Problem 8 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Magnesium powder burns in air to give: (a) \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) (b) \(\mathrm{MgO}\) and \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) (c) \(\mathrm{Mg} \mathrm{O}\) only (d) \(\mathrm{MgO}\) and \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

The correct answer is (b) \(\text{MgO}\) and \(\text{Mg}_3\text{N}_2\).

Step 1: Identify the Reactants

The reactant in this chemical reaction is magnesium powder, which is represented as \(\text{Mg}\). Magnesium reacts with oxygen in the air, denoted as \(\text{O}_2\). The question asks us to determine the products formed when magnesium reacts with air.

Step 2: Determine the Possible Products

When magnesium burns in air, it primarily reacts with oxygen to form magnesium oxide, \(\text{MgO}\). However, if nitrogen from the air is considered, magnesium might form magnesium nitride, \(\text{Mg}_3\text{N}_2\). Therefore, the potential products could be \(\text{MgO}\), \(\text{Mg}_3\text{N}_2\), or both.

Step 3: Analyze the Given Options

Compare the potential products from step 2 with the options provided: (a) \(\text{Mg}(\text{NO}_3)_2\) and \(\text{Mg}_3\text{N}_2\) - Magnesium nitrate usually requires a complex reaction with nitric acid, not air and magnesium alone.(b) \(\text{MgO}\) and \(\text{Mg}_3\text{N}_2\) - Possible as both \(\text{MgO}\) and \(\text{Mg}_3\text{N}_2\) are formed during burning in air.(c) \(\text{MgO}\) only - Possible if only oxygen is considered.(d) \(\text{MgO}\) and \(\text{Mg}(\text{NO}_3)_2\) - Unlikely for a similar reason as option (a).

Key Concepts

Magnesium Oxide Formation

When magnesium burns in air, it predominantly reacts with oxygen from the air. Magnesium powder, when exposed to sufficient heat, combines with oxygen molecules (\(\text{O}_2\)) to create magnesium oxide (\(\text{MgO}\)). This reaction is highly exothermic, meaning it releases a substantial amount of heat. This is part of why the reaction appears so bright and intense, resembling fireworks.

The balanced chemical equation for this reaction is:\[2 \text{Mg} + \text{O}_2 \to 2 \text{MgO}\]Here are some important points to take away:

• White ash: The end product, magnesium oxide, appears as a white ash-like substance.
• Heat and light: The reaction releases energy, creating a bright white flame.

This synthesis of magnesium oxide is commonly witnessed in lab settings and is an exemplary demonstration of combustion reactions in metals.

Magnesium Nitride Formation

While the formation of magnesium oxide is most prominent, nitrogen in the air can also react with magnesium, particularly when there is limited oxygen supply. This results in the formation of magnesium nitride (\(\text{Mg}_3\text{N}_2\)).

In this case, the magnesium combines with nitrogen gas (\(\text{N}_2\)) from the air. The chemical equation for this less dominant reaction is:\[3 \text{Mg} + \text{N}_2 \to \text{Mg}_3 \text{N}_2\]Points to remember include:

• Greenish-yellow tint: Magnesium nitride can sometimes give a greenish-yellow tint to the flame at high temperatures.
• Limited conditions: The formation of magnesium nitride is typically less noticeable unless oxygen is scarce.

This reaction is a good example of how different elements in the same environment can yield varied products, even with a single reactant like magnesium.

Combustion Reactions

Combustion reactions are fundamental chemical processes where a substance combines with oxygen, releasing energy in the form of light or heat. Magnesium's reaction with air is a classic example of a combustion reaction, marked by its bright white flame.

Key characteristics of combustion reactions include:

• Exothermic nature: They release energy, often resulting in flames or explosions.
• Reactants and products: In the context of magnesium, the primary reactant is magnesium metal, and the products can be magnesium oxide and sometimes magnesium nitride.
• Speed: These reactions are usually fast, especially with metals like magnesium.

Understanding combustion reactions is crucial, not just for academic purposes but also for their applications in various industries, from energy production to materials science. They demonstrate how powerful and efficient chemical transformations can be, embodying both destructive and beneficial potential.