In which of the following pairs , the first can reduce the second ?

Options

A. `Zn^(2+) |Zn , Ni^(2+)|Ni`
B. `Sn^(2+) |Sn , Mg^(2+) |Mg`
C. `Cu^(2+)|Cu , Ag^(+)|Ag`
D. `Fe^(2+)|Fe , Al^(3+) |Al`

Answer

To determine which of the given pairs can have the first element reduce the second, we need to compare their standard reduction potentials. The element with the higher reduction potential can reduce the one with the lower reduction potential.

### Step-by-Step Solution:

1. **Understand Reduction Potentials**:
- The standard reduction potential indicates the tendency of a species to gain electrons (be reduced). A higher reduction potential means a greater ability to reduce another species.

2. **Identify the Reduction Potentials**:
- We need to look up or recall the standard reduction potentials for the ions in each pair.

3. **Evaluate Each Pair**:
- **Pair A: Zn²⁺ and Ni²⁺**
- Zn²⁺ has a reduction potential of -0.76 V.
- Ni²⁺ has a reduction potential of -0.23 V.
- Since -0.76 V < -0.23 V, Zn²⁺ cannot reduce Ni²⁺.

- **Pair B: Zn²⁺ and Mg²⁺**
- Zn²⁺ has a reduction potential of -0.76 V.
- Mg²⁺ has a reduction potential of -2.36 V.
- Since -0.76 V > -2.36 V, Zn²⁺ can reduce Mg²⁺.

- **Pair C: Cu²⁺ and Ag⁺**
- Cu²⁺ has a reduction potential of +0.34 V.
- Ag⁺ has a reduction potential of +0.80 V.
- Since +0.34 V < +0.80 V, Cu²⁺ cannot reduce Ag⁺.

- **Pair D: Fe²⁺ and Al³⁺**
- Fe²⁺ has a reduction potential of -0.44 V.
- Al³⁺ has a reduction potential of -1.66 V.
- Since -0.44 V > -1.66 V, Fe²⁺ can reduce Al³⁺.

4. **Conclusion**:
- The pairs where the first can reduce the second are:
- Pair B: Zn²⁺ can reduce Mg²⁺.
- Pair D: Fe²⁺ can reduce Al³⁺.

### Final Answer:
The correct pairs where the first can reduce the second are B and D.

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