AP Chemistry – Unit 7 – Equilibrium
Practice equilibrium concepts including K, Q, Le Châtelier’s Principle, and shifts in equilibrium.
Question 1
At equilibrium, the concentrations of reactants and products:
A. are equal
B. remain constant
C. are zero
D. are changing
... Correct Answer: B Why: At equilibrium, concentrations remain constant over time, but they are not necessarily equal. Common Mistake: Students think equilibrium means equal amounts. Fix It: Equilibrium = constant, NOT equal
Question 2
If K > 1, the reaction favors:
A. reactants
B. products
C. neither
D. equilibrium shifts left
... Correct Answer: B Why: A large K means products are favored at equilibrium. Common Mistake: Confusing direction of shift. Fix It: K > 1 → products dominate
Question 3
If Q < K, the reaction will:
A. shift left
B. shift right
C. stay same
D. stop
... Correct Answer: B Why: When Q < K, more products must form to reach equilibrium. Common Mistake: Mixing up Q vs K comparison. Fix It: Q
Question 4
Adding more reactant will shift equilibrium:
A. left
B. right
C. no change
D. stop
... Correct Answer: B Why: System shifts to consume added reactant. Common Mistake: Forgetting Le Châtelier’s Principle. Fix It: Add reactant → shift right
Question 5
Increasing temperature for an endothermic reaction shifts:
A. left
B. right
C. no change
D. depends on pressure
... Correct Answer: B Why: Heat acts like a reactant in endothermic reactions. Common Mistake: Not identifying reaction type. Fix It: Endothermic → add heat → shift right
Question 6
A catalyst will:
A. change K
B. shift equilibrium
C. speed up reaction
D. increase products
... Correct Answer: C Why: Catalysts speed up both forward and reverse reactions equally. Common Mistake: Thinking catalyst changes equilibrium position. Fix It: Catalyst = faster, not different
Question 7
Increasing pressure favors side with:
A. more gas molecules
B. fewer gas molecules
C. no molecules
D. liquids
... Correct Answer: B Why: System shifts to reduce pressure → fewer gas particles. Common Mistake: Forgetting gas count matters. Fix It: High pressure → fewer moles side
Question 8
If K is very small, the reaction favors:
A. products
B. reactants
C. equilibrium
D. forward reaction
... Correct Answer: B Why: Small K means very little product is formed. Common Mistake: Confusing large vs small K. Fix It: K
Question 9
Equilibrium is reached when:
A. reaction stops
B. forward = reverse rate
C. products equal reactants
D. no molecules move
... Correct Answer: B Why: Dynamic equilibrium means reactions continue but at equal rates. Common Mistake: Thinking reaction stops. Fix It: Equilibrium = dynamic, not static
Question 10
Which change does NOT affect equilibrium?
A. concentration
B. pressure
C. catalyst
D. temperature
... Correct Answer: C Why: Catalysts do not change equilibrium position. Common Mistake: Thinking catalyst shifts reaction. Fix It: Catalyst = speed only
Question 11
For the reaction:
N₂ + 3H₂ ⇌ 2NH₃
If pressure increases, equilibrium shifts:
A. left
B. right
C. no change
D. depends on temperature
... Correct Answer: B Why: Left side has 4 moles gas, right side has 2 → shift to fewer moles. Common Mistake: Not counting gas moles correctly. Fix It: High pressure → fewer gas moles
Question 12
Which change will NOT shift equilibrium?
A. adding reactant
B. changing temperature
C. adding catalyst
D. removing product
... Correct Answer: C Why: Catalysts do not affect equilibrium position. Common Mistake: Thinking catalyst shifts equilibrium. Fix It: Catalyst = speed only
Question 13
If K is very large, the reaction:
A. favors reactants
B. favors products
C. does not occur
D. is slow
... Correct Answer: B Why: Large K means products dominate. Common Mistake: Confusing rate with equilibrium. Fix It: K = position, not speed
Question 14
If Q = K, the system:
A. shifts right
B. shifts left
C. is at equilibrium
D. stops
... Correct Answer: C Why: Q = K means equilibrium already reached. Common Mistake: Thinking reaction stops. Fix It: Equilibrium = dynamic
Question 15
Which factor changes the value of K?
A. concentration
B. pressure
C. catalyst
D. temperature
... Correct Answer: D Why: Only temperature changes equilibrium constant. Common Mistake: Thinking pressure changes K. Fix It: Only temp affects K
Question 16
Removing product causes equilibrium to:
A. shift left
B. shift right
C. no change
D. stop
... Correct Answer: B Why: System produces more product to replace removed amount. Fix It: Remove product → shift right
Question 17
If a reaction is endothermic, increasing temperature will:
A. decrease K
B. increase K
C. no change
D. stop reaction
... Correct Answer: B Why: Heat is reactant → adding heat increases product formation → larger K. Common Mistake: Not connecting temp and K. Fix It: Endothermic + heat → K increases
Question 18
Which expression is correct for equilibrium constant?
A. reactants/products
B. products/reactants
C. sum of concentrations
D. difference
... Correct Answer: B Why: K = products / reactants (each raised to coefficients). Common Mistake: Reversing fraction. Fix It: Products on top
Question 19
Which species are NOT included in K expression?
A. gases
B. aqueous
C. solids
D. ions
... Correct Answer: C Why: Pure solids and liquids are omitted. Common Mistake: Including everything. Fix It: Only (aq) and (g) count
Question 20
If volume decreases, pressure:
A. decreases
B. increases
C. stays same
D. becomes zero
... Correct Answer: B Why: Boyle’s Law: lower volume → higher pressure. Common Mistake: Forgetting inverse relationship. Fix It: Volume ↓ → Pressure ↑
FRQ
Explain how a system at equilibrium responds to an increase in temperature for an endothermic reaction.
... Key Points: - heat is a reactant - system shifts right - more products formed Sample Answer: In an endothermic reaction, heat acts as a reactant. Increasing temperature adds more heat, causing the equilibrium to shift to the right to consume the added heat, resulting in more product formation.