Question

For a gas phase reaction, A + 3B = 2C, if some inerts are added into the system under otherwise uniform conditions, conversion of A will

• A. Decrease
• B. Increase
• C. Remain unaffected
• D. May decrease or may increase, unpredictable

Hint:

When inert gases are added to a gas-phase reaction, they reduce the partial pressure of the reactants, which in turn decreases the reaction rate and the conversion.

Answer 1

The Correct Option is A

Step 1: Understanding the effect of inert gases.
Inert gases do not participate in the reaction but affect the partial pressures of the reactants and products. For a gas-phase reaction, adding inert gases reduces the partial pressure of the reactants, which in turn decreases the reaction rate. The conversion of A is directly related to the partial pressure of A.
Step 2: Analyzing the impact.
By introducing inert gases, the partial pressure of A decreases, leading to a decrease in the rate of reaction. As a result, the conversion of A will decrease.
Step 3: Conclusion.
The correct answer is (1) Decrease, as the addition of inert gases reduces the partial pressure of the reactants, lowering the conversion of A.

Answer 2

This question is about how adding an inert gas affects the equilibrium position of a gas-phase reaction where the total number of moles changes. For \( A + 3B \rightarrow 2C \), the moles of gas go from 4 (1 mol A + 3 mol B) to 2 (2 mol C), so the reaction proceeds with a decrease in the total number of moles. Let's check each option against this fact using Le Chatelier's principle applied to mole-number effects.

1. Decrease: At constant total pressure, adding an inert gas increases the total number of moles present, which dilutes every reacting species. Because the forward reaction reduces the total mole count (4 moles of reactant gas become 2 moles of product gas), diluting the system pushes the equilibrium toward the side with the greater number of moles, i.e. back toward the reactants, so the equilibrium conversion of A decreases. This matches the mole-number behaviour of this reaction.
2. Increase: This would be true only if the reaction increased the number of gas moles on going from reactants to products; here it is the opposite, moles decrease on reaction, so diluting the system cannot favour the forward direction.
3. Remain unaffected: This would only hold if the reaction did not change the number of moles of gas (\(\Delta n = 0\)); here \(\Delta n = 2 - 4 = -2 \neq 0 \), so adding inert must have an effect.
4. May decrease or may increase, unpredictable: The direction of the shift is fully determined once we know the sign of \(\Delta n\) for the reaction and that pressure is held constant, so the effect is entirely predictable, not ambiguous.

Since this reaction reduces the number of gas moles, adding an inert at constant pressure shifts equilibrium back toward the reactants, decreasing the conversion of A.

Therefore, the correct answer is Decrease.