Equilibrium & Le Chatelier
Some reactions run both ways at once. When the two directions tie, nothing seems to happen — and everything is happening.
Builds on: 5.2 Reaction Rates & Collisions
The two-way street
Many reactions are reversible: A turns into B while B turns back into A. Start with pure A and the forward traffic dominates; as B piles up, the reverse traffic grows. Eventually the two rates tie — and from the outside all change stops. This is dynamic equilibrium: not a ceasefire but a perfectly balanced exchange. Every second, millions of particles convert each way; the totals just no longer move.
Le Chatelier: the stubborn system
Disturb an equilibrium and it shifts to partially undo the disturbance. That one sentence — Le Chatelier’s principle — predicts an enormous amount of chemistry:
- Add reactant → the system burns some of it off: shifts toward products.
- Remove product (siphon it away as it forms) → the system replaces it: shifts toward products. This is industry’s favourite lever.
- Heat an exothermic reaction → heat is a product, so adding it pushes backwards. Cooling pulls forward.
- Compress a gas equilibrium → it shifts toward the side with fewer gas molecules, relieving the pressure.
N₂ + 3 H₂ ⇌ 2 NH₃ (ammonia for fertilizer) is exothermic and shrinks 4 gas molecules into 2. Le Chatelier prescribes: high pressure (shift right), moderate temperature (too cold is slow — kinetics again!), and constantly siphoning off the ammonia. The Haber-Bosch plants built on this reasoning feed roughly half of humanity. One principle, four billion lunches.
⚗️ Lab — The Two-Way Street
80 particles flicker between A (red) and B (cyan); the forward reaction is exothermic.
- Wait for Q ≈ K — the bars freeze while the particles never do. That’s “dynamic”.
- Dump in 20 A and watch the system eat most of them into B.
- Remove B repeatedly — can you starve the equilibrium? Now heat it and watch it run backwards.