Reaction quotient (Q) Knowledge-Based Practice

Questions

1. 1.

   In the context of chemical equilibrium, what does a reaction quotient value of $Q_c = 0$ signify about the initial composition of a reaction mixture and its spontaneous direction of change?

   • A) All reactants and products are present in equal molar amounts, so the system is already at dynamic equilibrium.
   • B) The reactants have been completely consumed, driving the reverse reaction spontaneously toward equilibrium.
   • C) The forward and reverse reaction rates are both equal to zero, preventing any chemical change from taking place.
   • D) At least one reaction product is entirely absent, meaning only the forward reaction can initially proceed to approach equilibrium.
2. 2.

   A gaseous mixture for the reversible reaction $\text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g)$ is initially at dynamic equilibrium with an equilibrium constant $K_c$ in a sealed container at constant temperature. If the volume of the container is suddenly doubled, what is the instantaneous value of the reaction quotient ($Q_c$) relative to $K_c$, and in which direction will the reaction proceed to restore equilibrium?

   • A) $Q_c = \frac{1}{2}K_c$; the reaction proceeds in the forward direction.
   • B) $Q_c = 2K_c$; the reaction proceeds in the reverse direction.
   • C) $Q_c = \frac{1}{4}K_c$; the reaction proceeds in the forward direction.
   • D) $Q_c = K_c$; the reaction remains at equilibrium without shifting.
3. 3.

   Which statement correctly distinguishes the reaction quotient ($Q$) from the equilibrium constant ($K$) for a reversible reaction?

   • A) $Q$ is calculated using instantaneous concentrations at any given moment, whereas $K$ is calculated using concentrations only at dynamic equilibrium.
   • B) $Q$ remains constant at a specific temperature regardless of concentration changes, whereas $K$ varies as the reaction progresses.
   • C) $Q$ includes the molar concentrations of pure solids and pure liquids, whereas $K$ excludes these condensed phases from its expression.
   • D) $Q$ is applicable only to homogeneous gaseous equilibria, whereas $K$ can be determined for both homogeneous and heterogeneous systems.
4. 4.

   When a reversible gaseous reaction mixture is in a state where the reaction quotient exceeds the equilibrium constant ($Q_c > K_c$), which statement correctly compares the instantaneous reaction rates and describes the direction of net change?

   • A) The forward reaction rate is greater than the reverse reaction rate, so reactants are consumed to form more products.
   • B) The forward reaction rate equals the reverse reaction rate, so the concentrations of all species remain constant.
   • C) The forward reaction rate is greater than the reverse reaction rate, so the reaction quotient increases until it matches the equilibrium constant.
   • D) The reverse reaction rate is greater than the forward reaction rate, so products are consumed to reform reactants.
5. 5.

   For the reversible gaseous reaction $\text{A}(g) + \text{B}(g) \rightleftharpoons 2\text{C}(g)$, the equilibrium constant is $K_c = 16$ at a given temperature. If a reaction vessel contains $0.20\text{ M}$ $\text{A}$, $0.20\text{ M}$ $\text{B}$, and $0.40\text{ M}$ $\text{C}$, what is the value of the reaction quotient ($Q_c$) and in which direction will the reaction proceed to achieve equilibrium?

   • A) $Q_c = 0.25$, and the reaction shifts toward the reactants.
   • B) $Q_c = 2.0$, and the reaction is at equilibrium.
   • C) $Q_c = 4.0$, and the reaction shifts toward the products.
   • D) $Q_c = 10$, and the reaction shifts toward the reactants.

Answers and step-by-step solutions appear after you finish the test.