Chapter 16 – Chemical Equilibria Objectives: 1.Determine equilibrium constant expressions. 2.Know the factors which affect equilibrium. 3.Calculate Q and K and predict effects on equilibrium. 4.Perform equilibrium calculations. 5.Recall the Le Chatelier’s principle and predict effects on equilibrium.

Chemical Equilibrium Equilibrium systems are: DYNAMIC (in constant motion) REVERSIBLE (can be approached from either direction). Pink to blue Co(H 2 O) 6 Cl 2 ---> Co(H 2 O) 4 Cl H 2 O Blue to pink Co(H 2 O) 4 Cl H 2 O ---> Co(H 2 O) 6 Cl 2

Dynamic Equilibrium Fe 3+ + SCN -  FeSCN 2+ After a period of time, the concentrations of reactants and products are _____________.After a period of time, the concentrations of reactants and products are _____________. The forward and reverse reactions __________ after equilibrium is attained.The forward and reverse reactions __________ after equilibrium is attained. +

Chemical Equilibrium In Phase Changes: In the formation of stalactites and stalagmites: CaCO 3 (s) + H 2 O(liq) + CO 2 (g) Ca 2+ (aq) + 2 HCO 3 - (aq)

Reaction Quotient & Equilibrium Constant Equilibrium achieved Product concentration increases; it becomes constant at equilibrium Reactants concentration decrease; then become constant at equilibrium

Reaction Quotient & Equilibrium Constant At any point in the reaction: Q = [HI] 2 [H 2 ][I 2 ] At Equilibrium: K = [HI] 2 = 55.3 [H 2 ][I 2 ] ALL reacting chemical systems are characterized by their REACTION QUOTIENT, Q. When Q = K, the system is at equilibrium. In general, for a reaction: aA + bB cC + dD Q = [C] c [D] d [A] a [B] b Product concentrations Reactant concentrations

Reaction Quotient & Equilibrium Constant For any time of chemical equilibrium of the type: The following is a constant: If K is known, it is possible to predict concentrations of reactants and products. aA + bB cC + dD K =

Equilibrium Expressions Solids and liquids never appear in equilibrium expressions. S (s) + O 2 (g) SO 2 (g) K = NH 3 (g) + H 2 O (l) NH 4 (aq) + OH - (aq) K =

Meaning of K 1. Can tell if a reaction if product-favored or reactant-favored. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) The concentration of products is ________________ than the reactants; the reaction is _____________ favored. AgCl (s) Ag + (aq) + Cl – (aq) K = The concentration of products is ________________ than the reactants; the reaction is _____________ favored. K =

Product or Reactant Favored

Meaning of K K comes from Thermodynamics. In chapter 19: If  G o < 0 ; reaction is product favored. If  G o > O ; reaction is reactant favored. If K is really big; lnK > 0  G o is __________. If K is really small; lnK < O  G o is __________.  G o = -RT lnK

Meaning of K 2. Can tell if a reaction is at equilibrium, or predict with way it will go to reach it.

Practice For the isomerization of butane ( ) into isobutene ( ), the Kc = 2.50 at 298K. Determine if the following systems are at equilibrium and if not, in which direction will the reaction proceed to achieve equilibrium. Is the system at equilibrium when [butane] = 0.75 M and [isobutene] = 2.60 M? If it is not, in which direction will the reaction proceed to achieve equilibrium? a) b)c)

An aqueous solution of ethanol and acetic acid, each at an initial concentration of M, is heated to 100 o C. At equilibrium, the acetic acid concentration is M. Calculate K for the reaction. CH 3 CH 2 OH + CH 3 COOH CH 3 COOCH 2 CH 3 + H 2 O

A L flask is filled with 1.00 mol of H 2 and 1.00 mol of I 2 at 448 o C. The value of the equilibrium constant Kc for the reaction at 448 o C is What are the equilibrium concentrations of H 2, I 2 and HI in moles per liter? H 2 + I 2 HI2

Manipulating K When two reactions are added to give a third net, the third K is the ____________ of the first two. S(s) + O 2 (g) SO 2 (g) S(s) + O 2 (g) SO 2 (g) SO 2 (g) + 1/2 O 2 (g) SO 3 (g) Find K for: S(s) + 3/2 O 2 (g) SO 3 (g)

Manipulating K When a reaction is multiplied by a number to change its coefficients, the K must __________________. S(s) + 3/2 O 2 (g) SO 3 (g) X 2 : 2 S(s) + 3 O 2 (g) 2 SO 3 (g)

Manipulating K When a reaction is reversed in direction, the new K if found by calculating _____________. S(s) + O 2 (g) SO 2 (g) S(s) + O 2 (g) SO 2 (g) SO 2 (g) S(s) + O 2 (g)

Manipulating K Concentration Units We have been writing K in terms of mol/L. These are designated by K c But with gases, P = (n/V)RT = conc RT P is proportional to concentration, so we can write K in terms of P. These are designated by K p. K c and K p may or may not be the same. K using concentration and pressure units Kp = Kc (RT) ∆n ** Change in moles For S(s) + O 2 (g) SO 2 (g) ∆n = 0 and Kp = Kc For SO 2 (g) + 1/2 O 2 (g) SO 3 (g) ∆n = –1/2 and Kp = Kc(RT) -1/2

Practice – Determine K Given the following equilibria, a) MgCO 3 (s) Mg 2+ (aq) + CO 3 2- (aq)K 1 = 6.8  b) MgF 2 (s) Mg 2+ (aq) + 2 F - (aq)K 2 = 5.2  Determine the equilibrium constant for the following reaction. c) MgCO 3 (s) + 2 F - (aq) MgF 2 (s) + CO 3 2- (aq) K 3 =?

Equilibrium and External Effects Temperature, catalysts, and changes in concentration affect equilibria.Temperature, catalysts, and changes in concentration affect equilibria. The outcome is governed by ______________________________The outcome is governed by ______________________________ “...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”“...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.” Henri Le Chatelier Studied mining engineering. Interested in glass and ceramics.

Equilibrium and External Effects Temperature changeTemperature change Consider the fizz in a soft drinkConsider the fizz in a soft drink CO 2 (aq) CO 2 (g) + H 2 O(liq) CO 2 (aq) CO 2 (g) + H 2 O(liq) Increase T. What happens to equilibrium position? To value of K?Increase T. What happens to equilibrium position? To value of K? Decrease T. Now what happens to equilibrium position?Decrease T. Now what happens to equilibrium position?

Equilibrium and External Effects Add catalystAdd catalyst A catalyst only affects the RATE of approach to equilibrium.A catalyst only affects the RATE of approach to equilibrium. A catalyst affects both reactions forward and reverse.A catalyst affects both reactions forward and reverse. Catalytic exhaust system

Equilibrium and External Effects Concentration changesConcentration changes –Only the equilibrium composition changes. –Reaction adjusts to new equilibrium “position”

Le Chatelier’s Principle Adding a “reactant” to a chemical system. Removing a “reactant” from a chemical system. Equilibrium shifts___________.

Le Chatelier’s Principle Adding a “product” to a chemical system. Removing a “product” from a chemical system. Equilibrium shifts___________.

Predict effect on K

Nitrogen Dioxide Equilibrium Increase P in the system by reducing the volume (at constant T). N 2 O 4 (g) 2 NO 2 (g) K = [NO 2 ] 2 N 2 O 4 = at 298 K In gaseous system the equilibrium will shift to the side with fewer molecules (in order to reduce the P). Therefore, reaction shifts ________ and P of NO 2 _____________ and P of N 2 O 4 _____________.

Cobalt Complexes Equilibrium Is the following reaction endothermic, or exothermic? In hot water, the solution looks blue, in ice-water bath the solution looks pink. Co(H 2 O) 6 2+ (aq) + 4 Cl - CoCl H 2 O pink blue

Remember Go over all the contents of your textbook. Practice with examples and with problems at the end of the chapter. Practice with OWL tutor. Practice with the quiz on CD of Chemistry Now. Work on your OWL assignment for Chapter 16.