Rucete ✏ AP Chemistry In a Nutshell
9. Chemical Equilibrium — Practice Questions 3
This chapter introduces the concept of dynamic equilibrium in chemical reactions and explores how changes in concentration, temperature, pressure, and catalysts affect the equilibrium position as described by Le Châtelier’s Principle.
(Multiple Choice — Click to Reveal Answer)
1. Which statement best explains why a reaction reaches equilibrium?
(A) The reactants are all consumed
(B) The rate of the forward and reverse reactions become equal
(C) The products decompose completely
(D) There are no more collisions between particles
Answer
(B) — At equilibrium, the rates of the forward and reverse reactions are equal, resulting in no net change in concentrations.
2. Which factor does NOT affect the position of equilibrium?
(A) Temperature
(B) Catalyst
(C) Pressure (for gases)
(D) Concentration
Answer
(B) — A catalyst speeds up both forward and reverse reactions equally but does not shift the equilibrium position.
3. Which direction will the equilibrium shift if more product is added?
(A) To the right
(B) To the left
(C) No shift
(D) Depends on temperature
Answer
(B) — Adding product causes the reaction to shift left to re-establish equilibrium.
4. The equilibrium expression for the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is:
(A) [NH₃]² / [N₂][H₂]³
(B) [N₂][H₂] / [NH₃]³
(C) [NH₃]³ / [N₂][H₂]²
(D) [N₂]²[H₂]³ / [NH₃]
Answer
(A) — The equilibrium expression is written using products over reactants, each raised to the power of its coefficient.
5. What is the value of Q for the reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) if [SO₂] = 0.20 M, [O₂] = 0.10 M, and [SO₃] = 0.05 M?
(A) 0.03125
(B) 6.25
(C) 0.25
(D) 1.6
Answer
(A) — Q = [SO₃]² / ([SO₂]²[O₂]) = (0.05)² / ((0.20)² × 0.10) = 0.0025 / 0.004 = 0.625
6. If Q < K, which direction will the reaction proceed to reach equilibrium?
(A) Left
(B) Right
(C) No change
(D) Cannot determine
Answer
(B) — If Q < K, the reaction shifts to the right to form more products and reach equilibrium.
7. The reaction A + B ⇌ C + D is exothermic. What happens to the equilibrium position when temperature increases?
(A) Shifts right
(B) Shifts left
(C) No shift
(D) K increases
Answer
(B) — Heat acts as a product in exothermic reactions, so increasing temperature shifts equilibrium left.
8. What is the effect of decreasing the volume in a reaction involving gases?
(A) Shifts toward side with fewer moles of gas
(B) Shifts toward side with more moles of gas
(C) No effect
(D) Depends on temperature
Answer
(A) — Decreasing volume increases pressure, shifting equilibrium toward fewer moles of gas to relieve pressure.
9. Which of the following would cause no change to the value of the equilibrium constant (K)?
(A) Change in temperature
(B) Addition of a catalyst
(C) Removal of products
(D) Addition of reactants
Answer
(B) — Only temperature affects the value of K; catalysts only change the rate of reaching equilibrium.
10. A solution of Fe³⁺ and SCN⁻ turns deep red when equilibrium is established. What does this color change indicate?
(A) Precipitate formed
(B) Endothermic reaction
(C) Product complex formed
(D) Gas evolved
Answer
(C) — The deep red color is due to the FeSCN²⁺ complex ion, indicating the formation of product.
11. In a reversible reaction, how does the system respond if products are removed?
(A) Shifts left
(B) Shifts right
(C) No effect
(D) Reaction stops
Answer
(B) — Removing products causes the system to shift right to make more product.
12. For the reaction: H₂(g) + I₂(g) ⇌ 2HI(g), what happens if more HI is added?
(A) Shift right
(B) Shift left
(C) K increases
(D) No change
Answer
(B) — Adding product causes the system to shift left to consume excess HI.
13. What happens to Kc when a reaction is reversed?
(A) Stays the same
(B) Kc becomes negative
(C) Kc is inverted (1/K)
(D) Kc is squared
Answer
(C) — Reversing a reaction inverts the equilibrium constant.
14. A system is at equilibrium. If the volume is increased, how will it shift?
(A) Toward more moles of gas
(B) Toward fewer moles of gas
(C) No shift
(D) Toward solid phase
Answer
(A) — Increasing volume decreases pressure, so the system shifts toward the side with more gas molecules.
15. What is the role of a catalyst in a reaction at equilibrium?
(A) Changes Kc
(B) Increases product concentration
(C) Speeds up forward reaction only
(D) Lowers activation energy for both directions
Answer
(D) — A catalyst lowers the activation energy for both forward and reverse reactions, affecting rate but not equilibrium.
16. In the expression for Kc, why are pure solids and liquids excluded?
(A) They don’t participate
(B) Their concentrations are constant
(C) They increase Kc
(D) They are unstable
Answer
(B) — Pure solids and liquids have constant concentration and are omitted from the equilibrium expression.
17. Which condition increases the value of K for an endothermic reaction?
(A) Increase temperature
(B) Decrease temperature
(C) Increase pressure
(D) Add product
Answer
(A) — For endothermic reactions, raising temperature shifts equilibrium right, increasing K.
18. What is required for a chemical system to reach equilibrium?
(A) Closed system
(B) Constant temperature
(C) Reversible reaction
(D) All of the above
Answer
(D) — All of these are necessary for dynamic equilibrium to be established.
19. If the reaction quotient Q is greater than K, what must happen?
(A) System is at equilibrium
(B) Reaction shifts right
(C) Reaction shifts left
(D) No change
Answer
(C) — Q > K means too much product; the reaction shifts left toward reactants.
20. What does a small equilibrium constant (Kc ≪ 1) imply about the reaction?
(A) Forward reaction is fast
(B) Reactants are favored
(C) Products dominate
(D) Catalyst is needed
Answer
(B) — A very small K indicates that reactants are much more abundant than products at equilibrium.
21. What is the correct unit for Kc in the reaction: A(g) ⇌ B(g) + C(g)?
(A) mol/L
(B) mol²/L²
(C) Unitless
(D) Depends on reaction coefficients
Answer
(D) — The units of Kc depend on the change in moles of gas (Δn) in the balanced equation.
22. Why does equilibrium shift when temperature changes?
Answer
Temperature alters the energy balance of the system, affecting the rate of forward and reverse reactions differently and shifting equilibrium accordingly.
23. In the reaction: C(s) + CO₂(g) ⇌ 2CO(g), what effect does increasing pressure have?
(A) Shift right
(B) Shift left
(C) No effect
(D) K changes
Answer
(A) — There are more gas moles on the right (2CO), so increasing pressure shifts equilibrium toward fewer gas moles — but since solids are present, effect is more specific to gas partial pressures.
24. Which principle explains the shift in equilibrium when stress is applied?
(A) Hess’s Law
(B) Le Châtelier’s Principle
(C) Dalton’s Law
(D) Graham’s Law
Answer
(B) — Le Châtelier’s Principle predicts the response of an equilibrium system to external changes.
25. What condition is necessary for a reversible reaction to reach equilibrium?
(A) Catalyst must be present
(B) Products must be removed
(C) Reaction must occur in open container
(D) System must be closed to matter
Answer
(D) — A closed system is essential so that reactants and products are not lost, allowing equilibrium to be established.
26. Consider the reaction: 2NO₂(g) ⇌ N₂O₄(g). What effect will decreasing the temperature have?
(A) Shift left
(B) Shift right
(C) No shift
(D) K increases
Answer
(B) — Since the reaction is exothermic, decreasing temperature shifts the equilibrium toward products (right).
27. If Kc = 0.01 for a certain reaction, what does this imply about the concentration of products at equilibrium?
(A) Products dominate
(B) Reactants dominate
(C) Equal amounts
(D) Cannot determine
Answer
(B) — A small Kc means the equilibrium favors reactants; little product is formed.
28. What is the effect of doubling all coefficients in a balanced equilibrium equation?
(A) K remains unchanged
(B) K is squared
(C) K is halved
(D) K becomes zero
Answer
(B) — Multiplying the reaction coefficients by 2 results in squaring the equilibrium constant.
29. In the Haber process (N₂ + 3H₂ ⇌ 2NH₃), how does increasing pressure affect the equilibrium?
(A) Shifts right
(B) Shifts left
(C) No effect
(D) K increases
Answer
(A) — Increasing pressure shifts equilibrium toward the side with fewer gas molecules, which is the product side.
30. What is the value of Δn (change in moles of gas) in the reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)?
(A) 0
(B) +1
(C) –1
(D) –2
Answer
(C) — Δn = (2) – (2 + 1) = –1
31. If the value of Kc for a reaction is 10⁶, what does that tell you about the extent of the reaction?
(A) Reactants dominate
(B) Reaction barely proceeds
(C) Products dominate
(D) Reaction is endothermic
Answer
(C) — A very large Kc means the reaction goes almost to completion; products are heavily favored.
32. The Kc for a reaction increases with temperature. What does this tell you about the reaction?
(A) Reaction is exothermic
(B) Reaction is endothermic
(C) No heat involvement
(D) Reaction is irreversible
Answer
(B) — If K increases with temperature, the reaction absorbs heat and is endothermic.
33. Which of the following equilibrium expressions is correct for: CaCO₃(s) ⇌ CaO(s) + CO₂(g)?
(A) Kc = [CaO][CO₂] / [CaCO₃]
(B) Kc = [CO₂]
(C) Kc = [CaO][CO₂]
(D) Kc = [CO₂] / [CaO]
Answer
(B) — Solids do not appear in the equilibrium expression, so Kc = [CO₂]
34. A reaction has an equilibrium constant of 1. What does this indicate?
(A) Products favored
(B) Reactants favored
(C) Equal concentrations
(D) Reaction does not occur
Answer
(C) — When K = 1, neither side is favored; the system has appreciable amounts of both reactants and products.
35. What change will increase the amount of product in an exothermic reaction at equilibrium?
(A) Add product
(B) Decrease temperature
(C) Increase temperature
(D) Add catalyst
Answer
(B) — Lowering temperature favors the exothermic direction (product side).
36. Explain why a reaction with a high Kc value is considered to go to completion.
Answer
Because a high Kc value means that, at equilibrium, the concentration of products greatly exceeds that of reactants, indicating extensive conversion to products.
37. Describe how Le Châtelier’s Principle applies when pressure is increased in a gaseous system.
Answer
The system will shift toward the side with fewer moles of gas to reduce the pressure.
38. What does it mean if a system has reached dynamic equilibrium?
Answer
The forward and reverse reactions occur at equal rates, and the concentrations of reactants and products remain constant.
39. Explain how temperature affects the equilibrium constant (K).
Answer
For endothermic reactions, increasing temperature increases K; for exothermic reactions, it decreases K.
40. Why is it important that a system be closed for equilibrium to be maintained?
Answer
In an open system, reactants or products could escape, preventing equilibrium from being established or maintained.
41. What happens to the equilibrium position if a catalyst is added?
Answer
The position of equilibrium remains unchanged, but the system reaches equilibrium faster.
42. For the reaction H₂ + I₂ ⇌ 2HI, explain the effect of removing some HI.
Answer
The equilibrium will shift to the right to produce more HI and counteract the removal.
43. Write the expression for Kc for the reaction: 2A(g) + B(g) ⇌ 3C(g)
Answer
Kc = [C]³ / ([A]²[B])
44. What does a reaction quotient (Q) greater than Kc indicate?
Answer
It indicates the system has too much product, so the equilibrium will shift to the left.
45. In the reaction: CO(g) + Cl₂(g) ⇌ COCl₂(g), what effect does increasing [CO] have?
Answer
The system shifts to the right, forming more COCl₂ to reduce the added CO concentration.
46. Describe a situation where no shift in equilibrium occurs despite a change in pressure.
Answer
If the number of moles of gas on both sides of the equation is equal, changing pressure has no effect.
47. What is the purpose of using an ICE table in equilibrium calculations?
Answer
To systematically track Initial concentrations, Changes, and Equilibrium concentrations to solve for unknowns.
48. If the Kp for a reaction is known, how can Kc be calculated?
Answer
Using the formula Kp = Kc(RT)^Δn, where Δn is the change in moles of gas, R is the gas constant, and T is temperature.
49. Why are solids and pure liquids excluded from K expressions?
Answer
Because their concentrations do not change during the reaction, they do not affect equilibrium.
50. What does it indicate if the equilibrium constant is very close to 1?
Answer
It suggests that significant amounts of both reactants and products are present at equilibrium, with neither side strongly favored.