Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions \(\mathrm{MgO}\) is not formed? (a) \(\mathrm{Mg}+\mathrm{B}_{2} \mathrm{O}_{3} \longrightarrow\) (b) \(\mathrm{Mg}+\mathrm{NO}_{2} \longrightarrow\) (c) \(\mathrm{Mg}+\mathrm{dil}_{.} \mathrm{HNO}_{3} \longrightarrow\) (d) \(\mathrm{Mg}+\mathrm{CO}_{2} \longrightarrow\)

Step-by-Step Solution

Answer

MgO is not formed in reaction (c) with dilute HNO3.

Step 1: Analyze Reaction (a)

The reaction in option (a) is \( \mathrm{Mg}+\mathrm{B}_{2} \mathrm{O}_{3} \longrightarrow \) which commonly forms \( \mathrm{MgO} \) and boron. The chemical equation could be: \( 3\mathrm{Mg} + \mathrm{B}_{2}\mathrm{O}_{3} \rightarrow 3\mathrm{MgO} + 2\mathrm{B} \). Therefore, \( \mathrm{MgO} \) is formed in this reaction.

Step 2: Analyze Reaction (b)

The reaction given in option (b) is \( \mathrm{Mg}+\mathrm{NO}_{2} \longrightarrow \). Typically, magnesium metal reacts with \( \mathrm{NO}_{2} \) to give \( \mathrm{MgO} \) and a nitrogen oxide. A possible balanced equation is \( \mathrm{Mg} + 2\mathrm{NO}_{2} \rightarrow \mathrm{MgO} + 2\mathrm{NO} \). Hence, \( \mathrm{MgO} \) is formed in this reaction as well.

Step 3: Analyze Reaction (c)

Option (c) involves the reaction \( \mathrm{Mg}+\mathrm{dil.} \mathrm{HNO}_{3} \longrightarrow \). With dilute nitric acid, magnesium primarily forms hydrogen gas and a soluble magnesium nitrate compound \( \mathrm{Mg}(\mathrm{NO}_{3})_{2} \), rather than \( \mathrm{MgO} \). Therefore, \( \mathrm{MgO} \) is not formed in this reaction.

Step 4: Analyze Reaction (d)

In option (d), the reaction is \( \mathrm{Mg}+\mathrm{CO}_{2} \longrightarrow \). This typically yields \( \mathrm{MgO} \) and carbon due to the reduction of carbon dioxide by magnesium. An example balanced reaction could be \( 2\mathrm{Mg} + \mathrm{CO}_{2} \rightarrow 2\mathrm{MgO} + \mathrm{C} \). Therefore, \( \mathrm{MgO} \) is formed in this reaction.

Key Concepts

Magnesium Reactions

Magnesium, a reactive, lightweight metal, is known for its ability to form compounds by interacting with various elements and compounds. When magnesium reacts with oxygen, it forms magnesium oxide (MgO), a common product in magnesium reactions. Here are some typical reactions involving magnesium in a simplified form:

• With oxygen: \( 2\,\text{Mg} + \text{O}_2 \rightarrow 2\,\text{MgO} \)
• With carbon dioxide: \( 2\,\text{Mg} + \text{CO}_2 \rightarrow 2\,\text{MgO} + \text{C} \)
• With water (slow reaction): \( \text{Mg} + 2\,\text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2 \)

Magnesium's reactivity is due to its ability to easily lose two electrons to achieve a stable electron configuration. This makes it effective in forming stable magnesium oxide, especially noticeable in its bright combustion in air. Understanding these basic reactions is key to grasping more complex chemical processes involving magnesium.

Oxidation

Oxidation is a chemical process where a substance loses electrons, increasing its oxidation state. In the context of magnesium reactions, oxidation occurs when magnesium metal transforms from the elemental state to a compound state like magnesium oxide. For instance, as magnesium reacts with oxygen or other oxidizing agents, it donates its outer electrons to the acceptor, such as oxygen, forming magnesium oxide:

• Oxidation of Mg: \( \text{Mg} \rightarrow \text{Mg}^{2+} + 2e^- \)

As magnesium loses electrons during these reactions, it is oxidized. An easy way to remember this is through the acronym OIL - Oxidation is Loss (of electrons). Understanding this process is crucial to analyze reactions where Mg is oxidized, especially in redox contexts, where one species is oxidized and another is reduced.

Reduction

In chemical terms, reduction is the gain of electrons by a molecule, atom, or ion, which often leads to a decrease in oxidation state. In magnesium reactions, this is seen when magnesium acts as a reducing agent. For example, when magnesium reacts with carbon dioxide, the carbon gains electrons, which indicates reduction:

• Reduction of \( \text{CO}_2 \): \( \text{CO}_2 + 4e^- \rightarrow \text{C} + 2\,\text{O}^{2-} \)

Magnesium effectively transfers electrons to the carbon, reducing it from a +4 state in \( \text{CO}_2 \) to a 0 state in elemental carbon. Remember the acronym RIG - Reduction is Gain (of electrons) to easily recall this concept. By understanding reduction, you gain insights into how substances in reactions change their oxidation states.

Chemical Reactions Analysis

Analyzing chemical reactions involves understanding what substances are involved and how they transform. In redox reactions, identify which components undergo oxidation and reduction. In magnesium reaction analysis, you look at equations to see changes in oxidation states. Take the reaction with nitric acid:

• \( \text{Mg} + \text{HNO}_3 \rightarrow \text{Mg(NO}_3\text{)}_2 + \text{H}_2 \)

Through analysis:

• Note that \( \text{Mg} \) does not form \( \text{MgO} \) directly.
• Focus on the formation of \( \text{Mg(NO}_3\text{)}_2 \) and \( \text{H}_2 \) as products.

Analyzing reactions helps predict products and understand the conditions under which specific compounds, like \( \text{MgO} \), are formed. This skill is fundamental in solving chemistry problems, as it requires recognizing reaction types and applying concepts like oxidation and reduction.