The equilibrium `2 Cu^(I)hArr Cu+Cu^(II)`
In aqueous medium at `25^@C` shifts towards the left in the presence of

Options

A. `NO_(3)^(ɵ)`
B. `Cl^(ɵ)`
C. `SCN^(ɵ)`
D. `CN^(ɵ)`

Explanation

To solve the problem regarding the equilibrium reaction \( 2 \text{Cu}^+ \rightleftharpoons \text{Cu} + \text{Cu}^{2+} \) and determine which ions cause the equilibrium to shift to the left, we can follow these steps:

### Step 1: Understand the Equilibrium Reaction
The equilibrium reaction involves the conversion of two copper(I) ions (\( \text{Cu}^+ \)) into one copper atom (\( \text{Cu} \)) and one copper(II) ion (\( \text{Cu}^{2+} \)). The reaction can be represented as:
\[ 2 \text{Cu}^+ \rightleftharpoons \text{Cu} + \text{Cu}^{2+} \]

### Step 2: Apply Le Chatelier's Principle
Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will adjust itself to counteract the disturbance and restore a new equilibrium. In this case, we want to shift the equilibrium to the left, which means we want to increase the concentration of the reactants (\( \text{Cu}^+ \)).

### Step 3: Identify the Ions
The ions provided in the question are \( \text{NO}_3^- \), \( \text{Cl}^- \), \( \text{SCN}^- \), and \( \text{CN}^- \). We need to determine which of these ions can react with \( \text{Cu}^{2+} \) to reduce it to \( \text{Cu}^+ \), thereby increasing the concentration of \( \text{Cu}^+ \) and shifting the equilibrium to the left.

### Step 4: Analyze Each Ion
- **Nitrate Ion (\( \text{NO}_3^- \))**: This ion does not have a strong tendency to reduce \( \text{Cu}^{2+} \).
- **Chloride Ion (\( \text{Cl}^- \))**: Chloride can form a complex with \( \text{Cu}^{2+} \) but does not effectively reduce it to \( \text{Cu}^+ \).
- **Thiocyanate Ion (\( \text{SCN}^- \))**: This ion can form a complex with \( \text{Cu}^{2+} \) and can also facilitate the reduction of \( \text{Cu}^{2+} \) to \( \text{Cu}^+ \).
- **Cyanide Ion (\( \text{CN}^- \))**: This ion has a strong tendency to form complexes with \( \text{Cu}^{2+} \) and can also reduce it to \( \text{Cu}^+ \).

### Step 5: Conclusion
The ions \( \text{SCN}^- \) and \( \text{CN}^- \) can effectively reduce \( \text{Cu}^{2+} \) to \( \text{Cu}^+ \), thus increasing the concentration of \( \text{Cu}^+ \) and shifting the equilibrium to the left. Therefore, the presence of \( \text{Cl}^- \), \( \text{SCN}^- \), or \( \text{CN}^- \) will shift the equilibrium towards the left.

### Final Answer
The equilibrium \( 2 \text{Cu}^+ \rightleftharpoons \text{Cu} + \text{Cu}^{2+} \) shifts towards the left in the presence of \( \text{SCN}^- \) or \( \text{CN}^- \).

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