Le Chatelier's principle describes how a system at dynamic equilibrium responds to a stress by doing what?

• A. The system changes to relieve the stress.
• B. The system speeds up the reaction.
• C. The system becomes completely unstable.
• D. The system stops reacting.

Answer: (A)

Le Chatelier's principle says a system at dynamic equilibrium will shift the balance of forward and reverse reactions to counteract the applied change. The idea is that the system acts to relieve the stress, not to speed up the reaction or become unstable. So when you mess with something like concentrations, pressure, or temperature, the reaction adjusts its position so that the effect of that disturbance is reduced. For example, adding more of a reactant pushes the equilibrium toward more products, removing some of the added reactant in effect. Raising the pressure of a gas mix makes the system favor the side with fewer gas moles, lowering the pressure change. Temperature changes tilt the balance depending on whether the reaction is exothermic or endothermic, again to offset the disturbance. In all cases, the observed result is the system rearranging itself to relieve the stress.