Question

Aluminium reacts with hot concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) to liberate (a) \(\mathrm{S}\) (b) \(\mathrm{SO}_{3}\) (c) \(\mathrm{SO}_{2}\) (d) \(\mathrm{H}_{2} \mathrm{~S}\)

Short answer

The correct answer is (c) \(\text{SO}_2\).

Step-by-step solution

Understand the Chemical Reaction

When aluminium (\(\text{Al}\)) reacts with hot concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)), the sulfuric acid acts as both an oxidizing agent and a dehydrating agent. Typically, concentrated sulfuric acid, when heated, will oxidize metals like aluminum.

Identify the Products

In this reaction, the sulfur in \(\text{H}_2\text{SO}_4\) is reduced as it is the oxidizing agent. Commonly, in such reactions, \(\text{SO}_2\) is a reduction product. Thus, the expected gaseous product from the reaction of aluminium with hot concentrated sulfuric acid is \(\text{SO}_2\), sulfur dioxide.

Confirm the Reaction Equation

The balanced chemical equation which represents this reaction is: \[2\text{Al} + 6\text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{SO}_2 + 6\text{H}_2\text{O}\]This confirms that the sulfur dioxide (\(\text{SO}_2\)) gas is released.

Key concepts

Reduction of Sulfuric Acid

When sulphuric acid is involved in a chemical reaction, it often plays a central supporting role due to its strong oxidizing capabilities. In the reaction of aluminium with hot concentrated sulfuric acid, the acid undergoes a process called reduction. Let's break it down.
In chemistry, reduction refers to the gain of electrons or a decrease in oxidation state by a chemical species. In this reaction, the sulfur component of sulfuric acid moves from an oxidation state of +6 in \(\text{H}_2\text{SO}_4\) to +4 in \(\text{SO}_2\).
This transition is due to the sulfur accepting electrons from aluminium, which itself is undergoing oxidation. As such, the sulfuric acid is reduced, forming \(\text{SO}_2\). This change mirrors one of the basic principles of redox reactions where the oxidizing agent (sulfuric acid in this case) is reduced during the reaction.

Oxidizing Agent

Sulfuric acid is often viewed as an oxidizing agent in various reactions due to its ability to gain electrons. But what exactly does that mean?
In redox chemistry, an oxidizing agent is a substance that oxidizes another substance by accepting electrons from it. This act of electron acceptance results in the oxidizing agent being reduced itself. In our specific reaction, sulfuric acid oxidizes aluminium by taking its electrons.
It does this quite effectively when concentrated and heated, leading to a successful exchange where aluminium loses its electrons and becomes oxidized to \(\text{Al}_2(\text{SO}_4)_3\). Meanwhile, the sulfur part of the sulfuric acid gains these electrons, creating \(\text{SO}_2\), a common product when sulfuric acid functions as an oxidizing agent.

Dehydrating Agent

Apart from being a powerful oxidizing agent, concentrated sulfuric acid also doubles as a dehydrating agent. What does this mean for the reaction with aluminium? Let's dive in.
A dehydrating agent is a substance that removes water molecules or hydrogen atoms from another compound. Concentrated sulfuric acid excels in this role due to its strong affinity for water. During the reaction with aluminium, this powerful agent helps facilitate the extraction of water molecules.
This explains how \(\text{H}_2\text{O}\), or water, is also a product in the balanced chemical reaction. The presence of \(6\text{H}_2\text{O}\) in the reaction equation showcases sulfuric acid's capacity to facilitate such transformations, proving its effectiveness as both an oxidizing and dehydrating agent.