3 does not contain any OH- ions, it reacts with water in an aqueous solution to form ammonium ions (NH4+) and hydroxide ions (OH-). The presence of hydroxide ions in the solution makes it basic. Therefore, the statement is True.

Statement (b)

To determine the truthfulness of the statement "HF is a strong acid," one should know the definition of a strong acid and if HF qualifies as a strong acid. A strong acid is a compound that completely dissociates into ions when dissolved in water, whereas a weak acid partially dissociates. HF (hydrofluoric acid) is classified as a weak acid because it does not completely dissociate in water. Therefore, the statement is False.

Statement (c)

To determine the truthfulness of the statement "Although sulfuric acid is a strong electrolyte, an aqueous solution of H2SO4 contains more HSO4- ions than SO4^2- ions," one should have knowledge of sulfuric acid's properties and its dissociation in water. Sulfuric acid (H2SO4) is a strong acid and a strong electrolyte, meaning it completely dissociates into ions. However, it is also a diprotic acid, which means it loses two protons (H+ ions) in two separate steps. In the first step, H2SO4 gives away its first proton (H+) to form HSO4- ions. This dissociation reaction occurs completely since it is a strong acid. In the second step, the HSO4- ion now acts as a weak acid and partially gives away its proton (H+) to form SO4^2- ions. As a result, there will be more HSO4- ions than SO4^2- ions in the aqueous solution. Therefore, the statement is True.

Key concepts

Ammonia Basicity

Ammonia, whose chemical formula is \(\mathrm{NH}_3\), is an interesting compound when it comes to its basicity. Unlike a traditional base that might contain \(\mathrm{OH}^-\) ions, ammonia itself does not have any \(\mathrm{OH}^-\) ions. However, once ammonia is added to water, an interesting reaction occurs.
When dissolved in water, ammonia reacts with water molecules to form ammonium ions \(\mathrm{NH}_4^+\) and hydroxide ions \(\mathrm{OH}^-\). This reaction can be written as:
\[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\]
Thus, the generation of \(\mathrm{OH}^-\) ions is what imparts basicity to the solution, making ammonia a base despite not containing \(\mathrm{OH}^-\) ions in its original form. This property makes it a classic example of a base that relies on its reaction with water to exhibit basic behavior.

• The lack of initial hydroxide ions does not prevent ammonia from acting as a base.
• This characteristic highlights the dynamic behavior of compounds in aqueous solutions.

Strong vs Weak Acids

Understanding the difference between strong and weak acids is fundamental in acid-base chemistry. A strong acid is defined by its complete dissociation in water, meaning that it fully breaks down into its ions. On the other hand, a weak acid only partially dissociates.
Take the example of HF, hydrofluoric acid. Contrary to what its "hydro-fluoro" name might suggest, HF is not a strong acid. It does not completely dissociate in water, but instead, it maintains equilibrium between its dissociated and undissociated forms. This characteristic identifies it as a weak acid in chemical terms.

• Strong acids dissociate 100% in water, producing maximum \(\mathrm{H}^+\) ions.
• Weak acids exist in equilibrium with their ions, resulting in a mix of ionic and molecular forms in solution.

Recognizing the nature of an acid is essential, as it directly affects the chemical reactivity and the pH of the solution.

Sulfuric Acid Dissociation

Sulfuric acid \(\mathrm{H}_2\mathrm{SO}_4\) is renowned for its strength as an acid in chemical reactions and solutions. However, its dissociation in water occurs in two distinct steps, as it is a diprotic acid.
In the first dissociation step, sulfuric acid loses one proton, forming hydrogen sulfate ions \(\mathrm{HSO}_4^-\). This step is complete because sulfuric acid is a strong acid:
\[\mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{H}^+ + \mathrm{HSO}_4^-\]
In the second step, \(\mathrm{HSO}_4^-\) ions can lose another proton to form sulfate ions \(\mathrm{SO}_4^{2-}\):
\[\mathrm{HSO}_4^- \rightleftharpoons \mathrm{H}^+ + \mathrm{SO}_4^{2-}\]
This second dissociation is not complete because \(\mathrm{HSO}_4^-\) acts as a weak acid. Due to this partial dissociation, there are more \(\mathrm{HSO}_4^-\) ions than \(\mathrm{SO}_4^{2-}\) ions in the solution.

• Sulfuric acid's two-step dissociation is key to its behavior in solutions.
• This process results in a greater concentration of \(\mathrm{HSO}_4^-\) ions over \(\mathrm{SO}_4^{2-}\) ions.

Grasping this concept helps in understanding the unique role that sulfuric acid plays across various chemical reactions.