Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following radicals are decomposed by conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) only? (a) \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) (b) \(\mathrm{HCO}_{3}^{-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) and (c) \(\mathrm{SO}_{3}^{2-}\).

Step 1: Understanding the Problem

The problem asks which radical ions can be decomposed by concentrated \( \mathrm{H}_{2}\mathrm{SO}_{4} \) only. We have four radicals to evaluate: \( \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \), \( \mathrm{HCO}_{3}^{-} \), \( \mathrm{SO}_{3}^{2-} \), and \( \mathrm{CH}_{3} \mathrm{COO}^{-} \). We need to determine if and how concentrated sulfuric acid can decompose each radical.

Step 2: Evaluating Each Radical

- \( \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \) (Oxalate ion): This ion is known to decompose in strongly acidic conditions into \( \mathrm{CO}_{2} \) and \( \mathrm{CO} \). \[\mathrm{C}_{2} \mathrm{O}_{4}^{2-} + \mathrm{H}^{+} \rightarrow \mathrm{CO}_{2} + \mathrm{CO} + \mathrm{H}_{2}O \]- \( \mathrm{HCO}_{3}^{-} \) (Bicarbonate ion): Generally decomposes into \( \mathrm{CO}_{2} \) and \( \mathrm{H}_{2}O \) but does not require concentrated sulfuric acid, as simple acid conditions are sufficient.- \( \mathrm{SO}_{3}^{2-} \) (Sulfite ion): Reacts with Concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \) but forms \( \mathrm{SO}_{2} \) and water, indicating decomposition.- \( \mathrm{CH}_{3} \mathrm{COO}^{-} \) (Acetate ion): Does not readily decompose under the action of concentrated sulfuric acid under the typical conditions.

Step 3: Identifying the Correctly Decomposed Radicals

From the evaluation, \( \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \) and \( \mathrm{SO}_{3}^{2-} \) are decomposed by concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \). The radical \( \mathrm{CH}_{3} \mathrm{COO}^{-} \) is not effectively decomposed solely by \( \mathrm{H}_{2} \mathrm{SO}_{4} \) under normal conditions. \( \mathrm{HCO}_{3}^{-} \) is not specific to decomposition by concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \).

Step 4: Conclusion

The radicals \( \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \) and \( \mathrm{SO}_{3}^{2-} \) decompose with concentrated sulfuric acid.

Key Concepts

Concentrated Sulfuric Acid

Sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)) is one of the strongest acids and is highly corrosive. It is also a dehydrating agent, meaning it can remove water molecules from other substances. This acid is used in various chemical reactions due to its strong acidic and oxidizing properties.
Concentrated sulfuric acid contains very little water, typically more than 98% sulfuric acid by weight. Its high concentration makes it a potent chemical for decomposing certain compounds. For instance, certain radicals, like oxalate ions (\( \mathrm{C}_2 \mathrm{O}_4^{2-} \)) and sulfite ions (\( \mathrm{SO}_3^{2-} \)), can experience significant transformations when exposed to it. This ability to decompose radicals helps facilitate many reactions in industrial settings.
In laboratory settings, safety precautions are essential when handling concentrated sulfuric acid, as it can cause severe burns and can react vigorously when it comes in contact with water or organic material.

Oxalate Ion Decomposition

The oxalate ion (\( \mathrm{C}_2 \mathrm{O}_4^{2-} \)) undergoes decomposition in the presence of concentrated sulfuric acid. When sulfuric acid donates protons (\( \mathrm{H}^{+} \)) to the oxalate ion, it triggers a breakdown of the oxalate ion into carbon dioxide (\( \mathrm{CO}_2 \)) and carbon monoxide (\( \mathrm{CO} \)).
This can be represented by the equation:
\[ \mathrm{C}_2 \mathrm{O}_4^{2-} + \mathrm{H}^{+} \rightarrow \mathrm{CO}_2 + \mathrm{CO} + \mathrm{H}_2\mathrm{O} \]

• Oxalate ion captures protons from sulfuric acid.
• Decomposition results in gas production: \( \mathrm{CO}_2 \) and \( \mathrm{CO} \).
• This process demonstrates how sulfuric acid can act as a catalyst for decomposition.

The activity of the oxalate ion in concentrated sulfuric acid highlights the acid's role in promoting chemical transformations that are often leveraged in chemical synthesis and analysis.

Sulfite Ion Reaction

The sulfite ion (\( \mathrm{SO}_3^{2-} \)) similarly reacts with concentrated sulfuric acid, leading to its decomposition. This reaction typically involves the sulfite ion gaining protons and rearranging to form sulfur dioxide (\( \mathrm{SO}_2 \)) and water.
The chemical equation for the decomposition is:
\[ \mathrm{SO}_3^{2-} + 2\mathrm{H}^{+} \rightarrow \mathrm{SO}_2 + \mathrm{H}_2\mathrm{O} \]

• Sulfite ion takes in protons from the acid, facilitating its breakdown.
• Produced sulfur dioxide is a gas, often used in different industrial processes.
• This reaction showcases the power of sulfuric acid in breaking down ions to simpler forms.

The interaction between concentrated sulfuric acid and sulfite ions is essential for understanding the versatile nature of chemical acids in decomposing various ions.

Chemical Reactions in Acidic Conditions

Not all radicals require concentrated sulfuric acid for their decomposition; some only need to be in a simple acidic environment. For example, the bicarbonate ion (\( \mathrm{HCO}_3^{-} \)) is decomposed into carbon dioxide and water merely by the presence of any strong acid. In such cases, the additional concentration of sulfuric acid is not necessary.
However, when concentrated sulfuric acid is involved, it significantly accelerates and alters chemical reactions compared to dilute acids. This is due to its intrinsic properties like high acidity and its ability to act as a dehydrating and oxidizing agent.

• Concentrated acids intensify reaction speeds and outcomes.
• They target specific ions, facilitating transformations that simpler acids can't achieve.
• Such reactions are relevant in many industrial applications where efficiency and speed are crucial.

Understanding these dynamics is vital for chemists to manipulate reactions finely, achieving desired products while maintaining safe handling measures.