A General Statement of the Laws of Chemical Equilibrium
Disturb a system at equilibrium, and it shifts to partly undo the disturbance.
Push on a chemical reaction that has settled into balance — heat it, squeeze it, pour in more of one ingredient — and it pushes back, shifting to partly undo what you did.
The idea, unpacked
Many reactions don't run to completion; they reach a balance point where the forward and reverse reactions go at the same rate, and the amounts stop changing. That's chemical equilibrium. Le Chatelier asked: if you disturb that balance, which way does it move?
His answer is a single rule of thumb that always works. The system shifts in whatever direction eases the disturbance. Heat it, and it moves the way that soaks up heat. Squeeze a gas mixture into less space, and it moves toward the side that has fewer gas molecules, relieving the pressure. Add more of an ingredient, and it gets used up. The reaction, in effect, resists being pushed.
An engineer after one rule
Henry Le Chatelier was a French mining engineer who spent his career on furnaces, cements, explosives and the safety of mines. Chemists of his day knew dozens of separate facts about how heat and pressure tipped reactions one way or the other, but no overarching law tied them together. In 1884, in a short note to the French Academy of Sciences, Le Chatelier stated one — a principle of moderation from which all the separate cases followed. The German physicist Karl Ferdinand Braun arrived at essentially the same rule a couple of years later, so it is sometimes named for both. Neither man could prove it from deeper theory at the time; that rigour came from the thermodynamics being built by Gibbs and van 't Hoff.
Why it mattered
The rule turned equilibrium from a thing to observe into a thing to engineer. If you know which way a reaction will move, you can choose the temperature and pressure that push it where you want. The supreme example is making ammonia from the nitrogen in the air — the Haber–Bosch process (see Haber 1909), which feeds roughly half the people alive by turning air into fertilizer. Le Chatelier's principle tells you exactly how to coax the reaction toward ammonia: high pressure, modest temperature. The same logic runs through the chemistry of the blood, the carbon dioxide balance of the oceans, and a thousand industrial reactors.
A crowded room
Picture a comfortable, evenly spread crowd in a room — that's the equilibrium. Now crank up the heat: people drift toward the cool spots, easing the change. Shove everyone into half the space: they spread back out as much as they can, pushing against the squeeze. Open a door and pour more people in on one side: they redistribute until it evens out again. In every case the crowd rearranges itself to soften whatever you did to it — which is exactly how a chemical equilibrium answers a disturbance.
Before and after
The deep theory came first but was hard to use: Gibbs (1876) had written the thermodynamics of equilibrium in forbidding generality, and van 't Hoff was quantifying how equilibrium constants change with temperature. Le Chatelier distilled a usable rule out of that world. A generation later it became the playbook for Haber (1909) and the whole chemical industry; today it explains ocean acidification and the fizz of your drink. In the Library it sits between the thermodynamics of Gibbs (1876) and the reactor of Haber (1909).
Tout système en équilibre chimique stable soumis à l'influence d'une cause extérieure qui tend à faire varier soit sa température, soit sa condensation (pression, concentration, nombre de molécules dans l'unité de volume) dans sa totalité ou seulement dans quelques-unes de ses parties, ne peut éprouver que des modifications intérieures, qui, si elles se produisaient seules, amèneraient un changement de température ou de condensation de signe contraire à celui résultant de la cause extérieure.