Problem 34 - Chemistry: The Central Science in SI Units, Global Edition

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.

Step-by-Step Solution

Answer

(a) True. Although NH3 does not contain OH- ions, it forms them when dissolved in water, making the solution basic. (b) False. HF is a weak acid, not a strong acid. (c) True. An aqueous solution of H2SO4 contains more HSO4- ions than SO42- ions, even though H2SO4 is a strong electrolyte.

Step 1: Statement (a): NH3 contains no OH- ions, and yet its aqueous solutions are basic.

This statement is true. Ammonia (NH3) does not contain hydroxide ions (OH-) in its molecular structure. However, when it is dissolved in water, it reacts with water to form ammonium ions (NH4+) and hydroxide ions (OH-). This increases the concentration of OH- ions in the solution, making it basic. The reaction is as follows: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq) An increase in OH- ion concentration raises the pH of the solution, indicating that it is indeed basic.

Step 2: Statement (b): HF is a strong acid.

This statement is false. Hydrofluoric acid (HF) is a weak acid. Strong acids dissociate completely in water, releasing a high concentration of hydrogen ions (H+) and corresponding anions. HF, on the other hand, is only partially dissociated in water. This leads to a lower concentration of H+ ions compared to strong acids, making HF a weak acid.

Step 3: Statement (c): Although sulfuric acid is a strong electrolyte, an aqueous solution of H2SO4 contains more HSO4- ions than SO42- ions.

This statement is true. Sulfuric acid (H2SO4) is a strong acid and a strong electrolyte that dissociates completely in water. However, it is important to note that it is diprotic, meaning it has two acidic protons that can be donated. When H2SO4 is dissolved in water, it ionizes in two steps: 1. H2SO4(aq) → H+(aq) + HSO4-(aq) 2. HSO4-(aq) ⇌ H+(aq) + SO42-(aq) In the first step, H2SO4 completely dissociates into H+ and HSO4-. But in the second step, HSO4- only partially dissociates into H+ and SO42- because it acts as a weak acid during the second dissociation. Therefore, the concentration of HSO4- ions in an aqueous solution of H2SO4 is greater than the concentration of SO42- ions.

Key Concepts

Ammonia and Water Reaction

Ammonia, a common household chemical, exhibits interesting behavior when dissolved in water. Although it doesn't have hydroxide ions (\(\text{OH}^-\)) in its structure, ammonia (\(\text{NH}_3\)) becomes basic in water due to its reaction with water molecules. This involves ammonia accepting a proton (\(\text{H}^+\)) from water, forming ammonium ions (\(\text{NH}_4^+\)) and increasing hydroxide ion concentration:

• \[\text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \leftrightarrow \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq})\]

This increase in hydroxide ions results in the solution becoming basic. A higher \(\text{OH}^-\) concentration means a higher pH, indicating a basic solution. This behavior is a great example of how substances can impact pH without direct introduction of hydroxide ions.

Hydrofluoric Acid

Hydrofluoric acid (\(\text{HF}\)) is often misunderstood as a strong acid, but it actually behaves differently. Unlike strong acids, which completely dissociate in water, HF is classified as a weak acid because it does not fully dissociate. This means that in an aqueous solution, not all of HF molecules release protons, leading to a lower concentration of hydrogen ions (\(\text{H}^+\)):

• In a dissociation equation, HF ⇌ \(\text{H}^+\) + \(\text{F}^-\), only some HF molecules ionize.

This partial ionization is a key characteristic of weak acids. The stronger the acid, the more it dissociates into \(\text{H}^+\) ions, contributing to solution acidity. HF's weaker ionization results in a less acidic solution compared to a strong acid of the same concentration.

Sulfuric Acid Dissociation

Sulfuric acid (\(\text{H}_2\text{SO}_4\)) is a classic example of a strong acid and a strong electrolyte with a unique dissociation pattern. It is a diprotic acid, meaning it can donate two protons, and this occurs in two steps:

• First step: \[\text{H}_2\text{SO}_4 (\text{aq}) \rightarrow \text{H}^+ (\text{aq}) + \text{HSO}_4^- (\text{aq}) \]
• Second step: \[\text{HSO}_4^- (\text{aq}) \leftrightarrow \text{H}^+ (\text{aq}) + \text{SO}_4^{2-} (\text{aq}) \]

In the first ionization step, sulfuric acid dissociates completely to release \(\text{H}^+\) and \(\text{HSO}_4^-\) ions. The second step is partial, because \(\text{HSO}_4^-\) itself is only a weak acid. As a result, there is a predominance of \(\text{HSO}_4^-\) ions compared to \(\text{SO}_4^{2-}\) ions in the solution. Understanding this helps explain why certain ions predominate in solution and is crucial for tasks involving pH adjustment and chemical reactions.