Question

Which of the following is only a redox reaction but not a disproportionation reaction?

• A. \( 4H_3PO_3 \rightarrow 3H_3PO_4 + PH_3 \)
• B. \( 2H_2O_2 \rightarrow 2H_2O + O_2 \)
• C. \( P_4 + 3NaOH + 3H_2O \rightarrow 3NaH_2PO_2 + PH_3 \)
• D. \( P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 2S_2Cl_2 + 4SO_2 \)

Hint:

In a disproportionation reaction, the same element is oxidized and reduced simultaneously. If different elements undergo oxidation and reduction, it is a regular redox reaction.

Answer 1

The Correct Option is D

Step 1: Understanding Disproportionation and Redox Reactions 
A disproportionation reaction is a special type of redox reaction in which a single element undergoes both oxidation and reduction. In contrast, a normal redox reaction involves oxidation and reduction of different elements. 
Step 2: Oxidation States Analysis 
Let's analyze the oxidation states of phosphorus (P) and sulfur (S) in the reaction: \[ P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 2S_2Cl_2 + 4SO_2 \] In elemental phosphorus (\( P_4 \)), P has an oxidation state of \( 0 \). 
In phosphorus trichloride (\( PCl_3 \)), P has an oxidation state of \( +3 \). 
In sulfur oxychloride (\( SOCl_2 \)), S has an oxidation state of \( +4 \). 
In disulfur dichloride (\( S_2Cl_2 \)), S has an oxidation state of \( +2 \). 
In sulfur dioxide (\( SO_2 \)), S has an oxidation state of \( +4 \). 
Step 3: Identify the Nature of the Reaction 
Phosphorus (\( P \)) is oxidized from \( 0 \) to \( +3 \). 
Sulfur (\( S \)) is reduced from \( +4 \) to \( +2 \). 
Since phosphorus only undergoes oxidation and sulfur only undergoes reduction, this is a simple redox reaction, NOT a disproportionation reaction.
Step 4: Verify Other Options 
Option (A) involves phosphorus undergoing both oxidation and reduction, making it a disproportionation reaction. 
Option (B) involves oxygen undergoing disproportionation from \( -1 \) to both \( -2 \) and \( 0 \). 
Option (C) also shows disproportionation of phosphorus. 
 

Answer 2

Step 1: Understand redox and disproportionation reactions
- A redox reaction involves oxidation and reduction — transfer of electrons between species.
- A disproportionation reaction is a special type of redox reaction where a single substance is both oxidized and reduced simultaneously.

Step 2: Analyze the given reaction
\( P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 2S_2Cl_2 + 4SO_2 \)

Step 3: Assign oxidation states
On the left:
- P in \( P_4 \) is in 0 oxidation state
- S in \( SOCl_2 \) is in +4 oxidation state

On the right:
- P in \( PCl_3 \) is in +3 oxidation state → oxidized
- S in \( S_2Cl_2 \) is in –1 oxidation state → reduced
- S in \( SO_2 \) is in +4 oxidation state → unchanged

Step 4: Determine the reaction type
- Phosphorus (P) is only oxidized: 0 → +3
- Sulfur (S) from SOCl₂ undergoes reduction to form S₂Cl₂ (S: +4 → –1)
- But there is no element that is simultaneously oxidized and reduced
⇒ So this is a redox reaction only, not a disproportionation reaction

Final Answer: \( P_4 + 8SOCl_2 \rightarrow 4PCl_3 + 2S_2Cl_2 + 4SO_2 \)