Zinc is used but not copper for the recovery for metallic silver from the complex `[Ag(CN)2]^-,` although electrode potentials of both zinc and copper are less than that of Ag. Explain why ?

Answer

To answer the question of why zinc is used instead of copper for the recovery of metallic silver from the complex \([Ag(CN)_2]^-\), we can break down the explanation into several key points:

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The complex \([Ag(CN)_2]^-\) contains silver ions that can be reduced to metallic silver. To achieve this reduction, a reducing agent is required.

2. **Electrode Potentials**:
Both zinc and copper have standard electrode potentials that are less than that of silver. This means that both metals can theoretically reduce silver ions to metallic silver. However, the effectiveness of a reducing agent also depends on its ability to donate electrons.

3. **Reducing Power**:
Zinc is a stronger reducing agent compared to copper. This means that zinc has a greater tendency to lose electrons and thus can more effectively reduce \([Ag(CN)_2]^-\) to metallic silver. The standard reduction potential of zinc is lower than that of copper, indicating that zinc can displace silver from its complex more readily.

4. **Formation of By-products**:
When zinc is used to reduce silver from the complex, it forms zinc cyanide as a by-product. Zinc cyanide is less toxic and can be easily disposed of or decomposed, making the process more environmentally friendly.

5. **Copper's Ineffectiveness**:
Although copper can also reduce silver, it is less effective than zinc. The reduction potential of copper is higher, meaning it is less likely to donate electrons compared to zinc. Therefore, using copper would not efficiently recover metallic silver from the complex.

6. **Conclusion**:
In summary, zinc is preferred over copper for the recovery of metallic silver from \([Ag(CN)_2]^-\) because it is a stronger reducing agent, leading to a more efficient reduction process and the formation of less hazardous by-products.