Le Chatelier's principle: Class 11 practice
A system at equilibrium shifts to oppose a change. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Balanced equations
- Exothermic/endothermic
The idea behind le chatelier's principle
A system at equilibrium shifts to oppose a change. Adding a reactant shifts it right; raising pressure shifts it to the side with fewer gas moles; raising temperature favours the endothermic direction. A catalyst does not shift equilibrium.
Step-by-step method
- Identify the change.
- Decide which direction reduces that change.
- State the shift and its effect on the product.
Worked example 1 (Basic)
N₂O₄(g) ⇌ 2NO₂(g), ΔH > 0. What is the effect of an increase in pressure?
- Gas moles: left 1, right 2
- Shift: left (fewer gas moles)
- Answer: Shifts left (fewer gas moles)
Worked example 2 (Standard)
N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH < 0. What is the effect of an increase in temperature?
- Forward reaction is exothermic
- Shift: left (endothermic direction)
- Answer: Shifts left (endothermic direction)
Worked example 3 (Challenge)
2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH < 0. What is the effect of an addition of a catalyst?
- A catalyst speeds both directions equally
- Shift: no shift; equilibrium is reached faster
- Answer: Shifts no shift; equilibrium is reached faster
Common mistakes and misconceptions
- Thinking catalysts shift equilibrium
- Counting solids in gas moles
Practice questions
- (Basic) N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH < 0. What is the effect of an increase in pressure?
- (Basic) CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH < 0. What is the effect of an increase in pressure?
- (Standard) PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), ΔH > 0. What is the effect of an increase in temperature?
- (Standard) N₂O₄(g) ⇌ 2NO₂(g), ΔH > 0. What is the effect of an increase in temperature?
- (Challenge) N₂O₄(g) ⇌ 2NO₂(g), ΔH > 0. What is the effect of an addition of a catalyst?
- (Challenge) PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), ΔH > 0. What is the effect of an addition of a catalyst?
Answer key with full working
- Q1: Gas moles: left 4, right 2 → Shift: right (fewer gas moles) → Answer: Shifts right (fewer gas moles)
- Q2: Gas moles: left 3, right 1 → Shift: right (fewer gas moles) → Answer: Shifts right (fewer gas moles)
- Q3: Forward reaction is endothermic → Shift: right (endothermic direction) → Answer: Shifts right (endothermic direction)
- Q4: Forward reaction is endothermic → Shift: right (endothermic direction) → Answer: Shifts right (endothermic direction)
- Q5: A catalyst speeds both directions equally → Shift: no shift; equilibrium is reached faster → Answer: Shifts no shift; equilibrium is reached faster
- Q6: A catalyst speeds both directions equally → Shift: no shift; equilibrium is reached faster → Answer: Shifts no shift; equilibrium is reached faster
Syllabus note
This skill is listed under "Equilibrium" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is le chatelier's principle in?
"Equilibrium", Class 11 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.