The standard reduction potentials at `25^(@)C` for the following half reactions are given against each:
`Zn^(2+) (a.q) + 2e^(-) hArr Zn (s) : " " - 0.762`
`Cr^(3+) (a.q) 3e^(-) hArr Cr (s) , " " - 0.740`
`2H^(+) + 2e^(-) hArr H_(2) (g), " " 0.00`
`Fe^(3+) + e^(-) hArr Fe^(2+) , " " 0.77`
Which is the strongest reducing agent?

Options

A. `Zn`
B. `Cr`
C. `H_(2)(g)`
D. `Fe^(2+) (aq)`

Answer

To determine the strongest reducing agent among the given half-reactions, we need to analyze the standard reduction potentials provided. The strongest reducing agent will be the species that has the highest tendency to lose electrons (i.e., undergo oxidation).

### Step-by-step Solution:

1. **Identify the Half-Reactions and Their Standard Reduction Potentials**:
- \( \text{Zn}^{2+} + 2e^- \rightleftharpoons \text{Zn} \) ; \( E^\circ = -0.762 \, \text{V} \)
- \( \text{Cr}^{3+} + 3e^- \rightleftharpoons \text{Cr} \) ; \( E^\circ = -0.740 \, \text{V} \)
- \( 2\text{H}^+ + 2e^- \rightleftharpoons \text{H}_2 \) ; \( E^\circ = 0.00 \, \text{V} \)
- \( \text{Fe}^{3+} + e^- \rightleftharpoons \text{Fe}^{2+} \) ; \( E^\circ = 0.77 \, \text{V} \)

2. **Convert Reduction Potentials to Oxidation Potentials**:
The oxidation potential is simply the negative of the reduction potential. Thus, we convert the given potentials:
- For \( \text{Zn} \): \( E_{\text{oxidation}} = +0.762 \, \text{V} \)
- For \( \text{Cr} \): \( E_{\text{oxidation}} = +0.740 \, \text{V} \)
- For \( \text{H}_2 \): \( E_{\text{oxidation}} = 0.00 \, \text{V} \)
- For \( \text{Fe}^{2+} \): \( E_{\text{oxidation}} = -0.77 \, \text{V} \)

3. **Compare the Oxidation Potentials**:
Now we compare the oxidation potentials:
- \( \text{Zn} \): \( +0.762 \, \text{V} \)
- \( \text{Cr} \): \( +0.740 \, \text{V} \)
- \( \text{H}_2 \): \( 0.00 \, \text{V} \)
- \( \text{Fe}^{2+} \): \( -0.77 \, \text{V} \)

4. **Identify the Strongest Reducing Agent**:
The strongest reducing agent corresponds to the species with the highest oxidation potential. From the comparison:
- The highest oxidation potential is for \( \text{Zn} \) at \( +0.762 \, \text{V} \).

5. **Conclusion**:
Therefore, the strongest reducing agent among the given options is **Zinc (Zn)**.