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

(c) \(\mathrm{Mg} + \mathrm{dil}.\ \mathrm{HNO}_3 \rightarrow \) does not form \(\mathrm{MgO}\).

Step 1: Analyze Each Reaction Option

We need to determine in which reaction option, the formation of \(\mathrm{MgO}\) is not possible. Consider each reaction and analyze if \(\mathrm{MgO}\) would be a product. For Reaction (a): \(\mathrm{Mg} + \mathrm{B}_2\mathrm{O}_3 \rightarrow \), magnesium reacts with boron trioxide to likely form \(\mathrm{MgO}\) and elemental boron. This reaction likely produces \(\mathrm{MgO}\).

Step 2: Consider Reaction (b)

Look at \(\mathrm{Mg} + \mathrm{NO}_2 \rightarrow \). In this reaction, magnesium could potentially reduce nitrogen dioxide to form \(\mathrm{MgO}\) alongside nitrogen gas \(\mathrm{N}_2\). This reaction also likely produces \(\mathrm{MgO}\).

Step 3: Examine Reaction (c)

For \(\mathrm{Mg} + \mathrm{dil}.\ \mathrm{HNO}_3 \rightarrow \), magnesium reacting with dilute nitric acid initially forms \(\mathrm{Mg(NO_3)_2}\), and not \(\mathrm{MgO}\). This is due to nitric acid being an aqueous solution and not forming \(\mathrm{MgO}\) directly.

Step 4: Evaluate Reaction (d)

In the reaction \(\mathrm{Mg} + \mathrm{CO}_2 \rightarrow \), the typical outcome is \(\mathrm{MgO}\) and \(\mathrm{C}\) being produced due to magnesium reducing carbon dioxide. This means \(\mathrm{MgO}\) would be formed in reaction (d) as well.

Step 5: Conclusion on Which Reaction Doesn't Form Magnesium Oxide

Based on the analyses above, Reaction (c) \(\mathrm{Mg} + \mathrm{dil}.\ \mathrm{HNO}_3 \rightarrow \) does not result in \(\mathrm{MgO}\) being formed, as it instead produces magnesium nitrate \(\mathrm{Mg(NO_3)_2}\).

Key Concepts

Chemical Reactions

Chemical reactions are processes in which substances, known as reactants, undergo a transformation to form different substances called products. These changes involve the breaking and forming of chemical bonds. Reactions can occur in various forms such as combustion, synthesis, decomposition, and redox reactions.

For example, when magnesium (Mg) reacts with different compounds, it can engage in different types of chemical reactions. If magnesium reacts with boron trioxide (B_2O_3), it typically undergoes a reduction-oxidation (redox) reaction. During this process, magnesium reduces the boron trioxide by donating electrons to form magnesium oxide (MgO) and boron.

• Reactants are chemically transformed.
• Bond breaking and formation occur.
• Energy is often released or absorbed.

Recognizing the type of reaction is crucial to predicting the products.

Reaction Mechanisms

Understanding a reaction mechanism involves exploring the step-by-step sequence of elementary steps by which an overall chemical change occurs. Each step describes a single molecular event, such as a collision between molecules.

In the case of magnesium reacting with nitrogen dioxide (NO_2), several steps could occur before the final products are formed. Initially, magnesium may transfer electrons to nitrogen dioxide, reducing it, and ultimately forming magnesium oxide (MgO) and nitrogen gas (N_2).

• Elementary steps describe individual molecular events.
• Reactions proceed through a series of transitional states.
• Products form through these elementary steps.

Analyzing each step of the reaction provides insight into why certain products, like MgO, are formed as opposed to other potential compounds.

Oxides

Oxides are chemical compounds that consist of at least one oxygen atom bonded to another element. They are commonly formed through reactions with oxygen-containing compounds. The nature of the oxide formed depends on the element it binds with, which can influence properties like acidity and reactivity.

Magnesium oxide (MgO) is a typical product when magnesium reacts with oxygen or other oxygen-containing substances. Its formation is seen in the reactions with B_2O_3, NO_2, and CO_2, where Mg reduces these compounds to release oxygen and thereby form MgO.

• Composed of oxygen and another element.
• Magnesium oxide is a white solid with robust ionic bonds.
• Oxides vary widely in physical and chemical properties.

The versatility and simplicity of forming oxides make them a crucial class of compounds in inorganic chemistry.