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Chapter 16 – Chemical Equilibria

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

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Chapter 16 – Chemical Equilibria

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

  2. Chemical Equilibrium • Equilibrium systems are: • DYNAMIC (in constant motion) • REVERSIBLE (can be approached from either direction). Pink to blue Co(H2O)6Cl2 ---> Co(H2O)4Cl2 + 2 H2O Blue to pink Co(H2O)4Cl2 + 2 H2O ---> Co(H2O)6Cl2

  3. + Dynamic EquilibriumFe3+ + SCN- FeSCN2+ • After a period of time, the concentrations of reactants and products are _____________. • The forward and reverse reactions __________ after equilibrium is attained.

  4. Chemical Equilibrium In Phase Changes: In the formation of stalactites and stalagmites: CaCO3(s) + H2O(liq) + CO2(g) Ca2+(aq) + 2 HCO3-(aq)

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

  6. At any point in the reaction: Q = [HI]2 [H2][I2] At Equilibrium: K = [HI]2 = 55.3 [H2][I2] Reaction Quotient & Equilibrium Constant In general, for a reaction: aA + bB cC + dD ALL reacting chemical systems are characterized by their REACTION QUOTIENT, Q. When Q = K, the system is at equilibrium. Q = [C]c[D]d [A]a[B]b Product concentrations Reactant concentrations

  7. 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 =

  8. Equilibrium Expressions • Solids and liquids never appear in equilibrium expressions. K = S (s) + O2(g) SO2 (g) K = NH3(g) + H2O (l) NH4(aq) + OH- (aq)

  9. Meaning of K 1. Can tell if a reaction if product-favored or reactant-favored. K = N2 (g) + 3 H2 (g) 2 NH3 (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.

  10. Product or Reactant Favored

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

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

  13. 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? b) c) a)

  14. An aqueous solution of ethanol and acetic acid, each at an initial concentration of 0.810 M, is heated to 100oC. At equilibrium, the acetic acid concentration is 0.748 M. Calculate K for the reaction. CH3CH2OH + CH3COOH CH3COOCH2CH3 + H2O

  15. A 1.000 L flask is filled with 1.00 mol of H2 and 1.00 mol of I2 at 448oC. The value of the equilibrium constant Kc for the reaction at 448oC is 50.5. What are the equilibrium concentrations of H2, I2 and HI in moles per liter? H2 + I2 HI 2

  16. Manipulating K When two reactions are added to give a third net, the third K is the ____________ of the first two. S(s) + O2(g) SO2(g) SO2(g) + 1/2 O2(g) SO3(g) Find K for: S(s) + 3/2 O2(g) SO3(g)

  17. Manipulating K When a reaction is multiplied by a number to change its coefficients, the K must __________________. S(s) + 3/2 O2(g) SO3(g) X 2 : 2 S(s) + 3 O2(g) 2 SO3(g)

  18. Manipulating K • When a reaction is reversed in direction, the new K if found by calculating _____________. S(s) + O2(g) SO2(g) SO2(g) S(s) + O2(g)

  19. Manipulating K Concentration Units We have been writing K in terms of mol/L. These are designated by Kc 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 Kp. Kc and Kp may or may not be the same. K using concentration and pressure units Kp = Kc (RT)∆n ** Change in moles For S(s) + O2(g) SO2(g) ∆n = 0 and Kp = Kc For SO2(g) + 1/2 O2(g) SO3(g) ∆n = –1/2 and Kp = Kc(RT)-1/2

  20. Practice – Determine K Given the following equilibria, a)MgCO3(s) Mg2+(aq) + CO32-(aq) K1 = 6.8  10-6 b)MgF2(s) Mg2+(aq) + 2 F-(aq) K2 = 5.2  10-11 Determine the equilibrium constant for the following reaction. c) MgCO3(s) + 2 F-(aq) MgF2(s) + CO32-(aq) K3 =?

  21. Equilibrium and External Effects • Temperature, catalysts, and changes in concentration affect equilibria. • 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.” Henri Le Chatelier 1850-1936 Studied mining engineering. Interested in glass and ceramics.

  22. Equilibrium and External Effects • Temperature change • Consider the fizz in a soft drink CO2(aq) CO2(g) + H2O(liq) • Increase T. What happens to equilibrium position? To value of K? • Decrease T. Now what happens to equilibrium position?

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

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

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

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

  27. Predict effect on K

  28. Nitrogen Dioxide Equilibrium Increase P in the system by reducing the volume (at constant T). N2O4(g) 2 NO2(g) [NO2]2 N2O4 K = = 0.0058 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 NO2 _____________ and P of N2O4 _____________.

  29. 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(H2O)62+ (aq) + 4 Cl- CoCl42- + 6 H2O blue pink

  30. 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.

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