Nitric Acid Preparation & Industrial Chemistry

Carrying out the reaction at approximately 200°C is crucial because nitric acid (HNO₃) is volatile and can easily vaporize. If the temperature is too high, HNO₃ would escape as a gas rather than being collected effectively. The chosen temperature allows for the controlled distillation of HNO₃ while preventing its decomposition and ensuring that the reaction proceeds efficiently. This balance is essential for maximizing yield and purity of the product.

Vacuum distillation operates under reduced pressure, which lowers the boiling point of concentrated HNO₃. This allows the distillation process to occur at a temperature that minimizes the decomposition of NO₂, a key factor contributing to the yellow color. Additionally, at these lower temperatures, the solubility of NO₂ in HNO₃ decreases, facilitating its separation from the acid. Consequently, vacuum distillation is more effective in removing the yellow impurity compared to atmospheric distillation, where higher temperatures may lead to increased NO₂ stability and solubility.

In the Birkland-Eyde process, the high temperature of 3500°C promotes the formation of nitric oxide (NO) from nitrogen (N₂) and oxygen (O₂) due to the endothermic nature of the reaction, in accordance with Le Chatelier's principle. By rapidly cooling the system to around 50°C, the equilibrium position is effectively 'frozen,' inhibiting the reverse reaction that would decompose NO back into N₂ and O₂. This strategy maximizes the yield of NO, making the process more efficient for industrial applications.

In the Birkland-Eyde process, HNO₂ serves as an intermediate that undergoes disproportionation, facilitating the conversion of reactants into useful products. This reaction sequence allows for the regeneration of NO, which can be oxidized back to NO₂, thus enhancing the overall efficiency of the process. The recycling of NO is crucial as it minimizes waste and improves atom economy, leading to a more sustainable industrial operation. By reusing NO, the process reduces the need for additional raw materials and limits environmental impact, making it economically and ecologically beneficial.

For efficient absorption of nitrogen oxides by NaOH, both NO and NO₂ are necessary because they react differently. NO, being a neutral oxide, does not react with NaOH on its own. However, when combined with NO₂, which does react, the presence of both gases ensures that the required 1:1 molar ratio is maintained, facilitating complete absorption. This combination allows the formation of sodium nitrite (NaNO₂) and ensures that the reaction proceeds effectively, maximizing the efficiency of the absorption process in tail-gas treatment.

Method (i) faces limitations due to the finite availability of Chile saltpetre, a key raw material, and generates NaHSO₄, which has limited utility. This restricts its scalability and sustainability. In contrast, the Birkland-Eyde process (method ii) is hindered by its high energy demands, primarily from the electric arc needed for nitrogen fixation, making it less economically viable compared to the more efficient Ostwald process. This critical analysis highlights the challenges of resource availability and energy consumption that impact the practicality of both methods in industrial applications.

In the Birkland-Eyde process, nitrogen undergoes a series of oxidation state changes. Starting from nitrogen gas (N₂) with an oxidation state of 0, it first forms nitric oxide (NO) with an oxidation state of +2. Then, it is further oxidized to nitrogen dioxide (NO₂) at +4, and finally, it reaches +5 in nitric acid (HNO₃). While the most significant single oxidation occurs during the first step (0 to +2), the overall change to +5 is achieved cumulatively through all three steps, highlighting the progressive nature of nitrogen's oxidation.

The claim is partially flawed because, although atmospheric nitrogen and oxygen are abundant and free resources, the Birkland-Eyde process requires significant energy to generate the high temperatures needed for the electric arc, leading to high operational costs. This energy demand raises concerns about sustainability unless renewable sources are utilized. In contrast, while the NaNO₃ method relies on finite mineral resources, it operates with considerably lower energy input, making it potentially more cost-effective despite its reliance on mining. Thus, the overall sustainability and cost-effectiveness of the Birkland-Eyde process are questionable.