3H$_{2(g)}$+ N$_{2(g)}$⇔ 2NH$_{3(g)}$ ; H= -ve In the reaction above, lowering of temperature will

Analysis

The question asks about the effect of temperature change on a chemical equilibrium system. Specifically, we are looking at the Haber process reaction: $3H_{2(g)} + N_{2(g)} \rightleftharpoons 2NH_{3(g)}$ The enthalpy change ($\Delta H$) is negative, which means the reaction is exothermic (it releases heat).

Concept: Le Chatelier’s Principle

Le Chatelier’s Principle states that if a dynamic equilibrium is disturbed by changing the conditions (such as temperature, pressure, or concentration), the position of equilibrium shifts to counteract the change.

For temperature:

• Exothermic reactions ($\Delta H < 0$): Heat is treated as a product. Increasing the temperature shifts the equilibrium to the left (reverse), while lowering the temperature shifts the equilibrium to the right (forward) to produce more heat.
• Endothermic reactions ($\Delta H > 0$): Heat is treated as a reactant. Increasing the temperature shifts the equilibrium to the right (forward), while lowering the temperature shifts it to the left (reverse).

Step-by-Step Solution

1. Identify the nature of the reaction: The equation shows $\Delta H = -ve$, confirming it is an exothermic reaction.
2. Treat heat as a component: We can rewrite the equation as: $3H_{2(g)} + N_{2(g)} \rightleftharpoons 2NH_{3(g)} + \text{Heat}$
3. Apply the change: The question states that the temperature is lowered.
4. Determine the shift: According to Le Chatelier’s Principle, the system will try to replace the heat that was removed. To do this, the equilibrium will shift in the direction that produces heat, which is the forward reaction.
5. Conclusion: By lowering the temperature, the system favours the production of ammonia ($NH_3$), which is the forward reaction.

Evaluation of Options

• A is incorrect because temperature changes always affect the equilibrium position of exothermic/endothermic reactions.
• B is incorrect because while temperature affects rates, the equilibrium position specifically shifts to favour one side.
• C is correct because lowering the temperature of an exothermic reaction shifts the equilibrium to the right (forward).
• D is incorrect because that would happen if the temperature were increased.

Exam Tip

Think of an exothermic reaction as a "heater." If you turn down the temperature (the environment gets cold), the "heater" works harder to produce more heat to compensate. Therefore, it pushes the reaction forward.