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.