Equilibrium Requires a Closed System

Steps:

• Identify the reaction: CaO(s) + CO2(g) ⇌ CaCO3(s), forward exothermic, 1 mol gas to 0 mol gas.
• Apply Le Chatelier's principle: increased pressure shifts equilibrium right (more CaCO3); increased temperature shifts left (less CaCO3).
• Note equilibrium definition: forward and reverse rates equal in a closed system, reaction continues dynamically.
• Evaluate options: only C ensures no loss of gases for equilibrium.

Why C is correct:

• Equilibrium in reversible gas reactions requires a sealed (closed) system to prevent reactant/product escape, per the definition of dynamic equilibrium.

Why the others are wrong:

• A: Higher pressure decreases gas moles, favoring forward reaction and producing more CaCO3 (Le Chatelier's principle).
• B: Exothermic forward reaction means higher temperature favors endothermic reverse, producing less CaCO3 (Le Chatelier's principle).
• D: At equilibrium, forward and reverse rates are equal but ongoing; net change is zero, not stopped.

Final answer: C