Mark the strongest reducing agent.

Options

A. `H_(2)F_(2)`
B. `HCI`
C. `HBr`
D. HI

Answer

To determine the strongest reducing agent among the given options (H2F2, HCl, HBr, HI), we need to analyze the ability of these compounds to release H⁺ ions, as a stronger reducing agent is one that can easily donate electrons or protons.

### Step-by-Step Solution:

1. **Understanding Reducing Agents**: A reducing agent is a substance that can donate electrons or protons (H⁺ ions) in a chemical reaction. The easier it is for a substance to release H⁺ ions, the stronger its reducing power.

2. **Analyzing the Given Compounds**:
- **H2F2 (Hydrogen difluoride)**: This compound contains fluorine, which is highly electronegative and forms strong bonds with hydrogen. The bond dissociation energy is high, making it less likely to release H⁺ easily.
- **HCl (Hydrochloric acid)**: This is a strong acid and can release H⁺ ions, but the bond between H and Cl is stronger than that in HI.
- **HBr (Hydrobromic acid)**: Similar to HCl, but the bond between H and Br is weaker than that in HCl, allowing for easier release of H⁺.
- **HI (Hydroiodic acid)**: This compound has the weakest bond between H and I due to the larger size of iodine, which leads to lower bond dissociation energy. Therefore, HI can release H⁺ ions more easily than the others.

3. **Comparing Bond Dissociation Energies**: As we move down the group of halogens (from F to I), the bond dissociation energy decreases:
- HF > HCl > HBr > HI
- This means that HI has the weakest bond and can release H⁺ ions more readily than HCl, HBr, or H2F2.

4. **Conclusion**: Based on the analysis, HI is the strongest reducing agent among the given options because it can easily release H⁺ ions due to the weak H-I bond.

### Final Answer:
The strongest reducing agent is **HI** (option D).