AP Chemistry Questions on Reaction Quotient (Q) and Equilibrium: Exam Prep

📚 Quick Study Guide

• ⚖️ Equilibrium is a state where the rates of the forward and reverse reactions are equal, and the net change in concentrations of reactants and products is zero.
• ⚛️ The equilibrium constant, $K$, is the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients. For the reaction $aA + bB \rightleftharpoons cC + dD$, $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$.
• 🌡️ $K$ is temperature-dependent; changing the temperature changes the value of $K$.
• ➗ The reaction quotient, $Q$, is a measure of the relative amount of products and reactants present in a reaction at any given time. For the reaction $aA + bB \rightleftharpoons cC + dD$, $Q = \frac{[C]^c[D]^d}{[A]^a[B]^b}$.
• 🧭 Comparing $Q$ and $K$ allows us to predict the direction a reversible reaction will shift to reach equilibrium:
  • If $Q < K$, the reaction will shift to the right (towards products).
  • If $Q > K$, the reaction will shift to the left (towards reactants).
  • If $Q = K$, the reaction is at equilibrium.
• 💡 Remember to only include gaseous and aqueous species in the $K$ and $Q$ expressions; solids and liquids are excluded.

🧪 Practice Quiz

1. Which of the following statements is true regarding the reaction quotient, Q?
   1. Q is only calculated at equilibrium.
   2. Q always equals the equilibrium constant, K.
   3. Q can be calculated at any point during a reaction.
   4. Q is independent of temperature.
2. For the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, the equilibrium constant K is 0.50 at a certain temperature. If the initial concentrations are $[N_2] = 1.0 M$, $[H_2] = 2.0 M$, and $[NH_3] = 0.50 M$, what is the value of Q?
   1. 0.0625
   2. 0.125
   3. 0.25
   4. 0.50
3. If Q > K, which direction will the reaction shift to reach equilibrium?
   1. Towards products
   2. Towards reactants
   3. No shift will occur; the reaction is already at equilibrium.
   4. The reaction will oscillate between products and reactants.
4. Consider the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$. At a certain temperature, K = 100. If $[SO_2] = 0.2 M$, $[O_2] = 0.1 M$, and $[SO_3] = 0.4 M$, what is the value of Q and which way will the reaction shift?
   1. Q = 40; shifts to the left
   2. Q = 40; shifts to the right
   3. Q = 80; shifts to the left
   4. Q = 80; shifts to the right
5. Which of the following factors does NOT affect the value of the equilibrium constant, K?
   1. Temperature
   2. Pressure
   3. Initial concentrations of reactants and products
   4. The presence of a catalyst
6. For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, K = 50. If Q = 75, what does this indicate about the reaction?
   1. The reaction is at equilibrium.
   2. The rate of the forward reaction is greater than the reverse reaction.
   3. The rate of the reverse reaction is greater than the forward reaction.
   4. The reaction will proceed spontaneously to form more products.
7. The reaction $aA(g) \rightleftharpoons bB(g)$ has an equilibrium constant K. If the reaction mixture consists only of $A$ initially, which of the following is true when the system reaches equilibrium?
   1. $[A] = [B]$
   2. $[A] > [B]$ if K > 1
   3. $[A] < [B]$ if K < 1
   4. The relationship between [A] and [B] depends only on the initial amount of A.