Question

Given below are the two reactions of \(\mathrm{H}_{2} \mathrm{O}_{2}\). Mark the correct statement which follows. (i) \(2 \mathrm{KMnO}_{4}+3 \mathrm{H}_{2} \mathrm{SO}_{4}+5 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{K}_{2} \mathrm{SO}_{4}+2 \mathrm{MnSO}_{4}\) \(+8 \mathrm{H}_{2} \mathrm{O}+5 \mathrm{O}_{2}\) (ii) \(2 \mathrm{Cr}(\mathrm{OH})_{3}+4 \mathrm{NaOH}+3 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2 \mathrm{Na}_{2} \mathrm{CrO}_{4}+\) (a) (i) Shows oxidising nature of \(\mathrm{H}_{2} \mathrm{O}_{2}\) and (ii) shows reducing nature of \(\mathrm{H}_{2} \mathrm{O}_{2}\) (b) In (i) \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as a reducing agent and in (ii) it acts as an oxidising agent. (c) In both (i) and (ii), \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as an oxidising agent. (d) In both (i) and (ii), \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as a reducing agent.

Short answer

The correct statement is (b): In reaction (i) H2O2 acts as a reducing agent and in reaction (ii) it acts as an oxidising agent.

Step-by-step solution

- Analyze Reaction (i)

Examine the oxidation states of manganese in the reactants and products of the first reaction. In the reactants, manganese is in +7 oxidation state (in KMnO4), and in the products, it is in +2 oxidation state (in MnSO4). As the oxidation state of manganese decreases, it is being reduced. Since H2O2 causes this reduction, H2O2 acts as a reducing agent in this reaction.

- Analyze Reaction (ii)

Examine the oxidation states of chromium in the reactants and products of the second reaction. In the reactants, chromium is in +3 oxidation state (in Cr(OH)3), and in the products, it is in +6 oxidation state (in Na2CrO4). As the oxidation state of chromium increases, it is being oxidized. Since H2O2 causes this oxidation, H2O2 acts as an oxidising agent in this reaction.

- Determine the Correct Statement

Based on the analysis from steps 1 and 2, in reaction (i) H2O2 acts as a reducing agent and in reaction (ii) H2O2 acts as an oxidising agent. Thus, the correct statement that follows is option (b): In (i) H2O2 acts as a reducing agent and in (ii) it acts as an oxidising agent.

Key concepts

Oxidizing and Reducing Agents

Understanding the roles of oxidizing and reducing agents is vital to mastering redox reactions, which are fundamental to chemical reactions such as combustion, corrosion, and metabolism. An oxidizing agent, also known as an oxidant, gains electrons and is reduced in the process, while a reducing agent, or reductant, loses electrons and is oxidized.

In the context of H₂O₂, or hydrogen peroxide, it can act as both an oxidizing and reducing agent, depending on the reaction it is involved in. This dual nature is due to its intermediate oxidation state, which allows it to either gain or lose electrons when reacting with other substances. When H₂O₂ causes another substance to lose electrons, it is acting as an oxidizing agent; conversely, when it gives up electrons to another substance, it behaves as a reducing agent.

For example:

• In the presence of a strong oxidizer like KMnO₄ (potassium permanganate), H₂O₂ will act as a reducing agent.
• However, when reacting with substances like Cr(OH)₃ (chromium hydroxide), H₂O₂ can function as an oxidizing agent.

It's crucial for students to be able to identify what role H₂O₂ is playing in a reaction to correctly describe its behavior.

Chemical Oxidation States

Chemical oxidation states indicate the degree of oxidation or reduction of an atom in a compound. These are typically represented by integers, which can be positive, negative, or zero. The oxidation state is a useful concept for understanding the electronic structure of chemical species during a reaction.

Hydrogen peroxide (H₂O₂) acts as a compelling case study in changes of oxidation states. By examining the change in oxidation state of other elements in a reaction with H₂O₂, we can identify whether H₂O₂ has been oxidized or reduced—and subsequently, whether it acted as an oxidizing or a reducing agent.

For instance:

• In the reaction with KMnO₄, the oxidation state of manganese shifts from +7 to +2, showing a reduction. This indicates that H₂O₂ served as a reducing agent.
• Conversely, in the reaction with Cr(OH)₃, the oxidation state of chromium changes from +3 to +6, signaling oxidation. Thus, H₂O₂ here acts as an oxidizing agent.

Understanding how to calculate and interpret oxidation states is essential for identifying redox processes in chemical reactions.

Redox Reactions

Redox reactions are a class of chemical reactions characterized by the transfer of electrons between two species, which results in changes in their oxidation states. These reactions encompass two key processes: oxidation, where a substance loses electrons, and reduction, where a substance gains electrons. It is important to note that oxidation and reduction always occur simultaneously in what is called a redox pair.

In our context, hydrogen peroxide (H₂O₂) participates in redox reactions as it can both accept and donate electrons. Deciphering whether H₂O₂ is being oxidized or reduced in a given reaction allows for a better understanding of its function as either an oxidizing or a reducing agent.

Key points for students:

• Oxidation is loss of electrons (OIL), and reduction is gain of electrons (RIG).
• Agents that cause other species to be oxidized, like H₂O₂ in reaction (ii) with Cr(OH)₃, are called oxidizing agents.
• Agents that cause other species to be reduced, as observed with H₂O₂ in reaction (i) with KMnO₄, are known as reducing agents.

Through practice and careful examination of oxidation state changes, students can hone their ability to predict and explain redox behavior in chemical reactions, including those involving versatile compounds like H₂O₂.