Hydrogen gas reduces which metal ion in its aqueous solution?

Options

A. `Mg^(2+)`
B. `Li^(+)`
C. `Pd^(2+)`
D. `Zn^(2+)`

Answer

To determine which metal ion can be reduced by hydrogen gas in its aqueous solution, we can follow these steps:

### Step 1: Understand the concept of reduction potential
Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. The higher the reduction potential, the more likely the species is to be reduced.

### Step 2: Reference point for hydrogen
In the electrochemical series, hydrogen is used as a reference point with a standard reduction potential of 0 volts. This means that any species with a higher reduction potential than hydrogen can be reduced by hydrogen gas.

### Step 3: Analyze the given metal ions
We have four metal ions to consider:
1. Mg²⁺
2. Li⁺
3. Pd²⁺
4. Zn²⁺

### Step 4: Look up the reduction potentials
- **Pd²⁺ + 2e⁻ → Pd**: Reduction potential = +0.915 V
- **Zn²⁺ + 2e⁻ → Zn**: Reduction potential = -0.76 V
- **Mg²⁺ + 2e⁻ → Mg**: Reduction potential = -2.37 V
- **Li⁺ + e⁻ → Li**: Reduction potential = -3.05 V

### Step 5: Compare reduction potentials
- Pd²⁺ has a reduction potential of +0.915 V, which is greater than 0 V (hydrogen).
- Zn²⁺, Mg²⁺, and Li⁺ all have negative reduction potentials, meaning they are less likely to be reduced than hydrogen.

### Step 6: Conclusion
Since hydrogen can only reduce species with a higher reduction potential than itself, it can reduce Pd²⁺ to Pd, but it cannot reduce Zn²⁺, Mg²⁺, or Li⁺. Therefore, the answer is:

**Option 3: Pd²⁺**