Chemical Equilibrium

"Can we influence an equilibrium reaction in any way? Are we stuck with whatever mixture is left after a reaction has come to its final equilibrium condition? That is, are we stuck with so much of each substance, A, B, C and D? Or can we tamper with those "final" equilibrium amounts? Can we force the reactions to give us more of what we want (for example, C and D) and less of what we don't want (for example, A and B)?"

Suppose we have a reaction that has already come to equilibrium:

There are certain equilibrium amounts of all four chemicals. But if the number of B molecules is increased, the As and Bs will continue to react because there are now more Bs for the As to collide into extra Cs and Ds are produced, and fewer As. The same thing would happen if we added some extra As instead of Bs: we'd still get an extra yield of Cs and Ds.

Why would we want to do this? Well, maybe we're a manufacturer of C or D, and the further we can push the reaction to produce more of them, the more we can sell. We have succeeded in pushing the equilibrium point in the direction we want—toward more C and D—by adding Bs alone; no extra As were needed. If we'd wanted to, we could have gotten the same result by adding some extra As instead of Bs, whichever one is cheaper.

We could also have accomplished the desired result—more C and D—not by speeding up the collision rate of As and Bs, but by slowing down the collision rate of Cs and Ds. That is, instead of adding extra As or Bs, we could keep removing some of the Cs or D's from the pot as they are formed. Then the A and B will keep colliding and producing more, unhindered by the building up of the backward process. We'd get a 100% reaction between A and B.

In both cases, it appears that when we changed the amount of any one of the chemicals (by adding or removing some) we imposed an unnatural stress upon the equilibrium system. The system then tried to relieve that stress by using up some of the added substance or by producing more of the removed substance. In 1884, French chemist Henry Le Châtelier (1850-1936) realized that these shifts in the equilibrium amounts of chemicals were part of a bigger principle, which now goes by his name. In general terms, Le Châtelier's principle can be stated this way: When any kind of stress is imposed upon a chemical system that has already reached its equilibrium condition, the system will automatically adjust the amounts of the various substances in such a way as to relieve that stress as much as possible.

Le Châtelier's principle is being applied every day in manufacturing industries to maximize efficiency of chemical reactions.