Ostwald process for nitric acid (HNO3):\nIn the platinum-catalyzed synthesis where conversion to NO is 95–97%, the subsequent gas-phase formation of nitrogen dioxide (2 NO + O2 → 2 NO2) is favored, from an equilibrium standpoint, by what change in conditions?

Introduction / Context:
The Ostwald process produces nitric acid (HNO3) via catalytic oxidation of ammonia over platinum gauze to nitric oxide (NO), followed by homogeneous gas-phase oxidation of NO to nitrogen dioxide (NO2) and subsequent absorption to form HNO3. After the first high-temperature catalytic step, plant designers tune temperature and pressure in the downstream gas path to maximize NO → NO2 conversion before absorption. This question focuses specifically on the oxidation equilibrium of NO to NO2.

Given Data / Assumptions:

• Main reaction considered: 2 NO(g) + O2(g) → 2 NO2(g).
• The first-stage ammonia conversion over Pt is high (about 95–97%).
• We are asking which change favors NO2 formation in the second step.
• Ideal gas behavior and standard thermochemical tendencies are assumed.

Concept / Approach:
The oxidation of NO to NO2 is exothermic and goes from 3 mol of gas on the left to 2 mol on the right. By Le Châtelier’s principle: lowering temperature favors an exothermic forward reaction, and increasing pressure favors the side with fewer moles. Thus, at fixed pressure, decreasing temperature increases the equilibrium fraction of NO2. Practically, plants cool the gas after the hot Pt gauze to promote NO oxidation before absorption towers. Although reaction rates slow as temperature decreases, in typical ranges the equilibrium benefit dominates and residence time/oxidant excess are adjusted accordingly.

Step-by-Step Solution:

Verification / Alternative check:
Plant practice includes gas cooling between stages and before absorption; cooler gas streams show higher NO2/NO ratios at equilibrium, improving nitric acid yields upon absorption.

Why Other Options Are Wrong:

• Decreasing pressure: would favor the side with more moles (reactants), opposing NO2 formation.
• Increasing temperature: shifts an exothermic equilibrium backwards toward NO.
• None of these: incorrect because decreasing temperature does favor NO2.
• Increasing moisture content: moisture affects absorption, not the gas-phase oxidation equilibrium directly.

Common Pitfalls:
Confusing equilibrium preference with reaction rate. While higher temperature can speed kinetics, the question asks what favors formation (equilibrium). Process control balances both by cooling and ensuring adequate residence time and oxygen availability.

Final Answer:
Decreasing the temperature.