Le Chatelier’s Principle
You may see an equilibrium question that asks you to use
or apply Le Chatelier’s principle on the SAT II Chemistry exam. Le
Chatelier’s principle basically states that if stress is applied
to a system at equilibrium, the position of the equilibrium will
shift in the direction that reduces the stress to reinstate equilibrium.
For example, if more reactants are added to the system, the reaction
will shift in the forward direction, and if more products are added, the
reaction will shift in the reverse direction. If heat is added to
the system and the reaction is exothermic, heat should be thought
of as a product and the reaction will shift to the left; if the
reaction is endothermic and heat is added, the reaction will shift
to the right. The addition of pressure will cause a shift in the
direction that results in the fewer number of moles of a gas, while
if pressure is relieved, the reaction will shift in the direction
that produces more moles of a gas.