Consider the following equilibrium reaction: 2AB 2 (g) + B 2 (g) 2AB 3 (g) . H= -X kJmol^ -1 . The backward…

Analyze the question

The reaction is 2AB₂(g) + B₂(g) ⇌ 2AB₃(g), with ΔH = −X kJ/mol, so the forward reaction is exothermic. We need the condition that favors the backward reaction, which is the reverse direction, from AB₃ back to AB₂ and B₂.

Concept: Le Chatelier's Principle

When a system at equilibrium is disturbed, it shifts to oppose the change.

Step-by-step analysis

Count the moles of gas on each side:

• Left side (reactants): 2 + 1 = 3 moles
• Right side (products): 2 moles

A. Decrease in pressure: The system shifts toward the side with more gas moles to raise the pressure again. That is the left side (3 moles), so the backward reaction is favored. ✔

B. Increase in pressure: The system shifts toward fewer moles, the right side. This favors the forward reaction. ✘

C. Decrease in temperature: The forward reaction is exothermic, so lowering the temperature favors the exothermic direction (forward). ✘

D. Positive catalyst: A catalyst speeds up both forward and backward reactions equally. It does not shift the equilibrium position. ✘

**Answer: A.

Tip: For gas equilibria, compare the moles of gas on each side.

• Pressure up → shifts to the side with fewer moles.
• Pressure down → shifts to the side with more moles.
• Temperature down → favors the exothermic direction.
• Catalysts never change the equilibrium position. They only help it be reached faster.