CHEMICAL EQUILIBRIUM 3/12/07 CHEMICAL EQUILIBRIUM: occurs in a reversible reaction, when the FORWARD reaction rate equals the REVERSE reaction rate. When equilibrium is established, the amounts of reactants and products present are termed the equilibrium concentrations. There will be un-reacted reactants at equilibrium. Many reactions produce little product before equilibrium is reached. The equilibrium constant ( K ) indicates the extent to which the reaction produces product. K values greater than 1 favor high product equilibrium concentrations. K values less than one indicate the reactants are favors to be the predominant species at equilibrium. Equilibrium reactions will have a double arrow  . NOTE : equilibrium is a reversible chemical change in a chemical reaction.

Le Chatelier’s Principle Equilibrium also occurs in physical changes such as phase changes. This is called phase equilibrium, the equilibrium of twp phases at the same time at the same temperature. Le Chatelier’s Principle States that a system (reaction) at equilibrium will resist any external change (stress) that is applied. Stresses include any change in Temperature, Pressure or changing the concentrations of any species in the reaction. Le Chatelier’s Algorithms: If you add to one side of the arrow, the equilibrium shifts (runs) to the opposite side of the arrow. If you remove something on one side of the arrow, the equilibrium will run to the same side.

EXAMPLES: THE EFFECT OF TEMPERATURE ON EQUILIBRIUM REACTIONS. *** What happens if you increase the temperature of the following EXOTHERMIC (Q=- ) reaction? A + 2B   C + D + RULE: Heating an exothermic reaction drives it in reverse, cooling it drives it in forward. Write heat on the PRODUCT side of an exothermic reaction, apply the algorithm as if heat were a chemical species. Heat is on the RIGHT of the reaction, you added to the RIGHT, therefore the reaction shifts LEFT, runs in reverse. The heat is consumed. What you add, the reaction tries to get “rid” of . HEAT +X +2X -X -X REACTION RUNS IN REVERSE, CONSUMES ADDED HEAT, shift away form an addition

RULE: cooling an endothermic reaction drives it in reverse, warming it drives it forward. *** What happens if you increase the temperature of the following ENDOTHERMIC (Q= + ) reaction? + A + 2B   C + D Heat is on the LEFT of the reaction, you added to the LEFT, therefore the reaction shifts RIGHT, runs in forward direction. The heat is consumed. What you add, the reaction tries to get “rid” of . For endothermic reactions write heat on the REACTANT side of the reaction, apply the algorithm as if it were a chemical species. HEAT -X -2X +X +X THIS REACTION RUNS FORWARD. Shift away from an addition.

RULE: cooling an endothermic reaction drives it in reverse, warming it drives it forward. *** What happens if you DECREASE the temperature of the following ENDOTHERMIC (Q= + ) reaction? + A + 2B   C + D Heat is on the LEFT of the reaction, you REMOVED from the LEFT, therefore the reaction shifts LEFT, runs in reverse direction. The heat is consumed. What you remove, the reaction tries to replace. For endothermic reactions write heat on the REACTANT side of the reaction, apply the algorithm as if it were a chemical species. HEAT +X +2X -X -X REACTION SHIFS LEFT, TO A REMOVAL (SUBTRACTION).

RULE: cooling an Exothermic reaction drives it in FORWARD, warming it drives it in REVERSE. *** What happens if you DECREASE the temperature of the following EXOTHERMIC (Q= - ) reaction? H2O(l)   H2O(s) + Heat is on the right of the reaction, you REMOVED from the RIGHT, therefore the reaction shifts LEFT, runs in forward direction. The heat is consumed. What you remove, the reaction tries to replace. For Exothermic reactions write heat on the PRODUCT side of the reaction, apply the algorithm as if it were a chemical species. HEAT -X +X THIS REACTION RUNS IN FORWARD. SHIFT TO A REMOVAL (COOLING REMOVES HEAT).

A IS REMOVED FROM THE LEFT, THERFORE THE REACTION SHIFTS TO THE LEFT RULE: IF A SPECIES IS REMOVED ONE SIDE OF THE REACTION, THE EQUILIBRIUM, WILL RUN TO THE SAME SIDE OF THE ARROW, THE SYSTEM WILL REPLACE WHAT WAS REMOVED. Example, for the reaction below, what will happen if the molarity of A is REDUCED? A + 3B   2C + D -X +3X -2X -X A IS REMOVED FROM THE LEFT, THERFORE THE REACTION SHIFTS TO THE LEFT THE CHANGES THAT OCCUR AFTER THE [A] IS DECREASED THE EQUILIBRIUM CONCENTRATION AFTER A NEW EQUILIBRIUM IS ESTABLISHED +X +3X -2X -X REACTION RUNS IN REVERSE, REPLACES SOME OF REMOVED A. SHIFTS TO A REMOVAL (DECREASED CONCENTRATION)

A IS ADDED TO THE LEFT, THERFORE THE REACTION SHIFTS TO THE RIGHT RULE: IF A SPECIES IS ADDED TO ONE SIDE OF THE REACTION, THE EQUILIBRIUM, WILL RUN TO THE OPPOSITE SIDE OF THE ARROW, THE SYSTEM WILL CONSUME WHAT WAS ADDED. Example, for the reaction below, what will happen if the molarity of A is increased? A + 3B   2C + D +X -3X +2X +X A IS ADDED TO THE LEFT, THERFORE THE REACTION SHIFTS TO THE RIGHT THE CHANGES THAT OCCUR AFTER THE [A] IS INCREASED THE EQUILIBRIUM CONCENTRATION AFTER A NEW EQUILIBRIUM IS ESTABLISHED -X -3X +2X +X THIS REACTION RUNS IN FORWARD. SHIF AWAY FROM AN ADDITION (INCREASED CONCENTRATION