Rucete ✏ AP Chemistry In a Nutshell
9. Chemical Equilibrium — Practice Questions
This chapter introduces the principles of chemical equilibrium, equilibrium constants, and Le Châtelier’s principle.
(Multiple Choice — Click to Reveal Answer)
1. Which of the following indicates a system at chemical equilibrium?
(A) Reactants have completely turned into products
(B) The reaction stops completely
(C) The rates of forward and reverse reactions are equal
(D) Product concentration exceeds reactant concentration
Answer
(C) — At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.
2. What does the equilibrium constant (K) indicate about a reaction?
(A) The time it takes to reach equilibrium
(B) The concentration of catalysts
(C) The ratio of product to reactant concentrations at equilibrium
(D) The rate of reaction
Answer
(C) — K expresses the ratio of products to reactants at equilibrium.
3. Which of the following would shift the equilibrium position to the right?
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat
(A) Decreasing pressure
(B) Removing NH₃
(C) Adding heat
(D) Adding a catalyst
Answer
(B) — Removing product causes the system to shift right to replace it.
4. In the expression for Kc, what is excluded?
(A) Gases
(B) Solids and liquids
(C) Aqueous ions
(D) Reactants
Answer
(B) — Solids and pure liquids are not included in the equilibrium constant expression.
5. What does it mean if Q > K for a reaction?
(A) The system is at equilibrium
(B) The reaction will shift to the right
(C) The reaction will shift to the left
(D) There is no reaction
Answer
(C) — Q > K means there are too many products, so the system shifts left to form more reactants.
6. Which of the following changes will NOT affect the value of the equilibrium constant K?
(A) Temperature change
(B) Adding more reactant
(C) Removing product
(D) Increasing pressure (for equal gas moles)
Answer
(B) — Only temperature changes affect the actual value of K; other changes shift equilibrium but not K.
7. For the reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), which stress would shift the reaction to the right?
(A) Increasing temperature (exothermic reaction)
(B) Removing O₂
(C) Decreasing pressure
(D) Increasing pressure
Answer
(D) — The reaction has fewer gas molecules on the right, so increasing pressure shifts equilibrium right.
8. What does a very large K value (K >> 1) indicate about the reaction at equilibrium?
(A) Reactants are favored
(B) Products are favored
(C) Reaction does not proceed
(D) Reaction is endothermic
Answer
(B) — A large K value means the equilibrium heavily favors products.
9. What is the effect of adding a catalyst to a reaction at equilibrium?
(A) It shifts the reaction to the right
(B) It shifts the reaction to the left
(C) It changes the value of K
(D) It speeds up the attainment of equilibrium
Answer
(D) — Catalysts do not shift equilibrium but help reach it faster by lowering activation energy.
10. Which is the correct equilibrium expression for the following reaction?
2NO₂(g) ⇌ 2NO(g) + O₂(g)
(A) K = [NO₂]² / ([NO]²[O₂])
(B) K = [NO]²[O₂] / [NO₂]²
(C) K = [NO] + [O₂] / [NO₂]
(D) K = [NO] / [NO₂][O₂]
Answer
(B) — K = [products]ⁿ / [reactants]ⁿ, so K = [NO]²[O₂] / [NO₂]².
11. What is the effect of increasing the temperature on an exothermic reaction at equilibrium?
(A) Shifts to the right
(B) Shifts to the left
(C) No effect
(D) Increases the value of K
Answer
(B) — Heat is a product in exothermic reactions, so adding heat shifts equilibrium to the left.
12. What does the expression “dynamic equilibrium” mean?
(A) Reactions have stopped completely
(B) Concentrations of products and reactants are changing
(C) Forward and reverse reactions continue at equal rates
(D) Equilibrium only exists in gaseous systems
Answer
(C) — Dynamic equilibrium means reactions are still occurring, but the concentrations remain constant.
13. What does it mean if K < 1 for a chemical reaction?
(A) Products are favored
(B) Reactants are favored
(C) The system is at equilibrium
(D) The temperature must be increased
Answer
(B) — A small K value indicates that reactants are favored at equilibrium.
14. Which of the following will affect the position of equilibrium but not the value of K?
(A) Temperature
(B) Catalyst
(C) Pressure
(D) Enthalpy of reaction
Answer
(C) — Pressure changes can shift equilibrium in gaseous systems, but do not change K.
15. For the equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g), what would happen if more H₂ is added?
(A) Shift to the right
(B) Shift to the left
(C) No shift
(D) Reaction stops
Answer
(A) — Adding reactants causes the equilibrium to shift to the right to form more products.
16. A reaction has an equilibrium constant K = 1.0 × 10⁻⁹. Which statement is true?
(A) The reaction lies far to the right
(B) The concentrations of reactants and products are nearly equal
(C) Reactants are highly favored
(D) The system is not at equilibrium
Answer
(C) — A very small K value indicates that little product forms; reactants dominate.
17. What happens to the equilibrium constant K if the reaction is reversed?
(A) It becomes 1/K
(B) It stays the same
(C) It becomes K²
(D) It becomes the square root of K
Answer
(A) — Reversing a reaction inverts the value of the equilibrium constant.
18. Which of the following best describes Le Châtelier’s principle?
(A) A change in pressure alters K
(B) A system at equilibrium resists all change
(C) A system at equilibrium shifts to oppose any imposed stress
(D) Reactions always favor products
Answer
(C) — Le Châtelier’s principle states that the system shifts to oppose any change.
19. In a gaseous equilibrium with equal numbers of moles on each side, how does pressure change affect the system?
(A) It shifts left
(B) It shifts right
(C) No change in equilibrium position
(D) K increases
Answer
(C) — If the number of gas moles is the same on both sides, pressure changes have no effect.
20. What is the effect of adding a catalyst to a system at equilibrium?
(A) Increases product concentration
(B) Decreases reactant concentration
(C) No change in concentrations
(D) Shifts equilibrium to the right
Answer
(C) — Catalysts do not affect equilibrium position; they only speed up attainment of equilibrium.
21. Which condition will cause a shift in the equilibrium position of a gas-phase reaction?
(A) Adding an inert gas
(B) Decreasing temperature in an exothermic reaction
(C) Adding a catalyst
(D) Diluting the solution with water
Answer
(B) — Lowering temperature favors exothermic direction; inert gases have no effect on equilibrium.
22. In the expression for Kc, which of the following is correct for the reaction: 2A + B ⇌ 3C?
(A) K = [C] / [A]²[B]
(B) K = [C]³ / [A]²[B]
(C) K = [A]²[B] / [C]³
(D) K = [C]³ / [A][B]²
Answer
(B) — K = [products]³ / ([A]²[B]) based on stoichiometry.
23. Which of the following equilibrium expressions is valid?
(A) K = [solid]/[aqueous]
(B) K = [gas]/[solid]
(C) K = [products]/[reactants], excluding solids
(D) K includes solids and pure liquids
Answer
(C) — Solids and pure liquids are excluded; only gases and aqueous species appear in K.
24. If Q < K for a system, what direction will the reaction proceed?
(A) Left
(B) Right
(C) No shift
(D) Cannot be determined
Answer
(B) — Q < K means too few products; the system shifts right to reach equilibrium.
25. A stress causes the equilibrium to shift and produce more reactants. What is likely true?
(A) Q > K
(B) Q = K
(C) K has changed
(D) The reaction is irreversible
Answer
(A) — If Q > K, there are excess products, and the system shifts left to make more reactants.
26. For the reaction: A + 2B ⇌ C + D, the equilibrium concentrations are [A] = 0.5 M, [B] = 0.3 M, [C] = 0.2 M, [D] = 0.4 M. What is the value of Kc?
(A) 0.89
(B) 1.78
(C) 2.67
(D) 3.56
Answer
Kc = [C][D] / ([A][B]²) = (0.2)(0.4) / (0.5)(0.3)² = 0.08 / (0.5 × 0.09) = 0.08 / 0.045 = (C) 1.78
27. The reaction: N₂O₄(g) ⇌ 2NO₂(g) has Kp = 0.25 at 298 K. What happens to the equilibrium if the volume is doubled?
(A) Shifts to the right
(B) Shifts to the left
(C) No shift
(D) Kp doubles
Answer
(A) — Increasing volume reduces pressure. Since there are more moles of gas on the right, the equilibrium shifts right.
28. Which of the following changes increases the value of the equilibrium constant (K) for an endothermic reaction?
(A) Decrease in pressure
(B) Increase in temperature
(C) Addition of inert gas
(D) Addition of product
Answer
(B) — For endothermic reactions, increasing temperature shifts equilibrium to the right and increases K.
29. The equilibrium constant Kc for a reaction is 4.0 at 25°C. If all reactants and products are doubled in concentration, what happens to Kc?
(A) Kc is halved
(B) Kc is doubled
(C) Kc is squared
(D) Kc remains the same
Answer
(D) — Kc is constant at a given temperature and does not change with concentration.
30. At equilibrium, the concentrations are [H₂] = 0.5 M, [I₂] = 0.5 M, and [HI] = 1.0 M. For the reaction H₂ + I₂ ⇌ 2HI, what is Kc?
(A) 1.0
(B) 2.0
(C) 4.0
(D) 16.0
Answer
Kc = [HI]² / ([H₂][I₂]) = (1.0)² / (0.5 × 0.5) = 1 / 0.25 = (C) 4.0
31. A reaction has ΔH < 0. Which of the following will increase the yield of products?
(A) Decreasing temperature
(B) Increasing temperature
(C) Adding inert gas
(D) Increasing volume
Answer
(A) — For exothermic reactions, lowering temperature shifts equilibrium toward products.
32. The equilibrium constant for a reaction is 1.0 × 10⁻⁸. Which of the following is true?
(A) The forward reaction is fast
(B) The reaction favors products
(C) The reaction lies far to the left
(D) The reverse reaction is negligible
Answer
(C) — A very small K indicates that reactants are heavily favored at equilibrium.
33. Which of the following does NOT change when a catalyst is added to a reversible reaction at equilibrium?
(A) Rate of forward reaction
(B) Rate of reverse reaction
(C) Time to reach equilibrium
(D) Position of equilibrium
Answer
(D) — Catalysts speed up both forward and reverse reactions equally but do not affect equilibrium position.
34. The equilibrium constant Kp is related to Kc by the expression Kp = Kc(RT)^Δn. What does Δn represent?
(A) Heat of reaction
(B) Change in concentration
(C) Change in number of gas moles
(D) Change in pressure
Answer
(C) — Δn = moles of gaseous products − moles of gaseous reactants.
35. What is the effect of decreasing the temperature on the equilibrium constant of an endothermic reaction?
(A) K increases
(B) K decreases
(C) K remains the same
(D) Reaction becomes spontaneous
Answer
(B) — Lowering temperature shifts endothermic reactions toward reactants, so K decreases.
36. Define the equilibrium constant (Kc) in terms of concentration for a general reaction: aA + bB ⇌ cC + dD
Answer
Kc = [C]^c [D]^d / [A]^a [B]^b — The equilibrium constant is the ratio of product concentrations to reactant concentrations, each raised to the power of their coefficients.
37. Explain why pure solids and liquids are omitted from equilibrium constant expressions.
Answer
Because their concentrations do not change — Pure solids and liquids have constant densities and are not included in K expressions.
38. What does it mean if Q > K for a system at a given moment?
Answer
The system has too many products — The reaction will shift left (toward reactants) to reach equilibrium.
39. Why does increasing pressure favor the side of a gaseous reaction with fewer moles?
Answer
To reduce pressure — According to Le Châtelier’s principle, the system counteracts pressure increase by favoring fewer gas molecules.
40. State the only condition that can change the value of the equilibrium constant (K).
Answer
Temperature — Changing temperature alters the value of K; pressure, concentration, and catalysts do not.
41. What is the role of a catalyst in a reversible reaction?
Answer
It speeds up both forward and reverse reactions equally — Thus it reduces the time to reach equilibrium but doesn’t affect K or equilibrium position.
42. For the reaction: 2NO₂(g) ⇌ N₂O₄(g), what will happen to equilibrium if NO₂ is removed?
Answer
The system will shift left — Removing a reactant causes the equilibrium to shift to make more of it.
43. How does increasing the temperature affect an exothermic equilibrium reaction?
Answer
Shifts the reaction left — Heat is treated as a product; increasing temperature favors the reverse direction.
44. Why does adding an inert gas at constant volume have no effect on equilibrium?
Answer
It does not change partial pressures of reactants or products — Therefore, the position of equilibrium remains unchanged.
45. A reaction has K = 100. What does this say about the relative amounts of products and reactants at equilibrium?
Answer
Products are much greater than reactants — A large K indicates a product-favored equilibrium.
46. What mathematical expression relates Kp and Kc?
Answer
Kp = Kc(RT)^Δn — Where R is the gas constant, T is temperature in K, and Δn is the change in moles of gas.
47. Why does decreasing volume favor the side of a reaction with fewer gas molecules?
Answer
To reduce pressure — The system responds by shifting toward the side with fewer moles of gas to minimize pressure.
48. For a reaction where ΔH is negative, what will happen to the equilibrium constant if temperature increases?
Answer
K will decrease — Heating an exothermic system shifts equilibrium to the left, reducing product concentration and K.
49. What does it mean if the concentrations of products and reactants are constant, but the system is not at equilibrium?
Answer
The system may be at steady state or reaction has stopped — At equilibrium, forward and reverse reactions must be equal, not just constant concentrations.
50. Give an example of a reversible reaction commonly used to illustrate equilibrium in chemistry classrooms.
Answer
Color change in cobalt(II) chloride with temperature: Co(H₂O)₆²⁺(aq) + 4Cl⁻(aq) ⇌ CoCl₄²⁻(aq) + 6H₂O(l)