The oxidation potentials of Zn, Cu, Ag, `H_(2)` and Ni are 0.76, -0.34, 0.80, 0 and 0.25 volt respectively. Which of the following reactions will provide maximum voltage ?

Options

A. `Zn+Cu^(2+) rarr Cu + Zn^(2+)`
B. `Zn+2Ag^(+) rarr 2Ag+zn^(2+)`
C. `H_(2) + Cu^(2+) rarr 2H^(+) + Cu`
D. `H_(2)+Ni^(2+) rarr 2H^(+) + Ni`

Answer

To determine which reaction will provide the maximum voltage, we need to analyze the oxidation and reduction potentials of the given half-reactions. The half-reactions are based on the oxidation potentials of the metals involved.

### Step-by-Step Solution:

1. **Identify the Oxidation Potentials**:
- Zn: 0.76 V
- Cu: -0.34 V
- Ag: 0.80 V
- H₂: 0 V
- Ni: 0.25 V

2. **Understand the Concept**:
- The maximum voltage of a cell is achieved when the difference between the oxidation potential of the anode and the reduction potential of the cathode is maximized.
- The oxidation potential of a metal indicates how easily it can lose electrons (be oxidized), while the reduction potential indicates how easily a species can gain electrons (be reduced).

3. **Calculate the Cell Potential for Each Reaction**:
- **Option A: Zn and Cu**
- Oxidation: Zn → Zn²⁺ + 2e⁻ (E° = 0.76 V)
- Reduction: Cu²⁺ + 2e⁻ → Cu (E° = -0.34 V)
- Cell potential (E°cell) = E°(reduction) - E°(oxidation) = (-0.34) - (0.76) = -1.10 V

- **Option B: Zn and Ag**
- Oxidation: Zn → Zn²⁺ + 2e⁻ (E° = 0.76 V)
- Reduction: Ag⁺ + e⁻ → Ag (E° = 0.80 V)
- Cell potential (E°cell) = E°(reduction) - E°(oxidation) = (0.80) - (0.76) = 0.04 V

- **Option C: H₂ and Cu**
- Oxidation: H₂ → 2H⁺ + 2e⁻ (E° = 0 V)
- Reduction: Cu²⁺ + 2e⁻ → Cu (E° = -0.34 V)
- Cell potential (E°cell) = E°(reduction) - E°(oxidation) = (-0.34) - (0) = -0.34 V

- **Option D: H₂ and Ni**
- Oxidation: H₂ → 2H⁺ + 2e⁻ (E° = 0 V)
- Reduction: Ni²⁺ + 2e⁻ → Ni (E° = 0.25 V)
- Cell potential (E°cell) = E°(reduction) - E°(oxidation) = (0.25) - (0) = 0.25 V

4. **Compare the Cell Potentials**:
- Option A: -1.10 V
- Option B: 0.04 V
- Option C: -0.34 V
- Option D: 0.25 V

5. **Conclusion**:
- The maximum voltage is provided by **Option D** (H₂ and Ni) with a cell potential of **0.25 V**.