Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Which substance chars when heated with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4} ?\) (a) protein (b) hydrocarbon (c) fat (d) carbohydrate

Step-by-Step Solution

Answer

Carbohydrate chars when heated with conc. \(\mathrm{H}_{2}\mathrm{SO}_{4}\).

Step 1: Understand the Problem

We need to determine which type of substance undergoes charring when heated with concentrated sulfuric acid (conc. \(\mathrm{H}_{2}\mathrm{SO}_{4}\)). We have four options: protein, hydrocarbon, fat, and carbohydrate.

Step 2: Review the Properties of Concentrated Sulfuric Acid

Concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is known for its dehydrating properties. It can remove water molecules from compounds, especially organic compounds, leading to charring or carbonization.

Step 3: Identify the Substance that Loses Water and Chars

Among the given options, carbohydrates (option d) are known to lose water when heated with \(\mathrm{H}_{2}\mathrm{SO}_{4}\), as they contain a large proportion of atoms capable of being removed as water. This results in the charring or carbonization process.

Step 4: Conclusion

Carbohydrates are particularly susceptible to dehydration and charring by \(\mathrm{H}_{2}\mathrm{SO}_{4}\) due to their molecular composition which easily loses water under these conditions.

Key Concepts

Concentrated Sulfuric Acid Properties

Concentrated sulfuric acid, often represented by the chemical formula \( \mathrm{H}_2\mathrm{SO}_4 \), is known for its aggressive nature. It is a dense, oily liquid that is colorless and highly corrosive. Its high concentration makes it a powerful reagent in many chemical reactions. One notable property of concentrated sulfuric acid is its ability to act as a strong acid and a dehydrating agent. It has a high affinity for water and can remove water molecules from substances that it reacts with. Additionally, concentrated sulfuric acid can conduct electricity due to the full dissociation of hydrogen ions in its solution.
Some essential properties of concentrated \( \mathrm{H}_2\mathrm{SO}_4 \) include:

• High boiling point and low vapor pressure, which means it evaporates at a very slow rate.
• Vigorous reaction with water, releasing heat.
• Corrosive effects on skin and materials, necessitating careful handling and storage.

Understanding these properties is crucial, as they play a significant role in the dehydration and charring reactions that concentrated \( \mathrm{H}_2\mathrm{SO}_4 \) can induce.

Dehydrating Properties

Sulfuric acid is widely recognized for its dehydrating properties. This means it has a strong tendency to absorb water from other compounds, causing the affected compounds to lose water and undergo significant changes.
This dehydration process is particularly impactful on organic compounds. For example, when sulfuric acid comes into contact with sugar, which is a type of carbohydrate, it absorbs water, leaving behind a charred carbon residue.
Some key aspects of sulfuric acid's dehydrating capabilities include:

• The acid absorbs water quickly and releases a significant amount of heat during this process.
• It can lead to the decomposition of the compound, such as breaking down carbohydrates into elemental carbon and water vapor.
• This property is utilized in industrial processes where water removal or drying is desired.

The dehydrating ability is an essential consideration because it can determine the outcome of reactions with various organic materials.

Carbohydrate Reaction with Sulfuric Acid

Carbohydrates are organic compounds consisting primarily of carbon, hydrogen, and oxygen. When these compounds react with concentrated sulfuric acid, a fascinating transformation takes place. The acid acts as a dehydrating agent and removes water molecules from the carbohydrate structure.
This rapid dehydration is responsible for the charring process, where the carbohydrate loses its water content and leaves behind a black, carbon-rich residue. This reaction clearly illustrates the powerful dehydrating nature of sulfuric acid.
Detailed reaction mechanics include:

• The acid pulls water (\(\mathrm{H}_2\mathrm{O}\)) molecules out, especially from polysaccharides, due to the presence of abundant hydroxyl (\(\mathrm{-OH}\)) groups in carbohydrates.
• This removal of water breaks the molecular structure, exposing carbon atoms, which appear darkened due to their oxidized state.
• A significant exothermic reaction occurs, which can cause the mixture to heat up rapidly, potentially leading to further reactions or damage.

This practical example underscores how sulfuric acid's dehydrating properties interact with organic compounds, demonstrating why carbohydrates are particularly susceptible to charring in its presence.