AP Chapter 15

 Chemical Equilibrium occurs when opposing reactions are proceeding at equal rates.  It results in the formation of an equilibrium mixture of the reactants and products of the reaction.  The composition mixture does not change with time.

 N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g)  This reaction involves the presence of a catalyst, a pressure of several hundred atmospheres and a temperature of several hundred degrees Celsius.  This equilibrium mixture can be reached regardless of where one starts.

 The relationship between the concentrations of the reactants and the products of a system in equilibrium is given by the law of mass action.  aA + bB ↔ dD + eE  K c = [D] d [E] e ← products [A] a [B] b ← reactants Kc = equilibrium constant

 N 2 O 4 ( g ) ↔ 2NO 2 ( g )  Kc = = = [NO 2 ] 2 [N 2 O 4 ] [0.0172] 2 [ ]

 When the equilibrium system consists of gases, it is convenient to express the concentrations of reactants and products in terms of gas pressures:  Kp =  Kc and Kp are related by: K p = K c (RT) Δn (P D ) d (P E ) e (P A ) a (P B ) b

 CO(g) + Cl 2 (g) ↔ COCl 2 (g)  Kc = = 4.56 x 10 9  For the equilibrium constant to be so large, the numerator must be much larger than the denominator. Therefore, the equilibrium concentration of COCl 2 must be greater than that of CO or Cl 2. [COCl 2 ] [CO][Cl 2 ]

 A large value for the equilibrium constant indicates that the mixture contains more products than reactants and therefore lies towards the product side of the equation.  A small value for the equilibrium constant means the mixture contains less products than reactants and therefore lies toward the reactant side.

 Equilibria for which all substances are in the same phase are called homogeneous equilibria.  Equilibria in which 2 or more phases are present are called heterogeneous equilibria.  The concentrations of pure solids and liquids are left out of the equilibrium constant expression for a heterogeneous equation.

The equilibrium pressure of CO2 (g) is the same in both bell jars, at the same temperature. The equilibrium constant expression is K p = P CO 2.

 If the concentrations of all species in an equilibrium are known, the equilibrium-constant expression can be used to calculate the value of the equilibrium constant.  The changes in the concentrations of reactants and products in the process of achieving equilibrium are governed by the stoichiometry of the equation.

 The reaction quotient, Q, is found by substituting reactant and product concentrations or partial pressures at any point during a reaction into the equilibrium-constant expression.  If the system is at equilibrium, Q = K.  Q ≠ K, the system is not at equilibrium.  Q < K, the reaction will move toward equilibrium by forming more products (it moves from left to right.)  Q > K, the reaction will proceed from right to left.

 Knowing the value of K makes it possible to calculate the equilibrium amounts of the reactants and products, often by solving an equation where the unknown is the change in a partial pressure or concentration.

Predicting the direction of a reaction by comparing Q and K.

 LeChatlier’s principle states: ◦ If a system at equilibrium is disturbed by a change in temperature, pressure or concentration of one of the components, the system will shift its equilibrium position to counteract the effect of the disturbance.

The effect of adding H 2 to an equilibrium mixture of N 2, H 2 and NH 3.

 If a chemical system is at equilibrium and the concentration of a substance is increased (either product or reactant), the system responds by consuming some of the substance.  If some of the concentration is decreased, the system will respond by producing some of the substance.  N 2 + 3H 2 ↔ 2NH 3

 At constant temperature, reducing the volume of a gaseous equilibrium mixture causes the system to shift in the direction that reduces the number of moles of gas.  The system will always favor the side with fewer moles of a gas in order to re-equilibrate.

 When the temperature of a system in equilibrium is increased, the system reacts as though a reactant were added to an endothermic reaction or a product was added to an exothermic reaction.  It will shift in the direction to consume the excess reactant or product, which is heat.  Endothermic:reactants + heat ↔ products  Exothermic:reactants ↔ products + heat

 Adding a catalyst increases the rate at which equilibrium is achieved, but it does not change the composition of the equilibrium mixture.