Which of the following shows disproportionation ?

Options

A. `Cl_(2) underset("dil. NaOH")overset("Cold")to`
B. `KCl underset(H_(2)SO_(4)) overset("Conc.")to `
C. `F_(2) overset(H_(2)O) to `
D. `H_(3)PO_(4) overset((Delta))to`

Answer

To determine which of the given reactions shows disproportionation, we first need to understand what a disproportionation reaction is.

### Step-by-Step Solution:

1. **Definition of Disproportionation Reaction**:
- A disproportionation reaction is a specific type of redox reaction where a single substance in an intermediate oxidation state is transformed into two different products: one with a higher oxidation state and the other with a lower oxidation state.

2. **Analyzing the Given Reactions**:
- We need to analyze each reaction provided in the question to identify if any of them fits the definition of disproportionation.

3. **Reaction 1**: Chlorine with dilute NaOH:
- The reaction is:
\[ \text{Cl}_2 + 2 \text{NaOH} \rightarrow \text{NaCl} + \text{NaOCl} + \text{H}_2\text{O} \]
- In this reaction, chlorine (Cl) starts at an oxidation state of 0.
- In NaCl, chlorine has an oxidation state of -1.
- In NaOCl, chlorine has an oxidation state of +1.
- Here, chlorine is both oxidized (to +1) and reduced (to -1), thus this reaction is a disproportionation reaction.

4. **Reaction 2**: KCl with concentrated sulfuric acid:
- The reaction is:
\[ \text{KCl} + \text{H}_2\text{SO}_4 \rightarrow \text{HCl} + \text{KHSO}_4 \]
- In this case, chlorine does not change its oxidation state significantly; hence, it does not show disproportionation.

5. **Reaction 3**: Chlorine with water:
- The reaction is:
\[ \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{O}_2 \]
- Here, chlorine is not in an intermediate oxidation state that leads to two different products with different oxidation states.

6. **Reaction 4**: Heating phosphoric acid:
- The reaction is:
\[ \text{H}_3\text{PO}_4 \rightarrow \text{HPO}_3 + \text{H}_2\text{O} \]
- This is a dehydration reaction and does not involve oxidation states changing in a way that fits the definition of disproportionation.

7. **Conclusion**:
- Among the reactions analyzed, only the first reaction (chlorine with dilute NaOH) shows disproportionation, as chlorine in the intermediate oxidation state (0) forms two products (NaCl and NaOCl) with different oxidation states (-1 and +1).

### Final Answer:
**The reaction that shows disproportionation is the first reaction: Chlorine with dilute NaOH.**