Le Chatelier's principle, as a way of thinking
Stop memorising shift directions. Le Chatelier's principle is one idea you can reason from every time, across concentration, pressure and temperature.
6 June 2026 · 2 min read
Le Chatelier's principle is usually taught as a table of "shifts left / shifts right" to memorise. That table is exam poison: under pressure you forget a row. The principle itself is a single sentence you can reason from every time.
The one idea
If you disturb a system at equilibrium, it responds in the direction that partly undoes your disturbance.
That is the whole thing. Every question is just "what did we disturb, and which way undoes it?"
The three levers
There are only three things an HSC question can change. For each, ask the same question.
Concentration
Add more of a species, and the system consumes some of it. Add reactant, the equilibrium shifts toward products. Remove product, it shifts to replace it. You are not memorising a direction, you are reading off the response that opposes the change.
Pressure and volume
This lever only matters when gases are involved, and only because pressure depends on the number of gas molecules. Squeeze the volume and you raise the pressure, so the system shifts toward the side with fewer gas molecules to relieve it. Count moles of gas on each side, then let the principle decide.
Temperature
Treat heat as if it were a reactant or product. For an exothermic reaction, heat is a product. Raise the temperature and you have added a product, so the system shifts back toward reactants. This is the lever students get wrong most, because it is the only one that also changes the value of the equilibrium constant.
A worked example
Consider the Haber process:
N2(g) + 3 H2(g) ⇌ 2 NH3(g), which is exothermic.
You want more ammonia. Reason through the levers:
- Pressure: four moles of gas become two. High pressure favours the smaller side, so favours ammonia.
- Temperature: the forward reaction releases heat, so low temperature favours ammonia. But too low and the rate crawls, which is why industry compromises.
- Concentration: removing ammonia as it forms keeps pulling the equilibrium forward.
No table required. You derived the industrial conditions from one sentence.
The trace: a marker can tell the difference between a memorised direction and a reasoned one. The reasoned answer survives the question you did not revise for.