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 (b) CO_{2}.

Step 1: Understanding the Problem

The problem provides a scenario where a burning strip of magnesium is placed into a jar of gas and later, the walls are coated with carbon. We need to identify which gas could result in carbon deposition.

Step 2: Evaluating the Reaction

When magnesium burns, it typically reacts with gases. If carbon is deposited, it indicates a reaction where carbon is reduced. Such reactions usually involve carbon-containing gases.

Step 3: Analyzing Options

(a) Water vapor ( H_{2}O ) does not contain carbon, so it cannot be the source of carbon deposition. (b) Carbon dioxide ( CO_{2} ) contains carbon and can reduce to deposit carbon. (c) Oxygen ( O_{2} ) and (d) Nitrogen ( N_{2} ) do not contain carbon and hence cannot be the source of carbon deposition.

Step 4: Determining the Correct Answer

Based on the analysis, carbon dioxide is the only gas among the options that can deposit carbon when a reaction takes place. Therefore, the gas in the container is (b) CO_{2}.

Key Concepts

Magnesium Reaction

When magnesium is burned, an intense white flame is produced. This reaction is a type of oxidation where magnesium metal reacts with oxygen, transforming into magnesium oxide. But magnesium can also react with other gases, which can lead to different chemical outcomes.
In the case of the original exercise, magnesium is placed in a gas-filled jar. If the outcome is carbon deposition on the jar walls, we suspect a different reaction pathway. Magnesium's high reactivity means it can reduce certain compounds, such as carbon dioxide, to elemental carbon. This reaction highlights magnesium's role in retrieving elemental forms from complex compounds through strong reduction capabilities.

Carbon Deposition

Carbon deposition refers to the process where carbon accumulates on a surface. This typically occurs when a compound containing carbon undergoes reduction or decomposition in specific chemical environments.
When a highly reactive substance like burning magnesium is introduced into carbon dioxide, the magnesium can extract oxygen from carbon dioxide, leaving carbon as a residual deposit. This process signifies the transformative power of chemical reactions resulting in solid carbon forming a visible layer on the container's walls.
Understanding carbon deposition is crucial for industrial applications where carbon build-up can affect the efficiency and operation of machinery.

Carbon Dioxide

Carbon dioxide ( CO_{2} ) is a colorless, odorless gas composed of one carbon atom and two oxygen atoms. It is a vital component of the Earth's atmosphere, essential for plant photosynthesis, and is one of the greenhouse gases responsible for keeping the Earth warm.
In chemical reactions, CO2 can act as both an oxidizing agent and a source of carbon. When reacting with magnesium, carbon dioxide can be reduced to carbon, especially evident under high-temperature conditions provided by burning magnesium. This results in carbon being deposited as discussed in the original exercise.
The role of carbon dioxide in various natural and industrial processes underscores its significant value, from biological functions to being a reactant in chemical industries.

Oxidation-Reduction Reactions

These reactions, often called redox reactions, involve the transfer of electrons between substances. In an oxidation process, a substance loses electrons, while in a reduction process, a substance gains electrons.
Burning magnesium in the presence of carbon dioxide demonstrates both oxidation and reduction. Here, magnesium is oxidized as it reacts, losing electrons while simultaneously reducing carbon dioxide by transferring electrons.

• The magnesium strip gains oxygen, forming magnesium oxide (MgO).
• Carbon dioxide is reduced to carbon (C), depositing on the jar walls.

Understanding these concepts is fundamental to grasping how chemical reactions can transform substances, explaining phenomena from energy production to material synthesis.