Le Chatelier's Principle on Equilibrium Shift

Steps:

• The reaction 2NO₂(g) ⇌ 2NO(g) + O₂(g) is reversible; NO₂ is dark brown, while NO and O₂ are colorless.
• The pale brown color indicates partial decomposition with mostly products present.
• Adding O₂ increases reactant concentration on the product side.
• Equilibrium shifts left to consume excess O₂, producing more NO₂.

Why A is correct:

• Shifting left increases NO₂ concentration, intensifying the brown color per Le Chatelier's principle.

Why the others are wrong:

• B: Shift produces more NO₂, not less, so color darkens rather than pales.
• C: Adding O₂ disturbs equilibrium, causing a shift and color change.
• D: Option incomplete, but adding O₂ triggers a predictable shift.

Final answer: A