Le Chatelier's Principle- Practice Problems
Le Chatelier's Principle: Practice Problems That Actually Teach You Something
If you're staring at Le Chatelier's Principle problems and nothing's clicking, you're not alone. This concept trips up more chemistry students than almost any other equilibrium topic. The good news? Once you see the pattern, it becomes automatic.
Let's skip the theoretical lecture. You want practice problems with real explanations. That's what you're getting.
What Le Chatelier's Principle Actually Says
Here's the blunt version: when you mess with a system at equilibrium, it shifts to counteract your changes.
That's it. Three factors can shift equilibrium:
- Concentration changes
- Temperature changes
- Pressure/volume changes
Every problem you encounter will test one of these three. Memorize that list.
Problem #1: Concentration Shifts
The Reaction:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
The Question: What happens when you remove NH₃ from the system?
Solution:
You removed product. The system sees less product and tries to make more. Equilibrium shifts right—toward the products—to replace what you took away.
Simple rule: remove something, shift away from it. Add something, shift toward consuming it.
Problem #2: Concentration With Multiple Changes
The Reaction:
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
The Question: What happens when you add SO₂ AND remove O₂ simultaneously?
Solution:
Don't panic. Handle each change separately.
Adding SO₂ shifts equilibrium right. Removing O₂ shifts equilibrium left. The net effect depends on which change is "stronger"—but in most textbook problems, they want you to identify the competing directions.
Answer: ambiguous without more information. Both changes pull the equilibrium in opposite directions. Real chemists would need concentration values to determine the dominant shift.
Problem #3: Temperature Changes
The Reaction:
N₂(g) + O₂(g) ⇌ 2NO(g) ΔH = +180 kJ/mol
The Question: What happens when temperature increases?
Solution:
This is where students mess up. Temperature isn't just a "stress"—it changes K (the equilibrium constant itself).
Endothermic reactions (ΔH > 0): heat is a reactant. Increasing temperature shifts right, and K increases.
Exothermic reactions (ΔH < 0): heat is a product. Increasing temperature shifts left, and K decreases.
Our reaction has ΔH = +180 kJ/mol. It's endothermic. Increasing temperature shifts equilibrium right, and K gets larger.
Decreasing temperature would shift left and K would shrink.
Problem #4: Pressure/Volume Changes
The Reaction:
2NO₂(g) ⇌ N₂O₄(g)
The Question: What happens when you increase the pressure (decrease volume)?
Solution:
Count gas molecules on each side.
Left side: 2 moles of gas. Right side: 1 mole of gas.
Higher pressure favors the side with fewer gas molecules. Equilibrium shifts right toward N₂O₄.
If the number of gas molecules is equal on both sides, pressure changes don't affect equilibrium. This trips people up constantly.
Problem #5: Combined Stresses
The Reaction:
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) ΔH = +87.9 kJ/mol
The Question: The system is at equilibrium. You increase the temperature AND decrease the pressure. Predict the direction of shift.
Solution:
Break it down:
Temperature increase: Reaction is endothermic, so heat acts as a reactant. Shift right.
Pressure decrease: Left side has 1 mole of gas, right side has 2 moles. Lower pressure favors more gas molecules. Shift left.
Both changes oppose each other. Without quantitative data, you can't definitively say which wins. The honest answer: the shifts oppose, and the net effect is unclear.
Your professor might expect you to identify this as an "unpredictable" situation. Don't force an answer when the problem doesn't give you enough information.
Common Mistakes That Cost Points
| Mistake | Why It's Wrong | Correct Approach |
|---|---|---|
| Ignoring ΔH for temperature problems | Temperature changes K, not just position | Check if reaction is endo- or exothermic first |
| Assuming all pressure changes shift equilibrium | Only matters if gas moles differ | Count gas molecules on each side |
| Forgetting to handle changes separately | Multiple changes can oppose each other | Evaluate each stress independently |
| Confusing "shift" with K change | These are different concepts | Temperature changes K; others only shift position |
Getting Started: Your Problem-Solving Checklist
Before you write anything on an exam, run through this:
- What changed? Concentration, temperature, or pressure?
- Does ΔH matter? Only for temperature problems.
- Which direction? Use the rule: oppose the change.
- Is K affected? Only temperature changes the equilibrium constant.
- Are multiple stresses opposing? Say so if they are.
This isn't complicated once you stop overthinking it. The principle is straightforward—systems fight back against disturbance. Your job is identifying which way they fight.
The Bottom Line
Le Chatelier's Principle problems follow patterns. Concentration shifts are straightforward. Temperature shifts require checking the sign of ΔH. Pressure shifts only matter when gas moles differ. When changes conflict, admit you can't determine the outcome without numbers.
Work through the problems above until the pattern clicks. Then tackle your homework. You'll stop second-guessing yourself within a day or two.