Heterogeneous equilibria Knowledge-Based Download PDF

Questions

1. 1.

   The heterogeneous reaction $\text{C}(s) + \text{CO}_2(g) \rightleftharpoons 2\text{CO}(g)$ is at dynamic chemical equilibrium in a closed vessel fitted with a movable piston at constant temperature. If the volume of the vessel is decreased at constant temperature, which of the following statements correctly describes the system when equilibrium is re-established?

   • A) The equilibrium constant $K_c$ decreases because the total pressure increases.
   • B) The mass of solid carbon, $\text{C}(s)$, decreases as the system adjusts.
   • C) The equilibrium shifts to the right because the reaction produces more gaseous particles.
   • D) The total number of moles of $\text{CO}(g)$ present in the vessel decreases.
2. 2.

   Solid ammonium hydrogen sulfide decomposes according to the heterogeneous equilibrium reaction $\text{NH}_4\text{HS}(s) \rightleftharpoons \text{NH}_3(g) + \text{H}_2\text{S}(g)$ in an evacuated, rigid vessel at a constant temperature. If the total equilibrium gas pressure in the vessel is $P_{\text{total}}$, which expression gives the equilibrium constant $K_p$?

   • A) $K_p = P_{\text{total}}^2$
   • B) $K_p = 2 P_{\text{total}}^2$
   • C) $K_p = \frac{P_{\text{total}}^2}{2}$
   • D) $K_p = \frac{P_{\text{total}}^2}{4}$
3. 3.

   A closed, rigid container contains the heterogeneous system $\text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)$ at chemical equilibrium at a constant temperature. Which of the following describes the effect of adding an additional amount of pure $\text{CaCO}_3(s)$ to this container?

   • A) The equilibrium shifts toward the products, increasing the partial pressure of $\text{CO}_2(g)$.
   • B) The equilibrium shifts toward the reactants, decreasing the partial pressure of $\text{CO}_2(g)$.
   • C) The partial pressure of $\text{CO}_2(g)$ and the composition of the gas phase remain unchanged.
   • D) The value of the equilibrium constant $K_p$ increases because the quantity of reactant has increased.
4. 4.

   Why are pure solids and pure liquids excluded from the equilibrium constant expression of a heterogeneous reaction?

   • A) They do not participate in the chemical transformation between reactants and products.
   • B) Their forward and reverse reaction rates are both equal to zero at equilibrium.
   • C) Their concentrations remain constant because their densities and molar masses do not change.
   • D) Their chemical activities change proportionally with the total volume of the reaction vessel.
5. 5.

   Which of the following is the correct equilibrium constant expression ($K_c$) for the heterogeneous reaction $\text{Fe}_2\text{O}_3(s) + 3\text{CO}(g) \rightleftharpoons 2\text{Fe}(s) + 3\text{CO}_2(g)$?

   • A) $K_c = \frac{[\text{CO}_2]}{[\text{CO}]}$
   • B) $K_c = \frac{[\text{Fe}]^2 [\text{CO}_2]^3}{[\text{Fe}_2\text{O}_3] [\text{CO}]^3}$
   • C) $K_c = \frac{[\text{CO}]^3}{[\text{CO}_2]^3}$
   • D) $K_c = \frac{[\text{CO}_2]^3}{[\text{CO}]^3}$