Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In which of the following reactions \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as a reducing agent? (i) \(\mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}\) (ii) \(\mathrm{H}_{2} \mathrm{O}_{2}-2 \mathrm{e}^{-} \rightarrow \mathrm{O}_{2}+2 \mathrm{H}^{+}\) (iii) \(\mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{e}^{-} \rightarrow 2 \mathrm{OH}^{-}\) (iv) \(\mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{OH}^{-}-2 \mathrm{e}^{-} \rightarrow \mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) (a) (i), (iii) (b) (ii), (iv) (c) (i), (ii) (d) (iii), (iv)

Step-by-Step Solution

Answer

(b) (ii), (iv)

Step 1: Understanding the Role of Reducing Agent

A reducing agent donates electrons in a chemical reaction and is oxidized in the process. To determine in which reactions \( \mathrm{H}_2\mathrm{O}_2 \) acts as a reducing agent, it must lose electrons and get oxidized.

Step 2: Analyzing Reaction (i)

The reaction is \( \mathrm{H}_2\mathrm{O}_2 + 2 \mathrm{H}^+ + 2 \mathrm{e}^- \rightarrow 2 \mathrm{H}_2\mathrm{O} \). In this reaction, \( \mathrm{H}_2\mathrm{O}_2 \) is gaining electrons, thus it is reduced, not oxidized. Therefore, it is not acting as a reducing agent.

Step 3: Analyzing Reaction (ii)

The reaction is \( \mathrm{H}_2\mathrm{O}_2 - 2 \mathrm{e}^- \rightarrow \mathrm{O}_2 + 2 \mathrm{H}^+ \). Here, \( \mathrm{H}_2\mathrm{O}_2 \) is losing electrons and forms \( \mathrm{O}_2 \). \( \mathrm{H}_2\mathrm{O}_2 \) is oxidized, making it act as a reducing agent.

Step 4: Analyzing Reaction (iii)

The reaction is \( \mathrm{H}_2\mathrm{O}_2 + 2 \mathrm{e}^- \rightarrow 2 \mathrm{OH}^- \). In this case, \( \mathrm{H}_2\mathrm{O}_2 \) gains electrons and is reduced, meaning it cannot be a reducing agent.

Step 5: Analyzing Reaction (iv)

The reaction is \( \mathrm{H}_2\mathrm{O}_2 + 2 \mathrm{OH}^- - 2 \mathrm{e}^- \rightarrow \mathrm{O}_2 + 2 \mathrm{H}_2\mathrm{O} \). \( \mathrm{H}_2\mathrm{O}_2 \) loses electrons and forms \( \mathrm{O}_2 \). Thus, it is oxidized and acts as a reducing agent.

Step 6: Identifying Correct Reactions

From the analysis, reactions where \( \mathrm{H}_2\mathrm{O}_2 \) acts as a reducing agent are (ii) and (iv). Therefore, the correct choice is \( (b) (ii), (iv) \).

Key Concepts

Oxidation

Oxidation is a fundamental concept in chemistry that refers to the loss of electrons by a molecule, atom, or ion. This process is crucial in various chemical reactions, including redox reactions.
When a substance undergoes oxidation, it is said to be oxidized. As electrons carry negative charge, losing them means the oxidized species increase in oxidation state.
An easy way to remember oxidation is with the mnemonic "OIL RIG", which stands for "Oxidation Is Loss, Reduction Is Gain" of electrons.
Oxidation does not happen in isolation; it always occurs with a simultaneous reduction process where another substance gains the electrons lost by the oxidized species.

Reducing Agent

A reducing agent, also known as a reductant, plays a key role in redox reactions. Its primary function is to donate electrons to another substance.
In doing so, the reducing agent itself gets oxidized during the process. Essentially, it reduces another substance by giving away electrons and undergoes oxidation as a result.

• A good example from the exercise is the hydrogen peroxide (\( \mathrm{H}_2\mathrm{O}_2 \)) in reactions (ii) and (iv). It becomes a reducing agent because it loses electrons to form oxygen.
• The substance that gains the electrons originally donated by the reducing agent becomes the oxidizing agent.

This intricate dance of electron exchanging allows chemical reactions to occur, driving many biological and industrial processes.

Electron Transfer

Electron transfer is the movement of electrons from one molecule or atom to another. This transfer is at the heart of redox reactions, allowing for chemical changes.
During a redox reaction, one species loses electrons (oxidation) while another gains those electrons (reduction). This exchange alters the properties of the involved species.
In the exercise provided, hydrogen peroxide (\( \mathrm{H}_2\mathrm{O}_2 \)) participates in electron transfer in distinct manners depending on the reaction.

• In reaction (ii), \( \mathrm{H}_2\mathrm{O}_2 \) loses electrons, and transforms into oxygen (\( \mathrm{O}_2 \)), thus acting as a reducing agent.
• This illustrates how electron transfer dictates changes in chemical identity and roles, a concept vital to understanding chemical reactions on a molecular level.

Understanding electron transfer is essential for grasping how substances interact and change in chemistry.