Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

A bottle completely filled with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is left unstoppered for several days and we observe spontaneous overflow of acid. It is due to (a) dehydration of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (b) absorption of air by \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) hygroscopic nature of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) change in temperature

Step-by-Step Solution

Answer

The overflow is due to the hygroscopic nature of H_{2}SO_{4} (option c).

Step 1: Identify the Problem

The problem involves understanding why a bottle of concentrated H_{2}SO_{4} left open results in overflow. You're provided with multiple options as the cause.

Step 2: Review Properties of H_{2}SO_{4}

Concentrated sulfuric acid ( H_{2}SO_{4}) is known for being hygroscopic, meaning it can absorb water from its environment. This is a key property relevant to the question.

Step 3: Eliminate Incorrect Options

Consider each given option: (a) Dehydration implies loss of water, which would not cause overflow. (b) Absorption of air isn't significant enough to cause overflow. (d) A change in temperature could play a role if it was extreme, but the problem specifically highlights prolonged exposure.

Step 4: Select the Correct Response

Given that concentrated sulfuric acid ( H_{2}SO_{4}) absorbs water from the air due to its hygroscopic nature, this results in an increase in volume, leading to overflow. Therefore, option (c) is correct.

Key Concepts

Hygroscopic Nature of Sulfuric Acid

One of the most intriguing properties of concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)) is its hygroscopic nature. This means it has the ability to attract and hold water molecules from its surrounding environment. When left unstoppered, \(\text{H}_2\text{SO}_4\) will begin to absorb water vapor from the air, thereby increasing in volume.
This property results from sulfuric acid’s polar nature, which allows it to interact strongly with water molecules. As moisture accumulates in the liquid, the acid's capacity to hold more water increases until all available air-bound moisture is absorbed.

• This characteristic makes it very effective as a drying agent.
• Also, it can have safety implications, such as the risk of container overflow.

Understanding this hygroscopic nature helps explain phenomena like the spontaneous overflow observed when a bottle of concentrated sulfuric acid is left exposed to air.

Chemical Reactions Involving Sulfuric Acid

Sulfuric acid is not only hygroscopic but also highly reactive. It readily undergoes various chemical reactions due to its strong acidic properties. The reactivity of sulfuric acid comes from its ability to donate protons (\(\text{H}^+\) ions) and its role as a dehydrating agent.
Here are a few common reactions that sulfuric acid takes part in:

• Dehydration: \(\text{H}_2\text{SO}_4\) can remove water from other compounds, making it useful for reactions where water needs to be eliminated.
• Acid-Base Reactions: It will neutralize bases, forming salts and water. The chemical equation for its reaction with sodium hydroxide (\(\text{NaOH}\)) is given by: \[\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}\]
• Oxidizing Agent: In some chemical processes, sulfuric acid acts as an oxidizer, facilitating the exchange of electrons between reacting species.

Exploring these reactions reveals the versatility and industrial significance of sulfuric acid. However, it’s important to handle it with care, as these reactions can be vigorous.

Acid Overflow in Containers

When storing concentrated sulfuric acid, one must be aware of the potential for overflow, particularly when the container is left unstoppered. This overflow is primarily a result of the acid's hygroscopic nature, which leads to volume expansion as it absorbs moisture.
Several factors contribute to acid overflow:

• Mistake in Sealant: A proper seal prevents moisture ingress. If a container is improperly sealed, more air (and thus more moisture) can contact the acid, increasing overflow risk.
• Temperature Changes: Although less influential than hygroscopic characteristics, temperature variations can cause expansion or contraction in fluid volume, potentially affecting containment.
• Spillage Safeguards: Containers should always be kept in spill-containment trays to handle any overflow safely.

Being aware of these factors can help minimize risks associated with handling and storing sulfuric acid. Ensuring that containers are securely sealed and monitored is crucial to prevent any hazardous incidents.