Q: Same form as K, but uses any concentrations (not just equilibrium)
For reaction: aA + bB ⇌ cC + dD
Q=[A]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!
📚 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:
Q=[SO2]2[O2][SO3]2
Substitute values:
Q=(0.40)2(0.30)(1.2)
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 exothermic reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat. Predict how the equilibrium will shift (and explain) when: (a) N₂ is added, (b) NH₃ is removed, (c) pressure is increased, (d) temperature is increased.
💡 Show Solution
Solution:
(a) Adding N₂ (reactant):
Equilibrium shifts right (toward products)
System consumes added N₂ to re-establish equilibrium
More NH₃ is produced
(b) Removing NH₃ (product):
Equilibrium shifts right (toward products)
System produces more NH₃ to replace what was removed
More N₂ and H₂ are consumed
(c) Increasing pressure:
3Problem 3medium
❓ 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.
For the reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) with K_c = 0.042 at 250°C, a mixture is prepared with [PCl₅] = 0.20 M, [PCl₃] = 0.10 M, and [Cl₂] = 0.10 M. (a) Calculate Q and determine which direction the reaction will proceed. (b) Calculate the equilibrium concentrations.
Compare: Q = 0.050 vs K_c = 0.042
Q > K, so reaction shifts left (toward reactants)
(b) Calculate equilibrium concentrations:
Let x = change in concentration
PCl₅
5Problem 5hard
❓ 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.
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 5 problems provided, checking solutions as you go. Regular review and active practice are key to retention.
Is this Reaction Quotient and Le Chatelier's Principle study guide free?▾
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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.
Are there practice problems for Reaction Quotient and Le Chatelier's Principle?▾
Yes, this page includes 5 practice problems with detailed solutions. Each problem includes a step-by-step explanation to help you understand the approach.