Which of the following is/are correct statement(s)? — Chemical Equilibrium Chemistry Question

💡 Solution & Explanation

Step 1 - Analyze Statement (A): Dissociation of \ce{CaCO3} under high pressure The thermal decomposition of solid calcium carbonate: \[\ce{CaCO3(s) <=> CaO(s) + CO2(g)}\] Change in gaseous moles: $\Delta n_g = 1 - 0 = +1$ By Le Chatelier's Principle, high external pressure shifts equilibrium toward fewer gaseous moles (backward direction), suppressing the dissociation of \ce{CaCO3}. **Statement (A) is correct.** Step 2 - Analyze Statement (B): Apparent molecular mass of \ce{PCl5} on dissociation \[\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}\] Starting with 1 mol \ce{PCl5}, degree of dissociation = $\alpha$: - Total moles at equilibrium: $(1-\alpha) + \alpha + \alpha = 1 + \alpha$ By conservation of mass, total mass = $M_{\ce{PCl5}}$: \[M_{\text{apparent}} = \frac{M_{\ce{PCl5}}}{1 + \alpha}\] Since $\alpha > 0 \implies 1 + \alpha > 1 \implies M_{\text{apparent}} < M_{\ce{PCl5}}$ Apparent molecular mass shows a lower value on dissociation. **Statement (B) is correct.** Step 3 - Analyze Statement (C): Pressure dependence of melting ice Phase equilibrium: $\ce{H2O(s) <=> H2O(l)}$ Water has anomalous behavior: liquid water is denser than ice, so: \[\Delta V_{\text{melting}} = V_{\text{liquid}} - V_{\text{solid}} < 0\] By Le Chatelier's Principle, high pressure favors the state with smaller volume (liquid water). So high pressure, not low pressure, is favorable for melting ice. **Statement (C) is incorrect.** Step 4 - Analyze Statement (D): Temperature dependence of hydrogen combination \[\ce{2H(g) <=> H2(g)}, \quad \Delta H^\circ < 0 \text{ (exothermic)}\] Bond formation releases energy. By Le Chatelier's Principle, high temperature shifts equilibrium toward the endothermic (backward) direction, favoring dissociation of \ce{H2}, not combination. **Statement (D) is incorrect.** Step 5 - Summary * **(A) Correct** — $\Delta n_g = +1$; high pressure suppresses dissociation. * **(B) Correct** — dissociation increases total moles, decreasing apparent molecular mass. * **(C) Incorrect** — melting of ice is favored by high pressure (negative $\Delta V$). * **(D) Incorrect** — combination is exothermic; low temperature, not high, favors it. \[\text{Correct Options: } \boxed{\text{A, B}}\]