Bell Question: What is the general format for the equation used to calculate equilibrium constants? What does the equilibrium constant tell you about a reaction?

Equilibrium EQUILIBRIUM IN CHEMISTRY 1. CHEMICAL KINETICS (SPEED AND DIRECTION OF RXNS)

Chemical Equilibrium  Both the forward and the reverse reactions are occurring at the same rate of speed  At equilibrium, the amounts of products and reactants are constant  The reaction has reached a point where the forward and reverse reaction are occurring at an equal rate.  Reaction has not stopped on a microscopic level  Reaction has stopped on a macroscopic level

Chemical Equilibrium Determining quantities of reactants and products at equilibrium  ICE method Initial Change Equilibrium

Chemical Equilibrium CO + 3 H 2 ⇌ CH 4 + H 2 O 1 mole3 mole0 mole - X 1 - X - 3 X 3 – 3X + X X X Initial Change Equilibrium mole mole mole

Chemical Equilibrium 2NOCl ⇌ 2NO + Cl mole0 mole - 2 X X X Initial Change Equilibrium mole mole 0.90 mole

Bell Question: Define equilibrium When the speed of the forward and reverse directions of a process are equal.

Chemical Equilibrium Equilibrium Constants (K eq )  When (K eq ) is large (50 and larger), there are more products than reactants  When (K eq ) is small (0.05 and smaller) there are more reactants than products  When (K eq ) is around 1, there are approximately equal amounts of both

Equilibrium Constants

Chemical Equilibrium 2NOCl ⇌ 2NO + Cl 2 Equilibrium mole mole 0.90 mole

Chemical Equilibrium 2NOCl ⇌ 2NO + Cl 2 NO + ½ Cl 2 ⇌ NOCl

Reaction Quotients Reaction Quotients (Q): has the same form as the equilibrium constant, but the concentration values are not necessarily at equilibrium.  When Q > (K eq ), the reaction will go to the left  When Q < (K eq ), the reaction will go to the right  When Q = (K eq ), the reaction is at equillibrium

Reaction Quotients

Homework: p. 684  Reaction Stoichiometry at equilibrium: 21-26,  Values of K eq: 35-38, 39, 40

Bell Question  Have out homework  When Q is bigger than Keq, will the reaction be moving to the left (reactants) or the right (products)?

Reaction Quotient  The reaction quotient for an equilibrium system is the same as the equilibrium expression, but the concentrations are NOT at equilibrium! N 2 O 4 (g)  2NO 2 (g) Q = [NO 2 ] 2 [N 2 O 4 ]

Bell Question: When a reaction has a positive ∆H… was heat absorbed or released?  Review HW

Le Chatlier’s Principle  If a stress is applied to a system at equilibrium, the system will change to relieve that stress and re–establish equilibrium The reaction will try to partially restore itself to the original equilibrium concentrations

Factors that Affect Equilibrium  Concentration  Temperature  Pressure (For gases only) Catalysts don’t change equilibrium (they just change how quickly you reach equilibrium)

Le Chatlier - Concentration Increasing the amount of a product shifts equilibrium to the reactants Increasing the amount of a reactant shifts equilibrium to the products

Example #1: Arsenic, As 4, is obtained from the following reaction As 4 O 6 (S) + 6C  As 4 + 6CO(g)  Predict the direction of the shift in the equilibrium position for this reaction that occurs in response to each of the following changes in conditions. a) Addition of carbon monoxide b) Addition or removal of C(s) or As 4 O 6 (s) c) Removal of As 4 (g)

Le Chatlier: Pressure/Volume Increasing the pressure shifts equilibrium to the side of the reaction with less moles of gas. Decreasing the pressure shifts the equilibrium to the side of the reaction with more moles of gas.

Example #2: Predict the shift in equilibrium position that will occur for each of the following processes when the pressure is reduced. a) The preparation of gaseous phosphorus pentachloride according to the equation PCl 3 (g) + Cl 2 (g)  PCl 5 (g) b) The reaction of phosphorus trichloride with ammonia PCl 3 (g) + 3NH 3 (g)  P(NH 2 ) 3 (g) + 3HCl(g)

Le Chatlier: Temperature If a reaction is exothermic, consider heat as a product  A increase in temperature will favor the reactants If a reaction is endothermic, consider heat as a reactant  An increase in temperature will favor the products

Bell Ringer For each of the following reactions, predict how the equilibrium will shift as the temperature is increased. a) N 2 (g) + O 2 (g) ↔ 2NO(g) (endothermic) b) 2SO 2 + O 2 ↔ 2SO 3 (exothermic)

HW: p 687  # odd only (may need to calculate change in enthalpy)