Problem 34 - Chemistry The Central Science In Si Units

Question

State whether each of the following statements is true or false. Justify your answer in each case. (a) \(\mathrm{NH}_{3}\) contains no 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, (b) False, (c) True.

Step 1: Understanding basic solutions

Ammonia (\(\mathrm{NH}_3\)) does not contain hydroxide ions \((\mathrm{OH}^{-})\) in its structure, yet when \(\mathrm{NH}_3\) dissolves in water, it reacts with water molecules to form \(\mathrm{NH}_4^{+}\) and \(\mathrm{OH}^{-}\). This reaction:\[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \leftrightarrow \mathrm{NH}_4^{+} + \mathrm{OH}^{-} \]makes the solution basic. Therefore, statement (a) is true.

Step 2: Classifying HF as an acid

While hydrofluoric acid (HF) is acidic, it is not classified as a strong acid. A strong acid dissociates completely in aqueous solution, while HF only partially dissociates:\[\mathrm{HF} \rightleftharpoons \mathrm{H}^{+} + \mathrm{F}^{-} \]This means HF is a weak acid, not a strong one. Therefore, statement (b) is false.

Step 3: Sulfuric acid dissociation

Sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) is a strong acid and a strong electrolyte. However, it dissociates in two stages, primarily resulting in\[\mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{H}^{+} + \mathrm{HSO}_4^{-}\]In the first stage, which occurs completely, forming \(\mathrm{HSO}_4^{-}\). In the second, incomplete dissociation occurs as:\[\mathrm{HSO}_4^{-} \leftrightarrow \mathrm{H}^{+} + \mathrm{SO}_4^{2-} \]This leads to more \(\mathrm{HSO}_4^{-}\) than \(\mathrm{SO}_4^{2-}\) ions in solution. Therefore, statement (c) is true.

Key Concepts

Ammonia

Ammonia, with the chemical formula \( \mathrm{NH}_3 \), is a fascinating compound. Despite not containing hydroxide ions \( (\mathrm{OH}^{-}) \) directly, it can still behave as a base when dissolved in water. This is due to a special reaction that occurs with water molecules. When ammonia is added to water, an interaction causes the formation of ammonium ions \( (\mathrm{NH}_4^{+}) \) and hydroxide ions \( (\mathrm{OH}^{-}) \). The equation is as follows:\[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \leftrightarrow \mathrm{NH}_4^{+} + \mathrm{OH}^{-}\]These formed hydroxide ions are responsible for increasing the pH of the solution, making it basic. If you're wondering why ammonia acts this way, it's because it is a weak base—meaning it doesn’t fully ionize in water.
Important points about ammonia:

• It is a weak base.
• It partially ionizes in water to form hydroxide ions.
• The presence of \( \mathrm{OH}^{-} \) ions is what contributes to its basic nature.

Understanding the behavior of ammonia can be incredibly helpful in acid-base chemistry as it broadens our understanding of what makes a substance acidic or basic beyond the presence of \( \mathrm{OH}^{-} \) ions.

Hydrofluoric Acid

Hydrofluoric acid, or HF, can be a bit tricky when it comes to classifying its strength as an acid. Many may instinctively classify HF as a strong acid, but in reality, it is a weak acid. The distinction between strong and weak acids lies in their ability to dissociate completely in water. Strong acids dissociate entirely, but HF dissociates only partially:\[\mathrm{HF} \rightleftharpoons \mathrm{H}^{+} + \mathrm{F}^{-}\]This means that in a solution, not all HF molecules will release hydrogen ions \( \mathrm{H}^{+} \) to the water. Because of this partial dissociation, it is considered a weak acid.
Key characteristics of hydrofluoric acid include:

• Partial dissociation in water.
• Classified as a weak acid due to incomplete ionization.
• Still corrosive and can pose significant safety risks despite being a weak acid.

It's important to recognize the difference between concentration and strength. An acid can be highly concentrated but still weak if it doesn't fully dissociate.

Sulfuric Acid

Sulfuric acid, \( \mathrm{H}_2\mathrm{SO}_4 \), is one of the most well-known strong acids. It is indeed a strong electrolyte, meaning it conducts electricity well when dissolved in water due to its significant dissociation. However, the dissociation of sulfuric acid is a two-step process, which is interesting and a bit complex. In the first step, sulfuric acid dissociates completely:\[\mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{H}^{+} + \mathrm{HSO}_4^{-}\]However, in the second stage, \( \mathrm{HSO}_4^{-} \) ions do not completely dissociate:\[\mathrm{HSO}_4^{-} \leftrightarrow \mathrm{H}^{+} + \mathrm{SO}_4^{2-}\]This results in more \( \mathrm{HSO}_4^{-} \) ions being present than \( \mathrm{SO}_4^{2-} \) ions in the solution.
Essential points about sulfuric acid include:

• Initial dissociation is complete and releases \( \mathrm{H}^{+} \) ions.
• The second dissociation is only partial, leaving more \( \mathrm{HSO}_4^{-} \) ions.
• Even as a strong acid, the two-step dissociation affects ion concentration balance.

Understanding the two-stage dissociation of sulfuric acid is crucial for grasping its behavior in various chemical reactions and its application in industrial processes.