Le Chatelier's principle
If a system at equilibrium is disturbed, it shifts to partly cancel the change. Adding reactant shifts it toward products. Raising pressure shifts it toward fewer gas moles. Heating shifts it toward the endothermic direction.
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Analogy
A crowded seesaw: push one side down and the rider on the other side shifts to rebalance.
Example
Add N₂: shifts rightRaise pressure: shifts rightRaise temperature: shifts leftAdd catalyst: no shift
In practice
In the Haber process, high pressure favours ammonia because 4 moles of gas make 2.
Practice
Flashcards
- What does Le Chatelier's principle say?A system at equilibrium shifts to partly oppose any change applied to it.
Interview questions
- State Le Chatelier's principle and predict how changing pressure, temperature and adding catalyst affect N₂ + 3H₂ ⇌ 2NH₃ (exothermic).
- System opposes the change
- Higher pressure shifts right (fewer moles)
- Higher temperature shifts left (exothermic)
- Catalyst changes rate, not position
Questions people ask
Does a catalyst shift the equilibrium?
No. A catalyst speeds up the forward and reverse reactions equally. The system reaches equilibrium faster, but the final concentrations and K are the same as without it. Pressure, concentration and catalysts never change K. Only a change in temperature does, because it alters the balance between forward and reverse reactions.
How does temperature change K?
Heating an endothermic reaction increases K, because heat acts like a reactant and shifts the equilibrium to products. Heating an exothermic reaction decreases K, because heat acts like a product. Changes in concentration and pressure shift the position but leave K unchanged.