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 \( \mathrm{CO}_{2} \).

Step 1: Identify the Reaction

When magnesium burns, it typically reacts with oxygen to form magnesium oxide (\[ \mathrm{MgO} \]). In this scenario, we need to consider the possibility that it reacted with another component instead.

Step 2: Analyze the Resulting Product

If the walls of the container are coated with carbon, this indicates that carbon is forming as a result of a chemical reaction. The formation of carbon suggests that carbon dioxide (\[ \mathrm{CO}_{2} \]) might be involved because magnesium can reduce \[ \mathrm{CO}_{2} \] to carbon.

Step 3: Understand Magnesium's Reaction with the Gas

Magnesium burns in carbon dioxide, reducing it to carbon. This reaction can be represented by: \( 2 \mathrm{Mg} + \mathrm{CO}_{2} \rightarrow 2 \mathrm{MgO} + \mathrm{C} \). This explains how the carbon coating forms on the walls of the container.

Step 4: Verify the Options Against the Reaction

Check each gas option to see which one produces carbon when reacting with magnesium. \[ \mathrm{H}_{2} \mathrm{O} \], \[ \mathrm{O}_{2} \], and \[ \mathrm{N}_{2} \] won't result in carbon formation with burning magnesium. Only \[ \mathrm{CO}_{2} \] is reduced to carbon by magnesium.

Key Concepts

Magnesium Combustion

Magnesium combustion is a fascinating chemical reaction. When magnesium burns, it reacts with oxygen in the air, creating a bright white flame.
This is because magnesium is a very reactive metal and releases a lot of energy when it combusts. The typical product of this combustion is magnesium oxide, represented as \( \mathrm{MgO} \).
The reaction can be written as:

• \( 2 \mathrm{Mg} + \mathrm{O}_{2} \rightarrow 2 \mathrm{MgO} \)

Here, you see that two magnesium atoms react with one molecule of oxygen to produce two MgO units. This reaction is highly exothermic, which is why you observe such intense light and heat when magnesium burns. Magnesium combustion is not limited to oxygen. When it interacts with different gases, the products change, as we'll see in the next sections.

Carbon Dioxide Reduction

In the context of magnesium combustion, carbon dioxide (\( \mathrm{CO}_{2} \)) can play an interesting role.
Instead of aiding combustion like oxygen, carbon dioxide can be reduced by magnesium. But what does it mean to "reduce" carbon dioxide?
Reduction, in a chemical sense, means that a compound loses oxygen atoms. In this case, magnesium reduces carbon dioxide, turning it into carbon and magnesium oxide. The reaction proceeds as follows:

• \( 2 \mathrm{Mg} + \mathrm{CO}_{2} \rightarrow 2 \mathrm{MgO} + \mathrm{C} \)

This reaction highlights magnesium's ability to strip away oxygen from carbon dioxide, resulting in the formation of carbon.
This process is possible because magnesium has a stronger affinity for oxygen than carbon does, allowing it to effectively "snatch" the oxygen from the carbon dioxide, thereby producing elemental carbon.

Magnesium Oxide Formation

Magnesium oxide formation is a central aspect of chemical reactions involving magnesium. Whether in normal air or within carbon dioxide, the tendency of magnesium to form magnesium oxide is very strong.
When magnesium combusts, combining it with oxygen, either directly from the air or through reactions like carbon dioxide reduction, the outcome is almost always the same: magnesium oxide.
Magnesium oxide is a stable white powder that results from two oxygen atoms bonding with magnesium. In the case of the reaction with carbon dioxide:

• \( 2 \mathrm{Mg} + \mathrm{CO}_{2} \rightarrow 2 \mathrm{MgO} + \mathrm{C} \)

The production of magnesium oxide is a product of magnesium’s strong affinity for oxygen atoms, creating a bond that is energetically favorable.
This reaction not only explains the intense heat and light of burning magnesium but also illustrates why the walls of the container become coated with carbon after the reaction. Magnesium oxide continues to form, signifying how effectively magnesium can harness oxygen from its environment, regardless of whether it is present as a free gas or bound in a compound like carbon dioxide.