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

In reaction (c), MgO is not formed.

Step 1: Analyze Reaction (a)

Consider the reaction \( \mathrm{Mg} + \mathrm{B}_2\mathrm{O}_3 \longrightarrow \). In this reaction, magnesium (Mg) reacts with boron trioxide (\( \mathrm{B}_2\mathrm{O}_3 \)). Magnesium is known to reduce metal oxides to its metal state, forming MgO and elemental boron:\[ \mathrm{3Mg} + \mathrm{B}_2\mathrm{O}_{3} \rightarrow 2\mathrm{B} + 3\mathrm{MgO} \] Thus, MgO is formed.

Step 2: Analyze Reaction (b)

Consider the reaction \( \mathrm{Mg} + \mathrm{NO}_2 \longrightarrow \).When magnesium reacts with nitrogen dioxide (\( \mathrm{NO}_2 \)), it typically reduces \( \mathrm{NO}_2 \) to nitrogen and forms magnesium oxide:\[ \mathrm{Mg} + \mathrm{NO}_2 \rightarrow \mathrm{MgO} + \mathrm{N}_2 \] Thus, MgO is formed.

Step 3: Analyze Reaction (c)

Consider the reaction \( \mathrm{Mg} + \mathrm{dil} . \mathrm{HNO}_3 \longrightarrow \).Magnesium when reacts with dilute nitric acid (which is \( \mathrm{HNO}_3 \)), typically produces magnesium nitrate (\( \mathrm{Mg(NO}_3)_2 \)), hydrogen gas, and other nitrogen oxides but does not form \( \mathrm{MgO} \). The reaction generally results in:\[ \mathrm{Mg} + 2 \mathrm{HNO}_3 \rightarrow \mathrm{Mg(NO}_3)_2 + \mathrm{H}_2 \] Thus, MgO is not formed.

Step 4: Analyze Reaction (d)

Consider the reaction \( \mathrm{Mg} + \mathrm{CO}_2 \longrightarrow \).When magnesium reacts with carbon dioxide (\( \mathrm{CO}_2 \)), it forms magnesium oxide and carbon:\[ \mathrm{2Mg} + \mathrm{CO}_2 \rightarrow 2\mathrm{MgO} + \mathrm{C} \] Thus, MgO is formed.

Key Concepts

MgO formation

Magnesium oxide or MgO is a compound formed through a chemical reaction where magnesium (Mg) combines with oxygen (O). This compound is an important example of a simple chemical reaction.
Here's what typically happens:

• Magnesium, a shiny grayish solid, is exposed to oxygen in the air.
• When heated, magnesium reacts vigorously with oxygen.

The key reaction is:
\[ 2 ext{Mg} + ext{O}_2 ightarrow 2 ext{MgO} \]
This conversion produces magnesium oxide, a white powdery substance. MgO is widely used in industries, for example, as a refractory material in furnace linings due to its high melting point.

Redox reactions

Redox reactions, short for reduction-oxidation reactions, are processes where one substance loses electrons (oxidation) while another gains electrons (reduction). Understanding these reactions helps explain how substances such as magnesium oxide and other compounds are formed.

**Key Features of Redox Reactions**

• Occurs with movement of electrons between substances.
• Oxidation is the loss of electrons or increase in oxidation state.
• Reduction is the gain of electrons or decrease in oxidation state.

This is essential in understanding reactions like the formation of MgO. For instance, in the reaction \( ext{Mg} + ext{O}_2 \), magnesium is oxidized as it loses electrons, and oxygen is reduced as it gains electrons. Redox reactions are found in all aspects of chemistry, from metabolic processes in our body to industrial applications.

Magnesium reactions

Magnesium is a versatile element that reacts with different substances to form various compounds. These reactions highlight the reactive nature of magnesium and its ability to engage in redox processes.

**Examples of Magnesium Reactions**

• With oxygen, forming MgO.
• With water, producing hydrogen gas and magnesium hydroxide.
• With acids like hydrochloric acid, forming magnesium chloride and hydrogen gas.

In these reactions, magnesium often acts as a reducing agent. It loses electrons readily to form stable products like MgO or liberate gases. Understanding magnesium reactions can aid in predicting outcomes in chemical experiments and industrial processes.