Problem 2 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The redox reaction among the following is: [Main Jan. 07, 2020 (II)] (a) formation of ozone from atmospheric oxygen in the presence of sunlight (b) reaction of \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}\) with \(\mathrm{AgNO}_{3}\) (c) reaction of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) with \(\mathrm{NaOH}\) (d) combination of dinitrogen with dioxygen at \(2000 \mathrm{~K}\)

Step-by-Step Solution

Answer

Option (d) is the redox reaction.

Step 1: Understanding Redox Reactions

Redox reactions involve the transfer of electrons between two substances. In a redox reaction, one substance is oxidized (loses electrons) and the other is reduced (gains electrons). Let's analyze each option to identify which one is a redox reaction.

Step 2: Analyzing Option (a)

The formation of ozone from atmospheric oxygen can be represented as: \(3\, \text{O}_2 \rightarrow 2\,\text{O}_3\). During this process, oxygen molecules are recombined in the presence of sunlight, but there is no electron transfer involved. Thus, it is not a redox reaction.

Step 3: Analyzing Option (b)

The reaction of \([\text{Co(H}_2\text{O})_6]\text{Cl}_3\) with \(\text{AgNO}_3\) is a metathesis or double displacement reaction where ions are exchanged, but there is no change in oxidation states of any element. Therefore, it's not a redox reaction.

Step 4: Analyzing Option (c)

The reaction between \(\text{H}_2\text{SO}_4\) and \(\text{NaOH}\) is a neutralization reaction: \(\text{H}_2\text{SO}_4 + 2\,\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\,\text{H}_2\text{O}\). There is no change in oxidation states of any involved species, hence it's not a redox reaction.

Step 5: Analyzing Option (d)

The combination of dinitrogen and dioxygen at high temperature \(2000 K\) can be expressed as: \(\text{N}_2 + \text{O}_2 \rightarrow 2\,\text{NO}\). In this reaction, the oxidation state of nitrogen changes from 0 in \(\text{N}_2\) to +2 in \(\text{NO}\), and for oxygen, it changes from 0 in \(\text{O}_2\) to -2 in \(\text{NO}\). This indicates electron transfer, confirming it as a redox reaction.

Key Concepts

Electron Transfer

In the realm of redox reactions, electron transfer is the core concept. When these reactions occur, electrons are moved from one atom or molecule to another. This movement signals a change in chemical states and marks a process where one substance loses electrons—a process known as oxidation—while another gains these electrons, which is called reduction.

• Oxidation: Loss of electrons
• Reduction: Gain of electrons

To recognize a redox reaction, observe the transfer of electrons between reactants. A simple representation of this process is: \[ ext{A} ightarrow ext{A}^{+} + ext{e}^{-} \] for oxidation, and \[ ext{B} + ext{e}^{-} ightarrow ext{B}^{-} \] for reduction. Understanding this exchange is key to analyzing such reactions in chemical processes.

Oxidation States

Oxidation states, or oxidation numbers, help us track electron transfer in a chemical reaction. These numbers represent the number of electrons that an atom gains or loses during a reaction. In some sense, they help detail how electron-rich or electron-poor an atom becomes through the reaction.

• An increase in oxidation state: Indicates oxidation
• A decrease in oxidation state: Indicates reduction

Let's use nitrogen and oxygen as examples from the reaction where dinitrogen (\( ext{N}_2 \)) combines with dioxygen (\( ext{O}_2 \)) to form nitrogen monoxide (\( ext{NO} \)). Initially, both nitrogen and oxygen have an oxidation state of 0, which showcases their elemental forms. In \( ext{NO} \), the oxidation state of nitrogen shifts to +2, indicating it lost electrons, while oxygen changes to -2, indicating it gained electrons. This clearly shows the electron transfer mechanism of redox reactions.

Chemical Reactions Analysis

Analyzing chemical reactions involves identifying what type of reaction is taking place. There are several types, including synthesis, decomposition, single displacement, and, notably, redox. In the given exercise, we identified redox reactions by detecting changes in oxidation states and electron transfer principles.
When analyzing a reaction, consider:

• Structure of reactants and products
• Oxidation states of involved elements
• Type of bonds breaking and forming

In our example, the combination of dinitrogen and dioxygen forming nitrogen monoxide is a synthesis reaction and a redox reaction. The changes in oxidation states verified electron transfer, confirming the redox nature. By examining oxidation state shifts and the structure transformation from reactants to products, chemists can deduce the dynamics of the chemical process in question.