Rucete ✏ AP Chemistry In a Nutshell
9. Chemical Equilibrium — Practice Questions 2
This chapter introduces the principles of dynamic equilibrium, equilibrium constants, Le Châtelier’s principle, and quantitative problem solving related to chemical equilibria.
(Multiple Choice — Click to Reveal Answer)
1. Which of the following will NOT affect the position of a chemical equilibrium?
(A) Change in temperature
(B) Addition of a catalyst
(C) Change in concentration
(D) Change in pressure
Answer
(B) — A catalyst speeds up both forward and reverse reactions equally, so it doesn't shift the position of equilibrium.
2. In a reversible reaction at equilibrium, the concentration of products remains constant because:
(A) No more reactions are occurring
(B) Reactants are fully consumed
(C) Forward and reverse rates are equal
(D) Activation energy is zero
Answer
(C) — At equilibrium, the rates of the forward and reverse reactions are equal, keeping concentrations stable.
3. What does it indicate if Q < K in a reaction system?
(A) The system is at equilibrium
(B) The reverse reaction will proceed
(C) The forward reaction will proceed
(D) The value of K is incorrect
Answer
(C) — If Q < K, the forward reaction must occur to reach equilibrium.
4. For an endothermic reaction, which change favors product formation?
(A) Decreasing temperature
(B) Adding a catalyst
(C) Increasing temperature
(D) Increasing pressure
Answer
(C) — Heat acts as a reactant in endothermic reactions; increasing temperature shifts equilibrium toward products.
5. Which of the following is NOT true about dynamic equilibrium?
(A) Concentrations remain constant
(B) No net change occurs
(C) Forward rate equals reverse rate
(D) Reactants are completely converted to products
Answer
(D) — Reactants are not fully converted at equilibrium; both reactants and products remain.
6. Which of the following is the best definition of equilibrium?
(A) All reactions have stopped
(B) The amount of reactants equals the amount of products
(C) The rates of the forward and reverse reactions are equal
(D) There are no products left
Answer
(C) — At equilibrium, the forward and reverse reaction rates are equal, though amounts may differ.
7. Which of the following would cause the equilibrium of the reaction 2SO₂ + O₂ ⇌ 2SO₃ to shift to the right?
(A) Removing O₂
(B) Decreasing pressure
(C) Adding SO₃
(D) Adding SO₂
Answer
(D) — Adding reactant (SO₂) drives the reaction toward product formation.
8. Which term describes the state in which the forward and reverse reactions occur at the same rate?
(A) Completion
(B) Equilibrium
(C) Saturation
(D) Static balance
Answer
(B) — This is the definition of chemical equilibrium.
9. For the reaction: H₂(g) + Cl₂(g) ⇌ 2HCl(g), which change will NOT affect equilibrium?
(A) Increasing temperature
(B) Adding a catalyst
(C) Removing HCl
(D) Adding more Cl₂
Answer
(B) — Catalysts speed up equilibrium attainment but do not affect the equilibrium position.
10. If the equilibrium constant for a reaction is much greater than 1, what is favored?
(A) Reactants
(B) Products
(C) Catalyst
(D) Activation energy
Answer
(B) — A large K value means the products are heavily favored at equilibrium.
11. What change will shift the equilibrium of a gaseous reaction to the side with fewer moles of gas?
(A) Decrease in pressure
(B) Increase in volume
(C) Increase in pressure
(D) Addition of catalyst
Answer
(C) — According to Le Châtelier’s principle, increasing pressure favors the side with fewer gas molecules.
12. What effect does lowering the temperature have on an exothermic reaction at equilibrium?
(A) Shifts left
(B) Shifts right
(C) No effect
(D) Changes catalyst
Answer
(B) — Lowering temperature removes heat, so the system shifts to produce more heat — toward products.
13. What does it mean if K = 1 for a chemical reaction?
(A) No reaction occurs
(B) Only products are present
(C) Only reactants are present
(D) Reactants and products are balanced at equilibrium
Answer
(D) — When K = 1, the system has comparable concentrations of products and reactants at equilibrium.
14. Which of the following is an example of a stress that can disturb chemical equilibrium?
(A) Watching the reaction
(B) Removing products
(C) Adding a catalyst
(D) Measuring temperature
Answer
(B) — Removing products disrupts equilibrium and causes a shift to the right to compensate.
15. The reaction: A ⇌ B has K = 10. What will happen if more B is added?
(A) The system shifts right
(B) The system shifts left
(C) The system remains unchanged
(D) Reaction stops
Answer
(B) — Adding product shifts the equilibrium left to reduce the excess B.
16. What does it mean if the concentration of reactants is not changing over time?
(A) The reaction has stopped
(B) Equilibrium may have been reached
(C) The reaction is irreversible
(D) Products are being removed
Answer
(B) — Constant concentrations suggest the system is at equilibrium.
17. What effect does adding a catalyst have on the value of the equilibrium constant?
(A) Increases it
(B) Decreases it
(C) Has no effect
(D) Doubles it
Answer
(C) — Catalysts do not change the value of K; they only affect reaction rate.
18. Which of the following is true about a system at equilibrium?
(A) The forward reaction has stopped
(B) Reactants equal products
(C) Forward and reverse reactions occur at equal rates
(D) The reaction is endothermic
Answer
(C) — Equal rates of forward and reverse reactions define dynamic equilibrium.
19. Which statement best describes Le Châtelier’s principle?
(A) The reaction shifts to produce more heat
(B) A system resists changes by adjusting its equilibrium
(C) Reactants turn fully into products
(D) Products always increase over time
Answer
(B) — Le Châtelier’s principle states that a system at equilibrium adjusts to minimize imposed stress.
20. Which of the following changes will affect both the position of equilibrium and the value of K?
(A) Pressure
(B) Concentration
(C) Temperature
(D) Catalyst
Answer
(C) — Temperature is the only factor that changes the value of K.
21. A reaction has K = 0.001. Which statement is true?
(A) It lies far to the left
(B) It is product-favored
(C) It’s irreversible
(D) Reactants and products are equal
Answer
(A) — A very small K value indicates the reaction favors reactants strongly.
22. If the number of gas molecules on both sides is equal, what is the effect of pressure changes?
(A) Favors products
(B) Favors reactants
(C) No shift in equilibrium
(D) K changes
Answer
(C) — When gas moles are equal, pressure changes have no net effect on equilibrium position.
23. What does it mean if a system is “closed” in the context of equilibrium?
(A) Only forward reaction occurs
(B) Energy cannot enter
(C) No matter is exchanged with surroundings
(D) Only gases can participate
Answer
(C) — A closed system allows energy exchange but not matter, which is essential for maintaining equilibrium.
24. What happens if a product is continuously removed from an equilibrium mixture?
(A) K increases
(B) The reaction stops
(C) The system shifts right
(D) Pressure rises
Answer
(C) — Removing product shifts the system toward producing more product to reestablish equilibrium.
25. Which of the following situations will NOT change the concentrations of species at equilibrium?
(A) Adding more reactant
(B) Changing the temperature
(C) Adding catalyst
(D) Removing product
Answer
(C) — A catalyst only affects the rate of reaching equilibrium, not the final concentrations.
26. The equilibrium constant Kc for a reaction is 1.0 × 10⁻⁵. Which of the following best describes the mixture at equilibrium?
(A) Mostly products
(B) Mostly reactants
(C) Equal amounts
(D) Reaction does not occur
Answer
(B) — A very small Kc means the equilibrium strongly favors reactants.
27. In the reaction: 2NO₂(g) ⇌ N₂O₄(g), what would happen if the total pressure is increased at constant temperature?
(A) Shift to the right
(B) Shift to the left
(C) No shift
(D) K increases
Answer
(A) — Increased pressure favors the side with fewer gas molecules; here, the product side.
28. For a gaseous reaction at equilibrium, what is the effect of compressing the container?
(A) No effect
(B) Shifts to side with fewer moles of gas
(C) Shifts to side with more moles of gas
(D) Equilibrium constant increases
Answer
(B) — Decreasing volume increases pressure, shifting equilibrium to the side with fewer gas particles.
29. Which of the following is true when a system at equilibrium is disturbed by adding a product?
(A) Forward reaction rate increases
(B) Equilibrium shifts toward products
(C) Reverse reaction rate increases
(D) Kc changes
Answer
(C) — Adding product increases reverse rate, causing the system to shift toward reactants.
30. A catalyst is added to a reaction mixture at equilibrium. Which of the following changes?
(A) Value of K
(B) Position of equilibrium
(C) Time to reach equilibrium
(D) Reactant concentrations at equilibrium
Answer
(C) — A catalyst speeds up the attainment of equilibrium but does not change K or concentrations.
31. If Q > K for a system, which direction will the reaction proceed to reach equilibrium?
(A) Forward
(B) Reverse
(C) No change
(D) Reaction stops
Answer
(B) — The reaction will proceed in the reverse direction to decrease the amount of products.
32. The expression Kc = [products]/[reactants] applies only under which condition?
(A) Reactions at low temperature
(B) Reactions involving solids
(C) Reactions at equilibrium
(D) Reactions with catalysts
Answer
(C) — The equilibrium constant only applies to systems at equilibrium.
33. A reaction has Δn = -2. How will increasing temperature affect the value of Kp if the reaction is exothermic?
(A) Kp increases
(B) Kp decreases
(C) Kp stays the same
(D) Cannot determine
Answer
(B) — For an exothermic reaction, increasing temperature shifts equilibrium left and decreases Kp.
34. What does a large value of K (>1000) imply about the reaction?
(A) Reactants dominate
(B) Reaction hardly proceeds
(C) Products dominate
(D) No reverse reaction occurs
Answer
(C) — A large K means the equilibrium lies far to the right; mostly products are present.
35. In the equation Kp = Kc(RT)^Δn, what happens to Kp when Δn = 0?
(A) Kp > Kc
(B) Kp = Kc
(C) Kp < Kc
(D) Kp = 0
Answer
(B) — If Δn = 0, (RT)^0 = 1, so Kp = Kc.
36. Define dynamic equilibrium in your own words.
Answer
Dynamic equilibrium is a state in which the forward and reverse reactions occur at the same rate, resulting in no net change in the concentrations of reactants and products.
37. Explain why the value of the equilibrium constant changes with temperature.
Answer
Because temperature affects the relative rates of the forward and reverse reactions, altering the equilibrium position and thus the value of K.
38. Describe what happens to a system at equilibrium when more reactant is added.
Answer
The system shifts toward the products (right) to consume the added reactant and re-establish equilibrium.
39. Give one real-life example where Le Châtelier’s principle is applied.
Answer
The industrial synthesis of ammonia (Haber process), where pressure is increased to favor the production of ammonia.
40. What does it mean if the equilibrium constant (K) is less than 1?
Answer
It means the equilibrium favors the reactants, and there is more reactant than product at equilibrium.
41. Why is equilibrium described as “dynamic” instead of “static”?
Answer
Because the forward and reverse reactions are still occurring, even though the concentrations remain constant.
42. Write the general form of the equilibrium constant expression for the reaction: aA + bB ⇌ cC + dD
Answer
Kc = [C]^c [D]^d / [A]^a [B]^b
43. What happens to the equilibrium position of an endothermic reaction when temperature increases?
Answer
The equilibrium shifts to the right (products), as heat is treated as a reactant in an endothermic reaction.
44. Why are solids and pure liquids omitted from equilibrium constant expressions?
Answer
Because their concentrations are constant and do not affect the equilibrium position.
45. Describe the effect of decreasing volume on a gaseous equilibrium system.
Answer
It increases pressure, so the system shifts toward the side with fewer moles of gas to reduce pressure.
46. A catalyst is added to a reaction mixture. What changes and what remains the same?
Answer
The rate at which equilibrium is reached increases, but the equilibrium constant and position do not change.
47. What is the effect of removing product from a reaction at equilibrium?
Answer
The reaction shifts toward the products to replace what was removed.
48. For the reaction A + B ⇌ C + D, what would happen if both A and B are removed?
Answer
The reaction shifts to the left to produce more A and B from C and D.
49. Explain the difference between Kp and Kc.
Answer
Kp is the equilibrium constant for partial pressures of gases; Kc is for molar concentrations in solution.
50. Describe what the reaction quotient (Q) tells you compared to the equilibrium constant (K).
Answer
Q is calculated using initial concentrations or pressures and predicts which direction the system will shift to reach equilibrium; K uses equilibrium values.