Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a redox reaction? (a) \(\mathrm{NaCl}+\mathrm{KNO}_{3} \longrightarrow \mathrm{NaNO}_{3}+\mathrm{KCl}\) (b) \(\mathrm{CaC}_{2} \mathrm{O}_{4}+2 \mathrm{HCl} \longrightarrow \mathrm{CaCl}_{2}+\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) (c) \(\mathrm{Mg}(\mathrm{OH})_{2}+2 \mathrm{NH}_{4} \mathrm{Cl} \longrightarrow \mathrm{MgCl}_{2}+2 \mathrm{NH}_{4} \mathrm{OH}\) (d) \(\mathrm{Zn}+2 \mathrm{AgCN} \longrightarrow 2 \mathrm{Ag}+\mathrm{Zn}(\mathrm{CN})_{2}\)

Step-by-Step Solution

Answer

The redox reaction is (d) \(\mathrm{Zn}+2 \mathrm{AgCN} \rightarrow 2 \mathrm{Ag}+\mathrm{Zn}( ext{CN})_2\).

Step 1: Understand Redox Reactions

A redox (oxidation-reduction) reaction involves the transfer of electrons between two species. One species will lose electrons (oxidation), and another will gain electrons (reduction). To identify a redox reaction, we look for changes in oxidation states of elements.

Step 2: Analyze Reaction (a)

Examine the oxidation states of each element in the reaction \(\mathrm{NaCl}+\mathrm{KNO}_{3} \longrightarrow \mathrm{NaNO}_{3}+\mathrm{KCl}\).- Sodium remains as Na\(^+\).- Chlorine remains as Cl\(^-\).- Potassium remains as K\(^+\).- Nitrogen remains part of the nitrate ion NO\(_3^-\).- There is no change in oxidation states for any element.Therefore, this is not a redox reaction.

Step 3: Analyze Reaction (b)

Examine the oxidation states of each element in the reaction \(\mathrm{CaC}_{2}\mathrm{O}_{4}+2 \mathrm{HCl} \longrightarrow \mathrm{CaCl}_2+\mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4}\).- Calcium remains as Ca\(^{2+}\).- Chlorine remains as Cl\(^-\).- Oxalic acid and its conjugate ion remain unchanged.- There is no change in oxidation states for any element.Thus, this is not a redox reaction.

Step 4: Analyze Reaction (c)

Examine the oxidation states of each element in the reaction \(\mathrm{Mg}( ext{OH})_{2}+2\mathrm{NH}_{4}\mathrm{Cl} \longrightarrow \mathrm{MgCl}_{2}+2 \mathrm{NH}_{4}\mathrm{OH}\).- Magnesium remains as Mg\(^{2+}\).- Chlorine remains as Cl\(^-\).- Nitrogen remains in the ammonium ion NH\(_4^+\).- There is no change in oxidation states for any element.Thus, this is not a redox reaction.

Step 5: Analyze Reaction (d)

Examine the oxidation states of each element in the reaction \(\mathrm{Zn}+2 \mathrm{AgCN} \longrightarrow 2 \mathrm{Ag}+\mathrm{Zn}( ext{CN})_{2}\).- Initial state: Zn is in the elemental state (oxidation state 0), and Ag is part of AgCN with oxidation state +1.- Final state: Zn is part of \(\mathrm{Zn(CN)}_2\) with oxidation state +2, and Ag is elemental (oxidation state 0).- Zn undergoes oxidation (increase in oxidation state from 0 to +2), and Ag undergoes reduction (decrease in oxidation state from +1 to 0).Thus, this is a redox reaction.

Key Concepts

Oxidation States

Oxidation states, also known as oxidation numbers, are a vital concept in understanding redox reactions. They help us keep track of what is happening to electrons during chemical reactions. When we assign oxidation states, we attribute electrons to atoms in molecules or ions, assuming electrons are not shared equally. Normally, the oxidation state of an atom in its elemental form is zero. For instance, zinc metal, \(\text{Zn}_\), has an oxidation state of 0. As elements form compounds, they may gain or lose electrons, altering their oxidation states.

In ionic compounds, atoms typically assume oxidation states equivalent to their ionic charges. For instance, in sodium chloride (NaCl), sodium is Na\(^+\) and chlorine is Cl\(^-\). Oxidation states are useful because they help us identify whether a redox reaction has occurred by observing any changes in these states across reactants and products. If an atom's oxidation state changes, it has either gained or lost electrons, clearly indicating a redox process.

Electron Transfer

Electron transfer is the fundamental process in redox reactions, where electrons are moved from one chemical species to another. This movement occurs when one species loses electrons, undergoing oxidation, and another species gains them, undergoing reduction. For example, in the reaction involving zinc and silver cyanide (\(\text{Zn}+2\text{AgCN} \longrightarrow 2\text{Ag}+\text{Zn(CN)}_2\)), zinc donates electrons, causing an increase in its oxidation state from 0 to +2. This is known as oxidation.

Meanwhile, the silver ion (\( ext{Ag}^+\)) accepts these electrons and reduces its oxidation state from +1 to 0, becoming metallic silver (Ag). This simultaneous process of electron loss and gain is what drives redox reactions, allowing chemical transformations to occur. By tracking these electron movements, we can understand the nature of redox reactions better.

Electron transfer not only helps us identify the oxidizing and reducing agents in a reaction but also gives insights into the energetics and mechanism, making it a profound concept to grasp Redox reactions fully.

Oxidation-Reduction

Oxidation-Reduction, often abbreviated as redox, collectively refers to chemical reactions where oxidation and reduction occur simultaneously. In these reactions, one species (the reducing agent) loses electrons, while another species (the oxidizing agent) gains electrons. Taking a look at the given example, reaction (d) illustrates such a process where zinc serves as the reducing agent while silver ions serve as the oxidizing agent.

During these redox transformations, you'll often notice that changes in oxidation states act as indicators. For zinc, going from an oxidation state of 0 to +2 means it undergoes oxidation. Conversely, silver ions decreasing from +1 to 0 signifies reduction. Recognizing these changing states can help you identify which part of the reaction involves oxidation and which part involves reduction.

These concepts not only outline the role of individual elements in reactions but also emphasize the balance and transfer of electrons. Redox reactions are foundational for understanding various chemical processes, from industrial applications to biological systems, highlighting their significance in the broader scope of chemistry.