Question

For the reversible reaction \(\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightleftharpoons 2\text{NH}_3(\text{g}) + \text{Heat}\), the forward reaction is favored by:

• A. High temperature and high pressure
• B. Low temperature and high pressure
• C. High temperature and low pressure
• D. Low temperature and low pressure

Hint:

For any exothermic gaseous reaction where \(\Delta n_g < 0\) (moles of product < moles of reactant), maximum yield is theoretically obtained at low temperature and high pressure.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to determine the optimal conditions of temperature and pressure that will maximize the yield of the forward reaction (Haber's process) based on Le Chatelier's Principle.

Step 2: Key Formula or Approach:
Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change.

Step 3: Detailed Explanation:
The given reaction is:
\(\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightleftharpoons 2\text{NH}_3(\text{g}) + \text{Heat}\)

1. Effect of Temperature:
The forward reaction produces heat, meaning it is an

exothermic process (\(\Delta H < 0\)).
According to Le Chatelier's Principle, decreasing the temperature will cause the system to favor the exothermic direction to generate more heat. Therefore, a

low temperature favors the forward reaction.

2. Effect of Pressure:
Let's count the stoichiometric moles of gaseous substances on both sides.
Reactants: \(1 \text{ mole of } \text{N}_2 + 3 \text{ moles of } \text{H}_2 = 4 \text{ moles of gas}\).
Products: \(2 \text{ moles of } \text{NH}_3 = 2 \text{ moles of gas}\).
The forward reaction results in a decrease in the total number of gaseous moles.
According to Le Chatelier's Principle, an increase in pressure shifts the equilibrium towards the side with fewer moles of gas to reduce the pressure. Therefore, a

high pressure favors the forward reaction.
Combining these two observations, the forward reaction is favored by low temperature and high pressure.

Step 4: Final Answer:
The correct choice is (B).