Question

22.66 Why isn't nitric acid produced by oxidizing \(\mathrm{N}_{2}\) as follows? (1) $$ \mathrm{N}_{2}(g)+2 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g) $$ (2) \(3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(a q)+\mathrm{NO}(g)\) (3) \(\frac{2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)}{3 \mathrm{~N}_{2}(g)+6 \mathrm{O}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 4 \mathrm{HNO}_{3}(a q)+2 \mathrm{NO}(g)}\) (Hint: Evaluate the thermodynamics of cach step.)

Short answer

The overall process is thermodynamically inefficient due to the high energy demand needed to break the \(\mathrm{N}_2\) triple bond in the first step.

Step-by-step solution

- Identify the Reactions

First, write down all the reactions provided in the exercise:(1) \[ \mathrm{N}_{2}(g)+2 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g) \](2) \[ 3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(a q)+\mathrm{NO}(g) \](3) \[ \frac{2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)}{3 \mathrm{~N}_{2}(g)+6 \mathrm{O}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 4 \mathrm{HNO}_{3}(a q)+2 \mathrm{NO}(g)} \]

- Consider Thermodynamics of Reaction 1

Evaluate the thermodynamics of reaction (1): \[\mathrm{N}_{2}(g)+2 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g) \]Reaction (1) involves breaking the strong triple bond in \( \mathrm{N}_{2} \) molecules, which requires a substantial amount of energy. Typically, this reaction is endothermic and requires high temperatures to proceed.

- Consider Thermodynamics of Reaction 2

Evaluate the thermodynamics of reaction (2): \[3 \mathrm{NO}_{2}(g)+\mathrm{H}_2 \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(a q)+\mathrm{NO}(g) \]Reaction (2) generally proceeds under standard conditions and is exothermic, meaning it releases energy.

- Consider Thermodynamics and Stoichiometry of Reaction 3

Combine reactions (1) and (2) to get reaction (3): \[\frac{2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)}{3 \mathrm{N}_{2}(g)+6 \mathrm{O}_{2}(g)+2 \mathrm{H}_2 \mathrm{O}(l) \longrightarrow 4 \mathrm{HNO}_{3}(a q)+2 \mathrm{NO}(g)} \]Although \(\mathrm{HNO}_{3}\) is produced, the reactions are not energetically favorable overall. The endothermic nature of step (1) overshadows the exothermic nature of step (2). As a result, the overall process requires more energy than it releases, making it inefficient.

- Conclusion

Nitric acid is not efficiently produced by oxidizing \(\mathrm{N}_2\) through the given steps due to the overall high energy demand. The significant energy required to break the \(\mathrm{N}_2\) triple bond in step (1) makes the process thermodynamically unfavorable.

Key concepts

Thermodynamics

Thermodynamics is the study of energy changes that accompany chemical reactions. When evaluating whether a chemical process can produce nitric acid efficiently, we focus on the thermodynamic feasibility. The first step involves breaking the strong triple bond in the nitrogen molecule (N2). This bond is one of the strongest in chemistry and requires a large amount of energy to break. As a result, this step is endothermic, meaning it absorbs more energy than it releases. This makes the reaction thermodynamically unfavorable despite the exothermic nature of the subsequent reactions.

Chemical Reactions

Understanding the series of chemical reactions involved in nitric acid production is key. The initial reaction is:
\[\mathrm{N}_{2}(g) + 2 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)
\] This forms nitrogen dioxide (NO2). Then, NO2 reacts with water:
\[3 \mathrm{NO}_{2}(g) + \mathrm{H}_2 \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(a q) + \mathrm{NO}(g)
\] These reactions convert NO2 into nitric acid and nitric oxide (NO), respectively. While the second reaction is exothermic, meaning it releases energy, the first reaction's high energy demand overshadows this benefit.

Energy Requirements

The energy requirements of a process dictate its feasibility. Specifically, the production of nitric acid begins with the reaction:
\[\mathrm{N}_{2}(g) + 2 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)
\] This step requires significant energy due to the strong triple bond in diatomic nitrogen (\(N_2\)). For the overall process to be efficient, the energy released in subsequent steps must compensate for this high initial energy demand. However, the second reaction, though exothermic, does not release enough energy to make up for the energy absorbed in the first reaction.

Thus, the high initial energy requirement makes this method inefficient for large-scale nitric acid production.

Oxidation Process

The oxidation process involves increasing the oxidation state of an element. In the case of nitrogen gas (\(N_2\)), it undergoes oxidation when it reacts with oxygen gas (\(O_2\)) to form nitrogen dioxide (\(NO_2\)). This specific oxidation step can be represented as:
\[\mathrm{N}_{2}(g) + 2 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)
\] Oxidation processes generally involve energy changes, and in this situation, the creation of \(NO_2\) from \(N_2\) and \(O_2\) is energetically demanding. This high energy demand combined with the inefficiency in subsequent energy recovery makes the process challenging for efficient nitric acid production.

Stoichiometry

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It helps determine the exact amounts needed and produced in reactions. For nitric acid production, consider the stoichiometric equation incorporating all steps:
\[3 \mathrm{N}_{2}(g) + 6 \mathrm{O}_{2}(g) + 2 \mathrm{H}_2 \mathrm{O}(l) \longrightarrow 4 \mathrm{HNO}_{3}(a q) + 2 \mathrm{NO}(g)
\] This equation shows precise reactant proportions to yield nitric acid and nitric oxide. However, despite the balanced stoichiometry, the overall energy required, especially to break the \(N_2\) triple bond, makes the process inefficient. Therefore, stoichiometry alone cannot justify this nitric acid production method without considering the associated energy costs.