Q: Same form as K, but uses any concentrations (not just equilibrium)
For reaction: aA + bB ⇌ cC + dD
Q=[A]
📚 Practice Problems
1Problem 1easy
❓ Question:
For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), K_c = 4.0 at 1000 K. At a certain time, [SO₂] = 0.40 M, [O₂] = 0.30 M, [SO₃] = 1.2 M. Calculate Q and predict the direction of reaction.
💡 Show Solution
Given:
Reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
K_c = 4.0
Current: [SO₂] = 0.40 M, [O₂] = 0.30 M, [SO₃] = 1.2 M
Calculate Q:
K/Q expression:
Explain using:
📋 AP Chemistry — Exam Format Guide
⏱ 3 hours 15 minutes📝 67 questions📊 3 sections
Section
Format
Questions
Time
Weight
Calculator
Multiple Choice
MCQ
60
90 min
50%
✅
Free Response (Long)
FRQ
3
69 min
30%
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Free Response (Short)
FRQ
4
36 min
20%
✅
📊 Scoring: 1-5
5
Extremely Qualified
~12%
4
Well Qualified
~16%
3
Qualified
~24%
2
Possibly Qualified
~24%
1
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~24%
💡 Key Test-Day Tips
✓Memorize common polyatomic ions
✓Practice dimensional analysis
✓Know your gas laws
⚠️ Common Mistakes: Reaction Quotient and Le Chatelier's Principle
Avoid these 3 frequent errors
🌍 Real-World Applications: Reaction Quotient and Le Chatelier's Principle
See how this math is used in the real world
📝 Worked Example: Stoichiometry — Limiting Reagent
Problem:
2 mol of H2 reacts with 1 mol of O2. How many grams of water are produced? Which is the limiting reagent? (2H2+O2→2H2O)
What is Reaction Quotient and Le Chatelier's Principle?▾
Use Q to predict reaction direction and apply Le Chatelier's principle to predict equilibrium shifts.
How can I study Reaction Quotient and Le Chatelier's Principle effectively?▾
Start by reading the study notes and working through the examples on this page. Then use the flashcards to test your recall. Practice with the 3 problems provided, checking solutions as you go. Regular review and active practice are key to retention.
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What course covers Reaction Quotient and Le Chatelier's Principle?▾
Reaction Quotient and Le Chatelier's Principle is part of the AP Chemistry course on Study Mondo, specifically in the Chemical Equilibrium section. You can explore the full course for more related topics and practice resources.
a
[
B
]b
[C]c[D]d
Difference from K:
K: only at equilibrium
Q: at any moment
Same expression, different values
Comparing Q and K
Predicting direction:
Comparison
Direction
What happens
Q < K
Forward →
Make more products
Q = K
Equilibrium
No net change
Q > K
Reverse ←
Make more reactants
Memory aid:
Q < K: Need more products (go forward)
Q > K: Too many products (go backward)
Example:
For N₂ + 3H₂ ⇌ 2NH₃, K = 0.50
If [N₂] = 1.0, [H₂] = 1.0, [NH₃] = 0.50:
Q=[N2][H2]3[NH3]2=(1.0)(1.0)3(0.50)2=0.25
Q (0.25) < K (0.50) → Forward reaction proceeds
Le Chatelier's Principle
"When stress applied to equilibrium, system shifts to relieve stress"
Types of stress:
Concentration changes
Pressure/volume changes
Temperature changes
Concentration Changes
Add reactant:
Shift right (toward products)
Consume added reactant
Make more products
Add product:
Shift left (toward reactants)
Consume added product
Make more reactants
Remove reactant:
Shift left (toward reactants)
Replace removed reactant
Remove product:
Shift right (toward products)
Replace removed product
Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Change
Shift
Why
Add N₂
Right →
Consume excess N₂
Remove H₂
Left ←
Replace removed H₂
Add NH₃
Left ←
Consume excess NH₃
Note: Adding/removing solid or pure liquid has NO effect
Pressure/Volume Changes
For reactions with gases:
Decrease Volume (Increase Pressure):
Shifts toward side with fewer moles of gas
Relieves pressure by reducing gas molecules
Increase Volume (Decrease Pressure):
Shifts toward side with more moles of gas
Fills space with more gas molecules
Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Count moles gas:
Left: 1 + 3 = 4 moles
Right: 2 moles
Increase pressure: Shift right (fewer moles)
Decrease pressure: Shift left (more moles)
Special case: Δn = 0 (equal moles)
Pressure change: no shift
Example: H₂ + I₂ ⇌ 2HI (2 moles each side)
Adding Inert Gas:
At constant volume: No shift (partial pressures unchanged)
At constant pressure: Volume increases, shift to more moles
Temperature Changes
Temperature is different - actually changes K value!
Exothermic Reaction (ΔH < 0):
Think of heat as product:
A + B ⇌ C + D + heat
Increase T: Shift left (consume heat, favor reactants)
K decreases
Decrease T: Shift right (produce heat, favor products)
K increases
Endothermic Reaction (ΔH > 0):
Think of heat as reactant:
A + B + heat ⇌ C + D
Increase T: Shift right (use heat, favor products)
K increases
Decrease T: Shift left (less heat available, favor reactants)
K decreases
Summary:
Reaction Type
Increase T
K value
Exothermic (ΔH < 0)
Shift left
Decreases
Endothermic (ΔH > 0)
Shift right
Increases
Catalyst Effect
Catalyst:
Speeds up both forward AND reverse reactions equally
NO shift in equilibrium position
NO change in K
Reaches equilibrium faster
Summary of Le Chatelier
Effect on equilibrium position:
Stress
Equilibrium Shift
K Changes?
Add/remove substance
Yes
No
Change pressure/volume
Yes (if Δn ≠ 0)
No
Change temperature
Yes
YES
Add catalyst
No shift
No
Only temperature changes K value!
Q=[SO2]2[O2][SO3]2
Substitute values:
Q=(0.40)2(0.30)(1.2)2
Q=(0.16)(0.30)1.44
Q=0.0481.44
Q=30
Answer: Q = 30
Compare Q and K:
Q = 30, K = 4.0
Q > K
Meaning:
Too many products (SO₃) relative to equilibrium
System will shift LEFT (reverse)
Consume SO₃, make more SO₂ and O₂
Direction: ← Reverse (toward reactants)
Summary:
Value
Amount
Q
30
K
4.0
Comparison
Q > K
Direction
Reverse (←)
Until Q = K, reaction proceeds in reverse direction.
2Problem 2medium
❓ Question:
For the equilibrium: N₂O₄(g) ⇌ 2NO₂(g), ΔH° = +57.2 kJ. Predict how each change affects the equilibrium: (a) Add N₂O₄, (b) Decrease volume, (c) Increase temperature, (d) Add catalyst.
Answer: Shifts right, increases [NO₂], K increases
(d) Add catalyst
Type: Catalyst
Effect:
Speeds up both forward and reverse equally
NO shift in position
NO change in K
Reaches equilibrium faster only
Answer: No shift, no change in concentrations or K
Summary:
Change
Direction
[NO₂]
K Changes?
(a) Add N₂O₄
Right →
Increases
No
(b) Decrease volume
Left ←
Decreases
No
(c) Increase T
Right →
Increases
Yes (↑)
(d) Add catalyst
No shift
No change
No
3Problem 3hard
❓ Question:
For the exothermic reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH° = -198 kJ. To maximize SO₃ production, should you use: (a) high or low temperature? (b) high or low pressure? (c) Explain any trade-offs.
Key lesson: Industrial processes balance thermodynamics (K) and kinetics (rate)!
Are there practice problems for Reaction Quotient and Le Chatelier's Principle?▾
Yes, this page includes 3 practice problems with detailed solutions. Each problem includes a step-by-step explanation to help you understand the approach.