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Chemical Equilibrium

Chemical Equilibrium. Chapter 17. Equilibrium vs. Kinetics. Kinetics: speed of a reaction or process how fast? Equilibrium: extent of reaction or process how much?. Chemical Equilibrium. Reactant and product concentrations remain constant

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Chemical Equilibrium

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  1. Chemical Equilibrium Chapter 17

  2. Equilibrium vs. Kinetics • Kinetics: speed of a reaction or process • how fast? • Equilibrium: extent of reaction or process • how much?

  3. Chemical Equilibrium • Reactant and product concentrations remain constant • Molecular level: rapid activity (dynamic) • Macroscopic level: unchanging • At equilibrium: rateforward = ratereverse • Does not limit time

  4. Law of Mass Action • If: • Then: • And: K = equilibrium constant m, n = coefficients in balanced equation

  5. Law of Mass Action • For: • Equilibrium expression:

  6. Value of K Favors K < 0.01 Reactants K > 100 Products 0.01 < K < 100 Neither

  7. Direction of Reaction and Q • For: • Equilibrium expression: • Reaction Quotient:

  8. N2O4(g) 2 NO2(g)

  9. Practice • 1. Writing expression for K • 2. Q vs. K and reaction direction • 3. K for a multistep process • 4. K for reaction “multiples”

  10. Write K The decomposition of dinitrogen pentoxide: N2O5(g) NO2(g) + O2(g) The combustion of propane gas: C3H8(g) + O2(g) CO2(g) + H2O(g)

  11. Write K The decomposition of dinitrogen pentoxide: 2 N2O5(g) 4 NO2(g) + O2(g) The combustion of propane gas: C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

  12. Write K 4 NH3(g) + O2(g) 4 NO(g) + 6 H2O(g) 2 NH3(g) + 5/2 O2(g) 2 NO(g) + 3 H2O

  13. K vs. Q • For the reaction: N2O4(g) 2NO2(g) • Kc = 0.21 at 1000C. • At a point during the reaction, [N2O4] = 0.12M and [NO2] = 0.55M. • (a) Find Q. Is the reaction at equilibrium? • (b) If not, in which direction is it progressing?

  14. K vs. Q • N2O4(g) 2NO2(g) Kc = 0.21 at 1000C. • At a point, [N2O4] = 0.12M and [NO2] = 0.55M. • (a) Find Q. Is the reaction at equilibrium? • (b) If not, in which direction is it progressing?

  15. K for multistep reactions • Nitrogen dioxide, a toxic pollutant that contributes to photochemical smog, can develop in combustion engines from N2 and O2. (1) N2 + O2 2NO Kc1 = 4.3 x 10-25 (2) 2NO + O2 2NO2 Kc2 = 6.4 x 109 (a) Show that Qc for the overall reaction is the same as the product of Qcs of the individual reactions. (b) Calculate Kc for the overall reaction.

  16. K for multistep reactions (1) N2 + O2 2NO Kc1 = 4.3 x 10-25 (2) 2NO + O2 2NO2 Kc2 = 6.4 x 109 N2 + 2 O2 2 NO2 (a) Qc (b) Kc,overall

  17. K for multistep reactions • For the following (1) Br2 2 Br (2) Br + H2 HBr + H (3) H + Br HBr (a) Write the overall balanced reaction. (b) Write out the individual expressions for Qc and show that their product is equivalent to the overall Qc.

  18. Multiples of K • For the ammonia reaction: • N2(g) + 3H2(g) 2NH3(g) • Kc is 2.4x10-3 at 1000K. • Find K for the following: (a) 1/3 N2+ H2 2/3 NH3 (b) NH3 1/2 N2 + 3/2 H2

  19. Multiples of K • N2(g) + 3H2(g) 2NH3(g), Kc = 2.4x10-3 (a) 1/3 N2+ H2 2/3 NH3 (b) NH3 1/2 N2 + 3/2 H2

  20. Multiples N2(g) + O2(g) 2 NO(g)Kc = 1 x 10-30 Write the expression for Q and determine its value for ½ N2(g) + ½ O2(g) 2 NO(g) H2(g) + Cl2(g) 2 HCl(g)Kc = 7.6 x 108 Write the expression for Q and determine its value for 2/3 HCl(g)1/3 H2(g) + 1/3 Cl2(g)

  21. Heterogeneous Equilibrium PURE solids and liquids do not appear in expression for K (or Q).

  22. Kc vs. Kp • For:

  23. Kp and Kc • For the ammonia reaction, • N2(g) + 3H2(g) 2NH3(g), • Kc = 2.4x10-3 • Find Kp at 1000 K.

  24. Kp and Kc • N2(g) + 3H2(g) 2NH3(g), Kc = 2.4x10-3 • Find Kp at 1000 K.

  25. Kp and Kc • For the following reaction, • PCl3(g) + Cl2(g) PCl5(g), • Kc = 1.67 at 500 K • Find Kp at 500 K.

  26. K vs. Q • For the reaction: CH4(g) + Cl2 CH3Cl(g) + HCl • Kp = 1.6x104 at 1500 K. • At a point during the reaction, • PCH4 = 0.13 atm, PCl2= 0.035 atm, • PCH3Cl = 0.24 atm, and PHCl = 0.47 atm. • (a) Find Q. Is the reaction at equilibrium? • (b) If not, in which direction is it progressing?

  27. Problems • 1. Given equilibrium concentrations or pressures, find K or Q. • 2. Given K and initial conditions (conc’s or P’s), find equilibrium quantities (conc’s or P’s).

  28. Le Châtelier’s Principle • . . . if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.

  29. Le Châtelier’s Principle • 1. Concentration • 2. Temperature • 3. Pressure • 4. Volume • 5. Catalysts*

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