AP Chemistry Questions on Le Chatelier's Principle and Temperature Changes

🧪 Quick Study Guide

• 🔥 Le Chatelier's Principle: If a change of condition (like temperature) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.
• 🌡️ Temperature & Equilibrium:
  • For endothermic reactions (heat is absorbed, $\Delta H > 0$): Increasing temperature favors the forward reaction. Decreasing temperature favors the reverse reaction.
  • For exothermic reactions (heat is released, $\Delta H < 0$): Increasing temperature favors the reverse reaction. Decreasing temperature favors the forward reaction.
• ⚗️ Catalysts: Catalysts speed up the rate at which equilibrium is reached but do not affect the position of the equilibrium.
• 🧐 Important Note: Only temperature changes alter the equilibrium constant ($K$) value. Changes in concentration or pressure do not.

✍️ Practice Quiz

1. Which of the following changes will shift the equilibrium to the right for the reaction: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) + heat$?
   1. Increasing the temperature
   2. Decreasing the pressure
   3. Adding $N_2(g)$
   4. Removing $NH_3(g)$
2. Consider the following reaction at equilibrium: $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ $\Delta H = -198 \text{ kJ/mol}$. Which change will favor the formation of $SO_3(g)$?
   1. Increasing the temperature
   2. Decreasing the temperature
   3. Adding a catalyst
   4. Increasing the volume of the container
3. For an endothermic reaction, what is the effect of increasing the temperature on the equilibrium constant, $K$?
   1. $K$ increases
   2. $K$ decreases
   3. $K$ remains constant
   4. The reaction rate decreases
4. Which of the following statements is true regarding the effect of a catalyst on a system at equilibrium?
   1. The catalyst shifts the equilibrium towards the products.
   2. The catalyst shifts the equilibrium towards the reactants.
   3. The catalyst increases the rate at which equilibrium is reached but does not change the equilibrium position.
   4. The catalyst changes the value of the equilibrium constant, $K$.
5. The reaction $A(g) + B(g) \rightleftharpoons C(g)$ has $\Delta H > 0$. Which of the following will increase the amount of $C(g)$ at equilibrium?
   1. Decreasing the temperature
   2. Increasing the pressure
   3. Adding an inert gas
   4. Increasing the temperature
6. For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, increasing the temperature causes the equilibrium to shift slightly towards the reactants. What can you conclude about the reaction?
   1. The reaction is endothermic in the forward direction.
   2. The reaction is exothermic in the forward direction.
   3. The reaction is neither endothermic nor exothermic.
   4. The reaction involves a change in volume.
7. Consider the equilibrium: $CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g) + \text{heat}$. Which of the following conditions would favor the production of $CH_3OH(g)$?
   1. High temperature and high pressure
   2. Low temperature and low pressure
   3. High temperature and low pressure
   4. Low temperature and high pressure