Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following does not represent redox reactions? (a) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+2 \mathrm{OH}^{-} \longrightarrow \mathrm{CrO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{SO}_{4}^{2-}+2 \mathrm{I}+2 \mathrm{H}^{+} \longrightarrow \mathrm{I}_{2}+\mathrm{SO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O}\) (c) \(2 \mathrm{Ca}(\mathrm{OH})_{2}+2 \mathrm{Cl}_{2} \longrightarrow \mathrm{Ca}(\mathrm{ClO})_{2}\) \(+\mathrm{CaCl}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{PCl}_{5} \longrightarrow \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The reaction (a) is not a redox reaction.

Step 1: Understanding Redox Reactions

Identify what characterizes a redox reaction. Redox reactions involve the transfer of electrons between two species. One species will undergo oxidation (lose electrons) and the other will undergo reduction (gain electrons).

Step 2: Reviewing Reaction (a)

Examine the given reaction: \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + 2 \mathrm{OH}^{-} \rightarrow \mathrm{CrO}_{4}^{2-} + \mathrm{H}_{2} \mathrm{O} \). Assign oxidation states to all elements. Chromium changes from +6 in \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \) to +6 in \( \mathrm{CrO}_{4}^{2-} \), no change occurs in oxidation state.

Step 3: Reviewing Reaction (b)

Examine the reaction: \( \mathrm{SO}_{4}^{2-} + 2 \mathrm{I} + 2 \mathrm{H}^{+} \rightarrow \mathrm{I}_{2} + \mathrm{SO}_{4}^{2-} + \mathrm{H}_{2} \mathrm{O} \). Iodine changes from 0 to +1 in \( \mathrm{I}_{2} \), thus iodine is oxidized. This reaction includes redox changes.

Step 4: Reviewing Reaction (c)

Review the reaction: \( 2 \mathrm{Ca} ( \mathrm{OH})_2 + 2 \mathrm{Cl}_{2} \rightarrow \mathrm{Ca} ( \mathrm{ClO})_2 + \mathrm{CaCl}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \). Chlorine changes from 0 in \( \mathrm{Cl}_{2} \) to -1 in \( \mathrm{CaCl}_{2} \) and +1 in \( \mathrm{Ca (ClO)}_2 \), indicating an electron transfer.

Step 5: Reviewing Reaction (d)

Examine the reaction: \( \mathrm{PCl}_5 \rightarrow \mathrm{PCl}_3 + \mathrm{Cl}_2 \). Phosphorus goes from +5 in \( \mathrm{PCl}_5 \) to +3 in \( \mathrm{PCl}_3 \), and chlorine from -1 in \( \mathrm{PCl}_5 \) to 0 in \( \mathrm{Cl}_2 \), showing a transfer of electrons thus it is a redox reaction.

Step 6: Determining the Non-Redox Reaction

Based on the oxidation states analysis, reaction (a) \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + 2 \mathrm{OH}^{-} \rightarrow \mathrm{CrO}_{4}^{2-} + \mathrm{H}_{2} \mathrm{O} \) does not involve any change in oxidation states for the species involved and hence is not a redox reaction.

Key Concepts

Oxidation States

Oxidation states, sometimes referred to as oxidation numbers, help us to determine how electrons are distributed in a chemical compound. They are a vital tool in identifying redox reactions. To assign oxidation states, you follow certain rules. For example, the oxidation state of elements in their standard state is zero, and for simple ions, it is the charge of the ions.
When examining a compound, you start by assigning oxidation states to atoms according to a hierarchy of rules. For instance, in a compound like chromium hexavalent ion, Cr in \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \) has an oxidation state of +6.
Knowing this, you can analyze whether oxidation occurs by checking if the oxidation state changes over the course of the reaction. A change indicates a redox process is happening, since electrons are either gained or lost.

Electron Transfer

Electron transfer is central to understanding redox reactions. It related to how electrons move from one species to another during a chemical reaction. Oxidation involves the loss of electrons, while reduction involves the gain of electrons.
These processes are interconnected and happen simultaneously. If one species donates an electron, another species must accept it. This exchange is how ions or atoms reach a more stable arrangement. Let's look at Reaction (d) as an example: \( \mathrm{PCl}_5 \rightarrow \mathrm{PCl}_3 + \mathrm{Cl}_2 \).
Here, phosphorus is reduced from +5 to +3 while chlorine is oxidized from -1 to 0, indicating a clear electron transfer between these elements. Keeping track of electron transfer helps to identify which elements serve as reducing or oxidizing agents.

Chemical Reactions Analysis

Analyzing chemical reactions requires a systematic approach to identify if a process is a redox reaction. By breaking down each reaction, as evident in the step-by-step solution, it's essential to determine oxidation state assignments initially. This allows us to track electron shifts.
Consider Reaction (a): \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} + 2 \mathrm{OH}^{-} \rightarrow \mathrm{CrO}_{4}^{2-} + \mathrm{H}_{2} \mathrm{O} \). Despite the complexity, none of the atoms in this reaction experience a shift in oxidation state, confirming it's not a redox reaction.
Understanding specific patterns and balancing chemical equations while paying close attention to whether there are changes in an atom's oxidation state is integral to distinguishing between redox and non-redox processes.

• Track changes in oxidation states
• Ensure electron conservation
• Classify reactions effectively

These strategies enable a comprehensive analysis to ascertain the nature of chemical reactions.