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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
Question 30
The flowchart outlines the sequence of steps in the Ostwald process for the manufacture of nitric acid. Step 1 4NH₃(g) + 5O₂(g) ⇌ 4NO(g) + 6H₂O(g) ΔH = -950 kJ ... show full transcript
Worked Solution & Example Answer: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
Step 1: 4NH₃(g) + 5O₂(g) ⇌ 4NO(g) + 6H₂O(g)
Answer
For the first step of the Ostwald process, high temperatures (around 900°C) and high pressures are essential to drive the reaction forward, as it is exothermic (ΔH = -950 kJ). According to Le Chatelier's principle, increasing the temperature shifts the equilibrium towards the product side, thus enhancing the formation of nitric oxide (NO). However, high pressure is also crucial, as it favors the side of the reaction that produces fewer gas molecules. In this reaction, 6 moles of gas are produced from 5 moles, supporting the need for high pressure.
Step 2
Step 2: 2NO(g) + O₂(g) ⇌ 2NO₂(g)
Answer
In the second step, moderate temperatures should be maintained, as this reaction is again exothermic (ΔH = -114 kJ). A balance is required: while lower temperatures favor the formation of NO₂ due to equilibrium considerations, they also slow the reaction rate. Therefore, a temperature range that sufficiently promotes the reaction, perhaps around 50-70°C, along with adequate pressure, should be applied to maximize both yield and reaction speed.
Step 3
Step 3: 3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)
Answer
For the final step of the process, using liquid water facilitates the reaction and drives it to completion. This step is also exothermic (ΔH = -117 kJ), so maintaining moderate temperatures, similar to step 2, is ideal for maximizing yield. Moreover, the removal of nitric acid (HNO₃) as it forms helps shift the equilibrium to the right, according to Le Chatelier's principle, thereby maximizing production rate and concentration of the desired product.