COMEDK 2025 Evening Solved Paper

Concept:Le Chatelier's principle says that a change in concentration, pressure, or temperature disturbs equilibrium. Pure solids and liquids do not affect the equilibrium constant because their activities remain constant.Explanation:The reaction is $\mathrm{Fe}_2\mathrm{O}_3(s) + 3\mathrm{CO}(g) \rightleftharpoons 2\mathrm{Fe}(l) + 3\mathrm{CO}_2(g)$.The equilibrium constant $K_p$ is given by $$K_p = \frac{(P_{\mathrm{CO}_2})^3}{(P_{\mathrm{CO}})^3}.$$Solid $\mathrm{Fe}_2\mathrm{O}_3$ and liquid $\mathrm{Fe}$ have fixed activity (unity), so they are absent from $K_p$.Adding or removing a pure solid or liquid does not shift the equilibrium.Options A, B, and C change the amounts of gaseous $\mathrm{CO}_2$ or $\mathrm{CO}$, which disturb the equilibrium.Option D, addition of $\mathrm{Fe}_2\mathrm{O}_3$ (a solid), will not disturb the equilibrium.Answer:D (Addition of $\mathrm{Fe}_2\mathrm{O}_3$)