Understanding Reactions Producing NO Gas

The question asks which of the given chemical reactions produces nitric oxide, NO, gas as one of its products. These reactions involve the interaction of metals (Copper or Zinc) with nitric acid (HNO₃), which is a strong oxidizing agent. The products of these reactions depend significantly on the nature of the metal and the concentration of the nitric acid used.

Analyzing Each Reaction Option

Let's examine each option and determine the products formed, paying close attention to the gaseous nitrogen species produced.

1. Reaction 1: 3Cu + 8HNO₃(dilute) →

   This reaction involves copper (Cu) reacting with dilute nitric acid. Copper is a relatively less reactive metal. When copper reacts with dilute nitric acid, the principal gaseous product is nitric oxide (NO).

   The balanced chemical equation is:

   \[3\text{Cu(s)} + 8\text{HNO}_3\text{(aq, dilute)} \rightarrow 3\text{Cu(NO}_3)_2\text{(aq)} + 4\text{H}_2\text{O(l)} + 2\text{NO(g)}\]

   As seen from the equation, nitric oxide (NO) gas is produced in this reaction.

2. Reaction 2: Cu + 4HNO₃(conc.) →

   This reaction involves copper (Cu) reacting with concentrated nitric acid. Concentrated nitric acid is a stronger oxidizing agent than dilute nitric acid. When copper reacts with concentrated nitric acid, the principal gaseous product is nitrogen dioxide (NO₂), a reddish-brown gas.

   The balanced chemical equation is:

   \[\text{Cu(s)} + 4\text{HNO}_3\text{(aq, conc.)} \rightarrow \text{Cu(NO}_3)_2\text{(aq)} + 2\text{H}_2\text{O(l)} + 2\text{NO}_2\text{(g)}\]

   This reaction produces NO₂ gas, not NO gas.

3. Reaction 3: 4Zn + 10HNO₃(dilute) →

   This reaction involves zinc (Zn) reacting with dilute nitric acid. Zinc is more reactive than copper. When zinc reacts with dilute nitric acid, the products can vary depending on the concentration and temperature, but typically it produces nitrous oxide (N₂O), or sometimes even nitrogen gas (N₂), or ammonium nitrate (NH₄NO₃) in very dilute conditions.

   The balanced chemical equation often representing a reaction with moderately dilute HNO₃ is:

   \[4\text{Zn(s)} + 10\text{HNO}_3\text{(aq, dilute)} \rightarrow 4\text{Zn(NO}_3)_2\text{(aq)} + 5\text{H}_2\text{O(l)} + \text{N}_2\text{O(g)}\]

   Alternatively, with more dilute acid, ammonium nitrate can form:

   \[4\text{Zn(s)} + 10\text{HNO}_3\text{(aq, dilute)} \rightarrow 4\text{Zn(NO}_3)_2\text{(aq)} + \text{NH}_4\text{NO}_3\text{(aq)} + 3\text{H}_2\text{O(l)}\]

   Neither reaction produces NO gas.

4. Reaction 4: Zn + 4HNO₃(conc.) →

   This reaction involves zinc (Zn) reacting with concentrated nitric acid. Similar to copper with concentrated acid, the principal gaseous product is nitrogen dioxide (NO₂).

   The balanced chemical equation is:

   \[\text{Zn(s)} + 4\text{HNO}_3\text{(aq, conc.)} \rightarrow \text{Zn(NO}_3)_2\text{(aq)} + 2\text{H}_2\text{O(l)} + 2\text{NO}_2\text{(g)}\]

   This reaction produces NO₂ gas, not NO gas.

Based on the analysis of each reaction, only the reaction between copper and dilute nitric acid (Option 1) produces nitric oxide (NO) gas.

Conclusion

Comparing the products of the reactions with the question requirement (producing NO gas), we find that the reaction of copper with dilute nitric acid is the correct one.

Revision Table: Reactions with Nitric Acid

Additional Information: Nitric Acid as an Oxidizing Agent

Nitric acid (HNO₃) is a powerful oxidizing agent. It oxidizes most metals except noble metals like gold and platinum. Unlike other mineral acids (like HCl or H₂SO₄) which typically produce hydrogen gas when reacting with active metals, nitric acid usually produces oxides of nitrogen or other nitrogen compounds because the nitrate ion (NO₃^-) is reduced instead of hydrogen ions (H⁺).

The specific reduction product of nitric acid depends on:

• The concentration of the nitric acid (concentrated vs. dilute).
• The reactivity of the metal.
• Temperature.

Generally:

• Concentrated HNO₃ is reduced to NO₂ (Nitrogen Dioxide).
• Dilute HNO₃ can be reduced to NO (Nitric Oxide) with less reactive metals (like Cu, Ag, Hg).
• Very dilute HNO₃ can be reduced to N₂O (Nitrous Oxide), N₂ (Nitrogen), or even NH₄NO₃ (Ammonium Nitrate) with more reactive metals (like Zn, Fe).

The reaction of copper with dilute nitric acid is a classic example demonstrating the formation of NO gas.