Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is/are correct regarding the electrolytic preparation of \(\mathrm{H}_{2} \mathrm{O}_{2} ?\) (a) sulphuric acid undergoes oxidation (b) hydrogen is liberated at anode (c) \(50 \% \mathrm{H}_{2} \mathrm{SO}_{4}\) is used (d) lead is used as cathode [2005]

Step-by-Step Solution

Answer

(a) and (c) are correct.

Step 1: Understand the Electrolytic Process

In the electrolytic preparation of hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)), we use sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) as the electrolyte. The process involves the oxidation of sulfuric acid to form peroxodisulfuric acid (\(\mathrm{H}_2\mathrm{S}_2\mathrm{O}_8\)), which then hydrolyzes to produce \(\mathrm{H}_2\mathrm{O}_2\) and regenerate \(\mathrm{H}_2\mathrm{SO}_4\).

Step 2: Analyze Anode and Cathode Reactions

At the anode, sulfuric acid undergoes oxidation: \(\mathrm{2HSO}_4^- \rightarrow \mathrm{H}_2\mathrm{S}_2\mathrm{O}_8 + 2e^-\). Thus, hydrogen is not liberated at the anode; instead, oxidation occurs. Lead dioxide (\(\mathrm{PbO}_2\)) is often used as the anode material.

Step 3: Examine the Concentration of Sulfuric Acid Required

The process typically requires using around \(50\%\) sulfuric acid to ensure adequate conductivity and facilitate the reaction to produce \(\mathrm{H}_2\mathrm{O}_2\). Therefore, the statement about \(50\%\) \(\mathrm{H}_2\mathrm{SO}_4\) being used is correct.

Step 4: Identify the Role of Cathode

At the cathode, water is reduced: \(\mathrm{2H}_2\mathrm{O} + 2e^- \rightarrow \mathrm{H}_2 + 2\mathrm{OH}^-\). Lead is not typically used as a cathode. Generally, metals such as platinum or graphite are used instead.

Step 5: Conclusion on the Correct Statements

From the analysis, the correct statements regarding the electrolytic preparation of \(\mathrm{H}_2\mathrm{O}_2\) include: (a) sulfuric acid undergoes oxidation, and (c) \(50\%\) \(\mathrm{H}_2\mathrm{SO}_4\) is used. Statements (b) and (d) are incorrect.

Key Concepts

Oxidation of Sulfuric Acid

In the electrolytic preparation of hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)), oxidation of sulfuric acid is a central part of the process. During this reaction, sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) is oxidized at the anode, leading to the formation of peroxodisulfuric acid (\(\mathrm{H}_2\mathrm{S}_2\mathrm{O}_8\)). This compound is crucial as it later decomposes to yield hydrogen peroxide. The process can be summarized by the anode reaction: \(2\mathrm{HSO}_4^- \rightarrow \mathrm{H}_2\mathrm{S}_2\mathrm{O}_8 + 2e^-\). This reflects the role of oxidation in the transformation of sulfuric acid, highlighting its significance in producing the desired hydrogen peroxide product.

Anode and Cathode Reactions

A fundamental aspect of electrolysis involves understanding what occurs at the anode and cathode. In the preparation of hydrogen peroxide, at the anode, sulfuric acid is oxidized into peroxodisulfuric acid, with the release of electrons as shown in the previous section. At the cathode, the process involves water reduction: \(2\mathrm{H}_2\mathrm{O} + 2e^- \rightarrow \mathrm{H}_2 + 2\mathrm{OH}^-\). This reaction illustrates that hydrogen is actually liberated at the cathode rather than the anode. It’s important to note that these reactions are crucial for the electrochemical process that leads to hydrogen peroxide production, ensuring the correct flow of electrons and ionic species within the electrolyte solution.

Concentration of Sulfuric Acid

The concentration of sulfuric acid is a critical parameter in the electrolytic preparation of hydrogen peroxide. Typically, a concentration of 50% \(\mathrm{H}_2\mathrm{SO}_4\) is employed. This concentration is optimal as it provides sufficient ionic conductivity necessary for the electrolysis process to proceed efficiently. A lower concentration might not support the necessary ion flow and can result in poor electrochemical activity. Conversely, too high a concentration could pose risks of secondary reactions or unwanted side reactions. Hence, maintaining the correct concentration is essential for maximizing hydrogen peroxide yield and process efficiency.

Electrolysis Process

Electrolysis, in this context, involves passing a current through an electrolyte solution—in this case, sulfuric acid—to induce chemical reactions at the electrodes. The process is central to the production of hydrogen peroxide. It begins with the oxidation of sulfuric acid to peroxodisulfuric acid at the anode and concludes with this compound decomposing to release hydrogen peroxide and regenerate sulfuric acid. Understanding the sequence of reactions occurring during electrolysis helps clarify how electricity is used to drive non-spontaneous chemical reactions, transforming reactants into valuable products efficiently.

Electrode Materials

Selecting appropriate electrode materials is crucial for efficient electrolysis. In the context of hydrogen peroxide production, \(\mathrm{PbO}_2\) (lead dioxide) serves as the anode. This material is chosen due to its stability and resistance to oxidation, along with its capability to facilitate the necessary reactions without corroding or impeding current flow. At the cathode, materials like platinum or graphite are typically utilized. These materials are chosen because they offer good conductivity and are resistant to the chemical environment created during the electrolysis process. The careful selection of electrode materials ensures the longevity and efficiency of the electrolytic preparation of hydrogen peroxide.