Problem 41 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following reactions does not form gaseous product? (a) \(\mathrm{PbS}+\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow\) (b) \(\mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow\) (c) \(\mathrm{PbO}_{2}+\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow\) (d) acidified \(\mathrm{KMnO}_{4}+\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow\)

Step-by-Step Solution

Answer

Reaction (a) \( \mathrm{PbS} + \mathrm{H}_2 \mathrm{O}_2 \) does not form a gaseous product.

Step 1: Identify Reaction (a) Products

The reaction \( \mathrm{PbS} + \mathrm{H}_2 \mathrm{O}_2 \) is examined. Since \( \mathrm{PbS} \) is a lead(II) sulfide, it reacts with hydrogen peroxide to form \( \mathrm{PbSO}_4 \) and water, but no gaseous product is likely to form.

Step 2: Analyze Reaction (b) Products

For the reaction \( \mathrm{Cl}_2 + \mathrm{H}_2 \mathrm{O}_2 \), chlorine (\(\mathrm{Cl}_2\)) can react with hydrogen peroxide to form \( \mathrm{O}_2 \) (oxygen gas), along with hydrochloric acid (\(\mathrm{HCl}\)). So, a gas is produced here.

Step 3: Evaluate Reaction (c) Products

The reaction \( \mathrm{PbO}_2 + \mathrm{H}_2 \mathrm{O}_2 \) results in the formation of lead(IV) oxide being reduced by hydrogen peroxide. Oxygen gas \( \mathrm{O}_2 \) is formed as a product of this redox reaction.

Step 4: Consider Reaction (d) Products

For \( \mathrm{KMnO}_4 + \mathrm{H}_2 \mathrm{O}_2 \) under acidic conditions, the reaction typically involves the reduction of \( \mathrm{MnO}_4^- \) to \( \mathrm{Mn}^{2+} \) and oxidation of \( \mathrm{H}_2 \mathrm{O}_2 \) to \( \mathrm{O}_2 \), thus producing oxygen gas.

Step 5: Determine Reaction Without Gaseous Product

Based on the analysis, reaction (a) does not produce any gas, whereas reactions (b), (c), and (d) produce oxygen gas \((\mathrm{O}_2)\).

Key Concepts

Redox Reactions

Redox reactions, also known as oxidation-reduction reactions, involve the transfer of electrons between two substances. These reactions are essential in both biological and chemical processes. One substance will undergo oxidation, meaning it loses electrons, and another will undergo reduction, which means it gains electrons. This transfer of electrons results in a change in the oxidation states of the involved species.

• **Oxidation:** Loss of electrons, leading to an increase in oxidation state.
• **Reduction:** Gain of electrons, leading to a decrease in oxidation state.
• **Oxidizing agent:** The substance that gets reduced by accepting electrons.
• **Reducing agent:** The substance that gets oxidized by donating electrons.

To identify a redox reaction, look for changes in the oxidation states of elements between the reactants and the products. For example, when hydrogen peroxide ( \(\mathrm{H}_2\mathrm{O}_2\)) is used, it often acts as both an oxidizing and reducing agent, depending on the other reactants present.

Oxygen Gas Formation

Oxygen gas ( \(\mathrm{O}_2\)) is commonly formed as a product in various chemical reactions, especially those involving redox processes.

• **In reaction (b):** Chlorine ( \(\mathrm{Cl}_2\)) reacts with hydrogen peroxide, resulting in the formation of oxygen gas.
• **In reaction (c):** Lead(IV) oxide ( \(\mathrm{PbO}_2\)) undergoes a reaction with hydrogen peroxide, leading to the release of oxygen gas as part of its reduction process.
• **In reaction (d):** Acidified potassium permanganate ( \(\mathrm{KMnO}_4\)) reacts with hydrogen peroxide, producing oxygen gas, through the reduction of permanganate ions to manganese ions.

The formation of oxygen gas in these reactions is significant because it often acts as an indicator that a redox reaction is occurring. In addition to being a common gaseous product, oxygen gas plays important roles in various industrial and environmental processes.

Reaction Analysis

Analyzing chemical reactions involves examining the reactants, products, and the sequence of steps that lead to the formation of the product. This thorough examination can help predict the outcome of chemical processes and ensure safety and efficiency in applications.

• **Identify reactants and products:** Start by noting the chemical formulas of the reactants and the expected products. For example, in reaction (a), lead(II) sulfide ( \(\mathrm{PbS}\)) and hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)) were considered, producing lead sulfate ( \(\mathrm{PbSO}_4\)) but no gaseous byproducts.
• **Determine phase changes:** Note whether any of the products will be in a different phase, such as forming a gas when reactants were initially solid or liquid. The absence of such change in reaction (a) suggests no gas formation.
• **Assess reaction conditions:** Understanding factors like temperature, pressure, and presence of acids can affect the types of reactions and the products formed.

By analyzing these elements, chemists can not only predict products but also understand the underlying mechanisms of chemical reactions.