3.1 Le Châtelier's Principle

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Think back to our escalator example (section 1-2), with you walking up a downward moving escalator. With the rate of the moving stairs and your walking evenly matched, you appear to be at a standstill. But what happens if the escalator begins moving just a little faster? If you want to maintain the same position you had, at some specific point between the bottom and the top of the stairs, you'll also need to make some adjustments.

Chemical systems at equilibrium tend to make these adjustments as well. A French chemist, Henri Louis Le Châtelier, was the first to describe what we now call Le Châtelier's Principle.

Le Châtelier's Principle

If a system at equilibrium is subjected to an external stress, the equilibrium will shift to minimize the effects of that stress.

Equilibrium is all about rates - the rate of the forward reaction is equal to the rate of the reverse reaction. External stresses are factors that will cause the rate of either the forward or reverse reaction to change, throwing the system out of balance. Le Châtelier's Principle allows us to predict how this will affect our system.

In our unit on Kinetics we examined factors that influenced reaction rates. Recall these factors:

concentration
pressure and volume
temperature, and
catalysts

We'll see how changing these factors affects a system at equilibrium.