Question

The Haber-Bosch process for ammonia synthesis operates at:

• A. Low pressure (1-5 atm) and high temperature (800 \(^\circ\text{C}\))
• B. High pressure (150-300 atm) and moderate temperature (400-500 \(^\circ\text{C}\))
• C. Atmospheric pressure and 200 \(^\circ\text{C}\)
• D. High pressure (500 atm) and low temperature (100 \(^\circ\text{C}\))

Hint:

The Haber-Bosch process is a classic example of compromise conditions in industrial chemistry:
High Pressure (\(150\text{-}300\text{ atm}\)) to favor equilibrium conversion.
Moderate Temperature (\(400\text{-}500^\circ\text{C}\)) to ensure catalyst activity and a fast reaction rate.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the standard industrial operating conditions (temperature and pressure) of the Haber-Bosch process for the synthesis of ammonia.
These conditions are determined by balancing thermodynamic equilibrium and reaction kinetics.

Step 2: Key Formula or Approach:
The chemical equation for ammonia synthesis is:
\[ \text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \quad \Delta H = -92.4 \text{ kJ/mol} \] We apply Le Chatelier's principle to analyze the equilibrium:
1. Effect of Pressure: The forward reaction reduces the number of moles of gas from 4 to 2. Therefore, high pressure shifts the equilibrium to the right, increasing the ammonia yield.
2. Effect of Temperature: The reaction is exothermic (\(\Delta H \lt 0\)). Therefore, low temperature shifts the equilibrium to the right, increasing the theoretical ammonia yield.

Step 3: Detailed Explanation:
Let us analyze why the specific conditions of Option (B) are selected industrially:
- Thermodynamics vs Kinetics: While a low temperature theoretically maximizes the equilibrium yield of ammonia, the rate of the reaction at low temperatures (such as \(100^\circ\text{C}\)) is extremely slow, making it industrially impractical.
- To achieve a reasonable rate of reaction, a higher temperature is required, and an iron-based catalyst is used.
- The catalyst becomes active only above \(400^\circ\text{C}\).
- Therefore, a moderate temperature of \(400^\circ\text{C}\) to \(500^\circ\text{C}\) is selected as a compromise between acceptable reaction rate and reasonable equilibrium conversion.
- Since this higher temperature shifts the equilibrium back to the left, a very high pressure of \(150\) to \(300\text{ atm}\) is applied to force the reaction forward and achieve an economical per-pass yield (about \(15\%\) to \(20\%\)).
- Extremely high pressures like \(500\text{ atm}\) or more are avoided due to the high capital and operating costs of high-pressure compressors and thick-walled reactor vessels.
- This makes high pressure (\(150\text{-}300\text{ atm}\)) and moderate temperature (\(400\text{-}500^\circ\text{C}\)) the standard industrial window.

Step 4: Final Answer
Thus, the correct option is (B).