Nitric acid is reduced to nitrogen dioxide in which of the following reactions?
More than one of the above
Nitric Acid Reduction Explained
Reduction is a chemical process where a substance gains electrons or its oxidation state decreases. In the given reactions, we are looking for instances where nitric acid (\( \text{HNO}_3 \)) is reduced to nitrogen dioxide (\( \text{NO}_2 \)). To identify this, we need to examine the oxidation state of nitrogen (N) in both compounds.
In \( \text{HNO}_3 \), the oxidation state of Hydrogen (H) is +1 and Oxygen (O) is -2. Using the rule that the sum of oxidation states in a neutral molecule is zero:
\( (+1) + \text{Oxidation State of N} + 3 \times (-2) = 0 \)
\( 1 + \text{Oxidation State of N} - 6 = 0 \)
\( \text{Oxidation State of N} = +5 \)
In \( \text{NO}_2 \), the oxidation state of Oxygen (O) is -2. Again, the sum of oxidation states is zero:
\( \text{Oxidation State of N} + 2 \times (-2) = 0 \)
\( \text{Oxidation State of N} - 4 = 0 \)
\( \text{Oxidation State of N} = +4 \)
For nitric acid to be reduced to nitrogen dioxide, the oxidation state of nitrogen must decrease from +5 in \( \text{HNO}_3 \) to +4 in \( \text{NO}_2 \). We will now check each reaction.
Nitric Acid Reduction in Reaction 1
The first reaction is: \( \text{C} + 4\text{HNO}_3 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + 4\text{NO}_2 \)
- Reactant containing nitrogen: \( \text{HNO}_3 \) (Nitrogen oxidation state = +5)
- Product containing nitrogen from nitric acid: \( \text{NO}_2 \) (Nitrogen oxidation state = +4)
- The oxidation state of nitrogen changes from +5 to +4. This is a decrease in oxidation state, indicating reduction.
Therefore, in this reaction, nitric acid is reduced to nitrogen dioxide.
Nitric Acid Reduction in Reaction 2
The second reaction is: \( \frac{1}{8} \text{ S}_8 + 6\text{ HNO}_3 \rightarrow \text{H}_2\text{SO}_4 + 6\text{NO}_2 + 2\text{H}_2\text{O} \)
- Reactant containing nitrogen: \( \text{HNO}_3 \) (Nitrogen oxidation state = +5)
- Product containing nitrogen from nitric acid: \( \text{NO}_2 \) (Nitrogen oxidation state = +4)
- The oxidation state of nitrogen changes from +5 to +4. This is a decrease in oxidation state, indicating reduction.
Therefore, in this reaction, nitric acid is reduced to nitrogen dioxide.
Nitric Acid Reduction in Reaction 3
The third reaction is: \( \text{Cu} + 4\text{HNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + 2\text{NO}_2 + 2\text{H}_2\text{O} \)
- Reactant containing nitrogen: \( \text{HNO}_3 \) (Nitrogen oxidation state = +5)
- Product containing nitrogen from nitric acid: \( \text{NO}_2 \) (Nitrogen oxidation state = +4)
- The oxidation state of nitrogen changes from +5 to +4. This is a decrease in oxidation state, indicating reduction.
Therefore, in this reaction, nitric acid is reduced to nitrogen dioxide.
Conclusion
In all three given reactions (Reaction 1, Reaction 2, and Reaction 3), nitric acid (\( \text{HNO}_3 \)) acts as an oxidizing agent and is reduced to nitrogen dioxide (\( \text{NO}_2 \)), as shown by the decrease in the oxidation state of nitrogen from +5 to +4.
Since more than one of the listed reactions shows nitric acid being reduced to nitrogen dioxide, the correct answer encompasses multiple options.
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