Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following reaction \(\mathrm{Cr}(\mathrm{OH})_{3}+\mathrm{OH}^{-}+\mathrm{IO}_{3} \longrightarrow \mathrm{CrO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O}+\mathrm{I}^{-}\) (a) \(\mathrm{IO}_{3}\) is oxidizing agent (b) \(\mathrm{Cr}(\mathrm{OH})_{3}\) is oxidized (c) \(6 \mathrm{e}^{-}\)are being taken per 1 atom (d) all are true

Step-by-Step Solution

Answer

Options (a) and (b) are true; (c) is false.

Step 1: Understanding the Oxidation and Reduction Process

In a redox reaction, an oxidizing agent gains electrons and is reduced, while a reducing agent loses electrons and is oxidized. In this reaction, identify the oxidation states of elements in the reactants and products to see which atoms are oxidized and which are reduced.

Step 2: Determining Oxidation States

Calculate the oxidation states for relevant elements: Chromium in \( \mathrm{Cr(OH)}_3 \) starts at +3, in \( \mathrm{CrO}_4^{2-} \) it is +6; Iodine in \( \mathrm{IO}_3^{-} \) starts at +5 and is reduced to -1 in \( \mathrm{I}^- \).

Step 3: Establishing Roles of Agent

Identify that \( \mathrm{IO}_3^{-} \) is reduced to \( \mathrm{I}^- \) by gaining electrons, thus \( \mathrm{IO}_3^{-} \) acts as the oxidizing agent. Chromium goes from +3 in \( \mathrm{Cr(OH)}_3 \) to +6 in \( \mathrm{CrO}_4^{2-} \), showing it is oxidized.

Step 4: Examining Electron Transfer

Chromium's oxidation from +3 to +6 indicates a loss of 3 electrons per chromium atom, not 6 electrons per atom, contrarily to what was suggested in option c.

Step 5: Final Assessment of Statements

Option (a) is correct because \( \mathrm{IO}_3^{-} \) is indeed the oxidizing agent. Option (b) is also correct since \( \mathrm{Cr(OH)}_3 \) is oxidized. Option (c) is incorrect because 3 electrons per chromium atom are involved in oxidation, not 6. Therefore, not all statements are true.

Key Concepts

Oxidation States

Oxidation states are important numbers that help us understand how electrons are distributed in molecules involved in a reaction. They serve as a tool to track electron transfer during chemical reactions. In general, the oxidation state is the theoretical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. For redox reactions, identifying the oxidation states of elements helps us determine which atoms are oxidized and which are reduced.

For example, let's analyze the reaction given:

• Chromium in \(\mathrm{Cr(OH)}_3\) starts with an oxidation state of +3.
• It changes to +6 in \(\mathrm{CrO}_4^{2-}\).
• Iodine in \(\mathrm{IO}_3^{-}\) begins with an oxidation state of +5.
• It is reduced to -1 in \(\mathrm{I}^-\).

By examining the changes in oxidation states, we can see that chromium undergoes oxidation (increase in oxidation state) and iodine undergoes reduction (decrease in oxidation state). This exchange highlights the fundamental nature of redox reactions.

Oxidizing and Reducing Agents

In a redox reaction, identifying the roles of oxidizing and reducing agents is crucial. An oxidizing agent, or oxidant, is a substance that gains electrons, causing another substance to lose electrons and become oxidized. Conversely, a reducing agent, or reductant, loses electrons, enabling another substance to gain electrons and become reduced.

• In the provided reaction, \(\mathrm{IO}_3^{-}\) gains electrons and is reduced to \(\mathrm{I}^-\), making it the oxidizing agent.
• \(\mathrm{Cr(OH)}_3\) loses electrons to form \(\mathrm{CrO}_4^{2-}\), identifying it as the reduced agent.

Understanding these roles helps to see the balance and transfer of electrons during chemical reactions. Knowing which species is the oxidizing or reducing agent can aid in predicting reaction products and balancing redox equations.

Electron Transfer

Electron transfer is the key defining characteristic of redox reactions. It directly associates with how substances change their oxidation states. During a redox reaction, one substance loses electrons (oxidation), while another gains electrons (reduction).

Let's consider electron transfer in the provided reaction:

• Chromium is oxidized from +3 to +6, shedding 3 electrons per atom.
• Contrary to the misleading statement in option (c), it undergoes 3-electron transfer per chromium atom, not 6.
• Iodine gains electrons as it reduces from +5 to -1.

By following the electron flow, you can obtain more insights into the mechanism of reactions and understand how elements change their form and composition during a chemical process. This includes verifying the accuracy of statements concerning electron transfer based on calculated data.