The species which convert `Cu^(2+)` to `Cu^(+)`

Options

A. `I^(-)`
B. `HCHO //OH^(-)`
C. `CN^(-)`
D. All of these

Answer

To determine the species that convert \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \), we can analyze the reactions involving different reagents that facilitate this reduction.

### Step-by-Step Solution:

1. **Understanding the Stability of Copper Oxidation States**:
- \( \text{Cu}^{+} \) is known to be unstable in aqueous solutions and is typically stable in solid or complex forms. This is crucial for understanding how to reduce \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \).

2. **Identifying Possible Reducing Agents**:
- Several reagents can reduce \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \). Common reducing agents include:
- Iodide ions (\( \text{I}^- \))
- Formaldehyde in an alkaline medium
- Cyanide ions (\( \text{CN}^- \))

3. **Reaction with Iodide Ions**:
- When \( \text{Cu}^{2+} \) reacts with iodide ions, the following reaction occurs:
\[ \text{Cu}^{2+} + 2 \text{I}^- \rightarrow \text{CuI} + \text{I}_2 \]
- In this reaction, \( \text{Cu}^{2+} \) is reduced to \( \text{Cu}^{+} \).

4. **Reaction with Formaldehyde**:
- In an alkaline medium, formaldehyde can reduce \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \):
\[ \text{Cu}^{2+} + \text{HCHO} \rightarrow \text{Cu}^{+} + \text{oxidized products} \]
- This reaction also confirms the conversion of \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \).

5. **Reaction with Cyanide Ions**:
- When \( \text{Cu}^{2+} \) reacts with cyanide ions, it forms a complex:
\[ \text{Cu}^{2+} + 4 \text{CN}^- \rightarrow \text{Cu(CN)}_4^{3-} \]
- In this complex, the oxidation state of copper is effectively \( +1 \), indicating that \( \text{Cu}^{2+} \) has been reduced to \( \text{Cu}^{+} \).

6. **Conclusion**:
- All the mentioned species (iodide ions, formaldehyde in alkaline medium, and cyanide ions) can convert \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \). Therefore, the correct answer is that all of these species can perform the reduction.

### Final Answer:
The species that convert \( \text{Cu}^{2+} \) to \( \text{Cu}^{+} \) are:
- Iodide ions (\( \text{I}^- \))
- Formaldehyde in alkaline medium
- Cyanide ions (\( \text{CN}^- \))

Thus, the correct option is **D: All of these**.