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

Reactions (a) and (d) are not redox reactions.

Step 1: Understanding Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between two substances. In these reactions, one substance undergoes oxidation (loses electrons) and another undergoes reduction (gains electrons). To determine if a reaction is redox, we need to check if there are changes in oxidation states of elements before and after the reaction.

Step 2: Analyzing Reaction (a)

In reaction (a), \[\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+2 \mathrm{OH}^{-} \rightarrow\mathrm{CrO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O}\]we look at the chromium atom's oxidation states. In \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\), the oxidation state of chromium is +6. In \(\mathrm{CrO}_{4}^{2-}\), chromium also has an oxidation state of +6. Thus, there is no change in oxidation state, indicating no electron transfer and hence it is not a redox reaction.

Step 3: Analyzing Reaction (b)

In reaction (b), \[\mathrm{SO}_{4}^{2-}+2 \mathrm{I}^{-}+2 \mathrm{H}^{+} \longrightarrow \mathrm{I}_{2}+\mathrm{SO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O}\]Iodine changes from an oxidation state of -1 in \(\mathrm{I}^-\) to 0 in \(\mathrm{I}_{2}\), showing it undergoes oxidation. Sulfate ions and protons do not change in their oxidation states, but the reaction involves electron transfer from iodine. This is a redox reaction.

Step 4: Analyzing Reaction (c)

In reaction (c), \[2 \mathrm{Ca}(\mathrm{OH})_{2}+2 \mathrm{Cl}_{2} \longrightarrow\mathrm{Ca}(\mathrm{ClO})_{2}+\mathrm{CaCl}_{2}+2 \mathrm{H}_{2} \mathrm{O}\] Chlorine changes from an oxidation state of 0 in \(\mathrm{Cl}_{2}\) to -1 in \(\mathrm{CaCl}_{2}\) and +1 in \(\mathrm{Ca}(\mathrm{ClO})_{2}\). This shows oxidation and reduction of chlorine, making it a redox reaction.

Step 5: Analyzing Reaction (d)

In reaction (d), \[\mathrm{PCl}_{5} \longrightarrow \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\] Phosphorus remains in the same oxidation state of +5 in both \(\mathrm{PCl}_{5}\) and \(\mathrm{PCl}_{3}\), while chlorine is in the state of -1 in both \(\mathrm{PCl}_{5}\) and 0 in \(\mathrm{Cl}_{2}\). This means there is no net change in oxidation states, so the reaction doesn't involve electron transfer. Thus, this is not a redox reaction.

Key Concepts

Oxidation State

Understanding oxidation states is essential to identify redox reactions. The oxidation state, or oxidation number, of an element in a molecule indicates how many electrons have been lost or gained by an atom compared to its elemental state. It helps keep track of electrons in a chemical reaction.
For example:

• In reaction (a), the oxidation state of chromium in \(\) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) and \(\mathrm{CrO}_{4}^{2-}\) is +6. Since there's no change in the oxidation state, there's no electron transfer.
• In reaction (c), chlorine has an oxidation state of 0 in \(\mathrm{Cl}_{2}\). After the reaction, chlorine has different oxidation states: -1 in \(\mathrm{CaCl}_{2}\) and +1 in \(\mathrm{Ca}(\mathrm{ClO})_{2}\).

By analyzing the change in oxidation numbers, we can identify if a reaction is undergoing a redox process. Changes in oxidation states indicate electron transfer, which is a key characteristic of redox reactions.

Electron Transfer

Electron transfer is at the heart of redox reactions. During a chemical reaction, if electrons move from one atom to another, the reaction is typically observed as a redox reaction.
During the process:

• Oxidation occurs when an atom loses electrons, increasing its oxidation state.
• Reduction occurs when an atom gains electrons, decreasing its oxidation state.

In reaction (b), iodine goes from an oxidation state of -1 in \(\mathrm{I}^-\) to 0 in \(\mathrm{I}_{2}\). This indicates iodide ions have lost electrons, thus being oxidized. The movement of electrons from iodine is direct evidence that reaction (b) is a redox reaction.

Chemical Reactions

Chemical reactions involve changes in the structure and energy content of substances, leading to the formation of new products. In the context of redox reactions, these changes include electron transfers that modify oxidation states.
Consider:

• Reaction (a) is purely a chemical change without alteration of oxidation states. Such reactions are non-redox.
• Reaction (c) has both reduction and oxidation occurring, showcasing the dynamic nature of redox reactions in altering chemical structures.

Understanding the characteristics of chemical reactions helps identify what type of change is occurring—whether it's a redox process or another kind of chemical transformation.

Oxidation and Reduction

Oxidation and reduction are twin processes involved in redox reactions. They always occur together in chemical reactions, as one substance loses electrons (oxidation) and another gains those electrons (reduction).
Let's see how this applies:

• In reaction (b), iodine ions are oxidized as they lose electrons, while another species is reduced, though not directly seen, the balance of change confirms its occurrence.
• For reaction (d), no change in oxidation states implies neither oxidation nor reduction is happening. Thus, it is not characterized as a redox reaction.

Understanding how oxidation and reduction work together helps in identifying redox reactions and analyzing their properties.