Which of the following combination will produce `H_(2)` gas ?

Options

A. Zn metal and NaOH(aq)
B. Au metal and NaCN(aq) in the presence of air
C. Cu metal and conc. `HNO_(3)`
D. Fe metal and conc. `HNO_(3)`

Explanation

To determine which combination will produce hydrogen gas (H₂), we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Option 1: Zinc metal and NaOH (aqueous)**
- When zinc (Zn) reacts with sodium hydroxide (NaOH), a base, a reaction occurs where zinc displaces hydrogen from the hydroxide.
- The reaction can be represented as:
\[ \text{Zn} + 2 \text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \]
- In this reaction, sodium zincate (Na₂ZnO₂) is formed, and hydrogen gas (H₂) is released.
- **Conclusion**: This combination produces hydrogen gas.

2. **Option 2: Gold metal and sodium cyanide in the presence of air**
- When gold (Au) reacts with sodium cyanide (NaCN) in the presence of air, it forms a coordination complex and does not produce hydrogen gas.
- The reaction can be represented as:
\[ \text{Au} + 2 \text{NaCN} + \text{O}_2 \rightarrow \text{Na}_2\text{Au(CN)}_2 + \text{NaOH} \]
- **Conclusion**: This combination does not produce hydrogen gas.

3. **Option 3: Copper metal and concentrated HNO₃**
- When copper (Cu) reacts with concentrated nitric acid (HNO₃), it produces copper nitrate (Cu(NO₃)₂), nitrogen dioxide (NO₂), and water (H₂O).
- The reaction can be represented as:
\[ \text{Cu} + 4 \text{HNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + 2 \text{NO}_2 + 2 \text{H}_2\text{O} \]
- **Conclusion**: This combination does not produce hydrogen gas.

4. **Option 4: Iron metal and concentrated HNO₃**
- When iron (Fe) reacts with concentrated nitric acid (HNO₃), a protective oxide layer forms on the iron, preventing further reaction.
- The reaction can be represented as:
\[ \text{Fe} + \text{HNO}_3 \rightarrow \text{Fe}_2\text{O}_3 + \text{NO}_2 + \text{H}_2\text{O} \]
- **Conclusion**: This combination does not produce hydrogen gas.

### Final Answer:
The only combination that produces hydrogen gas is **Option 1: Zinc metal and NaOH (aqueous)**.

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