The flowchart outlines the sequence of steps in the Ostwald process for the manufacture of nitric acid - HSC - SSCE Chemistry - Question 30 - 2011 - Paper 1

Step 1: Reaction of Ammonia and Oxygen

To maximize the yield of nitrogen monoxide (NO) in this exothermic reaction, the process should operate at low temperatures. According to Le Chatelier's principle, lowering the temperature favors the formation of products in exothermic processes. Additionally, a high concentration of reactants (ammonia and oxygen) will drive the equilibrium towards product formation, thus maximizing the yield. A suitable catalyst can also be employed to increase the reaction rate.

Step 2: Formation of Nitrogen Dioxide

In the second step, the formation of nitrogen dioxide (NO₂) is also an exothermic reaction. Similar to the first step, operating at lower temperatures will favor product formation. However, since this step is reversible, it’s essential to maintain a high concentration of NO to shift the equilibrium to the right. This can be achieved by ensuring efficient mixing and optimizing reactant feed rates.

Step 3: Production of Nitric Acid

The final step involves the reaction of nitrogen dioxide with water to produce nitric acid (HNO₃). This reaction is not significantly impacted by temperature since it is a straightforward combination. However, maintaining a high concentration of NO₂ will enhance the production of HNO₃. Using excess water can also help drive the reaction towards the product side, thereby increasing the yield.

Overall Considerations

For all steps, the use of catalysts is vital to enhance reaction rates without influencing the position of equilibrium. Additionally, monitoring and controlling pressure conditions can further optimize the yield, particularly in the first two steps.