EQUILIBRIUM EXPRESSIONS The equilibrium expression for the general equation on the previous slide is written as follows: In this equation, the brackets,[ ], stand for molar concentrations of the reactants, A and B, and the products, W and X. It is seen that each reactant concentration is raised to a power equal to the stoichiometric coefficient of that reactant in the equilibrium equation.

EQUILIBRIUM EXPRESSIONS (continued) This is demonstrated for the following equilibrium: 2NO (g) + 2H 2 (g) ⇆ N 2 (g) + 2H 2 O (g)

EQUILIBRIUM CONSTANT The K in an equilibrium expression is called the equilibrium constant. As long as the temperature does not change, it has a constant value because none of the concentrations used to express it change with time once equilibrium is established. A relatively large value for K indicates that the equilibrium position is toward the right or products side of the equilibrium. A small K indicates an equilibrium position toward the left or reactant side of the equilibrium. productsright reactants left or

THE RANGE OF K VALUES The values for K that have been found experimentally range between wide extremes. Some, such as, K = 1.1 x , are so small that for all practical purposes an equilibrium mixture would contain only reactants and the equilibrium position is extremely far to the left. Others, such as K = 1.2 x10 40, are so large that for all practical purposes an equilibrium mixture would contain only products and the equilibrium position is extremely far to the right.

FACTORS THAT INFLUENCE THE POSITION OF EQUILIBRIUM According to Le Châtelier's principle, the position of equilibrium shifts in response to changes made in the equilibrium. The factors that will be considered are: concentrations of reactants and products reaction temperature catalysts In general, Le Châtelier's principle predicts a shift away from the side to which something is added and toward the side from which something is removed.

FACTORS THAT INFLUENCE THE POSITION OF EQUILIBRIUM (continued) If reactants are added, the position of equilibrium will shift toward the products. If reactants are removed, the position of equilibrium will shift toward the reactants. If products are added, the position of equilibrium will shift toward the reactants. If products are removed, the position of equilibrium will shift toward the products. When the temperature of a reaction is increased, heat is added. If the reaction is endothermic, heat is a reactant and the position of equilibrium will shift toward the products. If the reaction is exothermic, heat is a product and the position of equilibrium will shift toward the reactants.

FACTORS THAT INFLUENCE THE POSITION OF EQUILIBRIUM (continued) When the temperature of a reaction is decreased, heat is removed. If the reaction is endothermic, heat is a reactant and the position of equilibrium will shift toward the reactants. If the reaction is exothermic, heat is a product and the position of equilibrium will shift toward the products. Catalysts cannot change the position of equilibrium because they lower the energy barrier for both the forward and reverse reactions; therefore, catalysts speed up both forward and reverse reactions and cannot change the position of equilibrium.

Le Châtelier's PRINCIPLE EXAMPLE Consider the following endothermic reaction at equilibrium: heat + 4NO 2 (g) + 6H 2 O(g) 7O 2 (g) + 4NH 3 (g) If an equilibrium mixture was heated, the equilibrium position would shift toward the right to try to use up the added heat. If some NO 2 was added to an equilibrium mixture, the equilibrium position would again shift toward the right to try to use up the added NO 2. If some NH 3 was removed from an equilibrium mixture, the equilibrium position would again shift toward the right in an attempt to replace the NH 3 that was removed. If a catalyst were added, the equilibrium position would not change.