Metal which become passive with conc. `HNO_(3)`

Options

A. `Cr`
B. Zn
C. Al
D. Both (1) & (3)

Answer

To determine which metals become passive with concentrated HNO3, we will analyze the reactions of the given metals with concentrated nitric acid. The metals in question are chromium, zinc, and aluminum.

### Step-by-Step Solution:

1. **Understanding Passivation**:
- Passivation is a phenomenon where a metal becomes less reactive due to the formation of a protective oxide layer on its surface.

2. **Analyzing Chromium (Cr)**:
- When chromium reacts with concentrated HNO3, it forms chromium nitrate (Cr(NO3)3) and nitrogen oxides (NO) along with water (H2O).
- The reaction can be represented as:
\[ 4 \text{Cr} + 10 \text{HNO}_3 \rightarrow 4 \text{Cr(NO}_3\text{)}_3 + 2 \text{NO} + 4 \text{H}_2\text{O} \]
- The formation of chromium nitrate leads to the creation of a protective oxide layer, resulting in passivation.

3. **Analyzing Aluminum (Al)**:
- Similarly, when aluminum reacts with concentrated HNO3, it forms aluminum nitrate (Al(NO3)3) and nitrogen oxides (NO) along with water.
- The reaction can be represented as:
\[ 2 \text{Al} + 6 \text{HNO}_3 \rightarrow 2 \text{Al(NO}_3\text{)}_3 + 3 \text{NO} + 3 \text{H}_2\text{O} \]
- The aluminum also forms a protective oxide layer, leading to passivation.

4. **Analyzing Zinc (Zn)**:
- Zinc does react with dilute HNO3, but with concentrated HNO3, it does not passivate in the same way as chromium and aluminum. Instead, it continues to react without forming a protective layer.

5. **Conclusion**:
- The metals that become passive with concentrated HNO3 are chromium and aluminum. Therefore, the correct answer is option 4, which states that both chromium (1st option) and aluminum (3rd option) become passive.

### Final Answer:
The metals which become passive with concentrated HNO3 are chromium and aluminum (Option 4: both 1st and 3rd).