Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

A burning strip of magnesium is introduced into a jar containing a gas. After sometimes the walls of the container is coated with carbon. The gas in the container is (a) \(\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{CO}_{2}\) (c) \(\mathrm{O}_{2}\) (d) \(\mathrm{N}_{2}\)

Step-by-Step Solution

Answer

The gas in the container is CO_{2}.

Step 1: Identify the Reaction

When magnesium burns in the presence of oxygen, it forms magnesium oxide. However, if the product of the reaction leads to carbon coating the container walls, we must consider a different reaction involving carbon-containing gases.

Step 2: Analyze Carbon Formation

The formation of carbon indicates that carbon dioxide (CO_{2}) might be present, as the interaction with magnesium could reduce the CO_{2} to carbon (particularly when the reaction occurs at high temperatures).

Step 3: Evaluate Gas Options

Among the options, CO_{2} is the only gas that contains carbon and could lead to carbon deposit on the container when reacting with burning magnesium.

Step 4: Conclude the Gas Identity

Based on the information and chemical possibilities, the gas in the container that results in a carbon coating must be CO_{2}. Other options either do not directly form carbon or support the formation of carbon in the observed manner.

Key Concepts

Magnesium Combustion

Magnesium combustion is an exciting and bright spectacle in chemistry. When magnesium, a silvery metal, burns, it reacts vigorously with oxygen in the air. This reaction is not only bright but also exothermic, which means it releases a significant amount of heat. When magnesium burns in air, it primarily forms magnesium oxide, a white powder. The chemical equation for this reaction is: \[ 2 \, \text{Mg} + \text{O}_2 \rightarrow 2 \, \text{MgO} \] This intense reaction is why magnesium is often used in fireworks. Moreover, Magnesium can reach temperatures hot enough to cause reactions with other gases present, not just oxygen. This characteristic will be pivotal when considering reactions involving compounds like carbon dioxide, where additional products can form.

Carbon Deposition

Carbon deposition occurs when carbon molecules deposit onto a surface, creating a thin coating. In the context of burning magnesium in the presence of carbon dioxide \((\text{CO}_2)\), this process happens through a reduction reaction. Here, burning magnesium reacts with the carbon dioxide gas by stealing its oxygen atoms, leaving carbon behind as a solid deposit:

• Magnesium reduces carbon dioxide to elemental carbon.
• Magnesium oxide, a reaction byproduct, is also formed.

This process is noteworthy because it shows that even in a combustion reaction primarily producing an oxide, conditions can significantly alter the end result. The equation representing this reaction is: \[ 2 \, \text{Mg} + \text{CO}_2 \rightarrow 2 \, \text{MgO} + \text{C} \] Thus, the carbon deposited on the container walls after magnesium combustion in \(\text{CO}_2\) environment confirms the reduction-based chemical transformation.

Gas Identification

Identifying a gas involved in a reaction can sometimes be complex, but it becomes straightforward if you understand the reactions that certain gases undergo. Initially, you should know the properties and reactive nature of the gases. In the given magnesium burning scenario, the presence of a carbon coating on the container wall hints at carbon dioxide's involvement:

• Among the gases listed, \(\text{H}_2\text{O}\), \(\text{O}_2\), and \(\text{N}_2\) do not produce carbon on combustion.
• Carbon deposition aligns with carbon dioxide, which contains carbon atoms that can be stripped by high-energy reactions.

Therefore, the unique identifier here is the carbon deposit resulting from magnesium's interaction with \(\text{CO}_2\). Such chemical insights help confidently conclude the presence of \(\text{CO}_2\) within the initial mixture by matching the reaction products to known characteristics of possible gases involved.