1. Le Chatelier’s Principle When a system at equilibrium is subjected to a stress, the system “shifts” to relieve effects of the stress and restore equilibrium When a system at equilibrium is subjected to a stress, the system “shifts” to relieve effects of the stress and restore equilibrium “Shift” = forward or reverse reaction speeds up “Shift” = forward or reverse reaction speeds up “Stress” = something that causes a reaction (forward or reverse) to speed up or slow down. “Stress” = something that causes a reaction (forward or reverse) to speed up or slow down. Possible Stresses: Possible Stresses: Change in Conc. Change in Conc. Change in Pressure Change in Pressure Change in Temperature Change in Temperature http://www.learnerstv.com/animation/anim ation.php?ani=120&cat=chemistry https://www.youtube.com/watch?v=dIDgP FEucFMhttps://www.youtube.com/watch?v=dIDgP FEucFM (2 min)

2. Effects of Stresses on Equilibrium Ex: A + B ↔ C + D If forward rate increases If forward rate increases C + D produced at greater rate C + D produced at greater rate amounts increase amounts increase A + B used up faster A + B used up faster amounts decrease amounts decrease

3. Change in Concentration Will affect only gas and aqueous Will affect only gas and aqueous INCREASE Concentration Reaction shifts in direction that consumes extra amount Reaction shifts in direction that consumes extra amount A + B ↔ C + D (Increase [A], shifts ) DECREASE Concentration Reaction shifts in direction that produces more of what was taken away Reaction shifts in direction that produces more of what was taken away A + B ↔ C + D (Decrease [B], shifts )

4. Note: Changing amounts of a pure liquid or solid will not cause a shift Changing amounts of a pure liquid or solid will not cause a shift That will not change their concentrations, just their amounts. That will not change their concentrations, just their amounts.

5. Change in Temperature Increase Temp. Favors ENDOTHERMIC Favors ENDOTHERMIC Direction that consumes excess heat Direction that consumes excess heat A + B ↔ C + D + energy A + B ↔ C + D + energy Decrease Temp. Favors EXOTHERMIC Favors EXOTHERMIC Direction that produces heat Direction that produces heat A + B ↔ C + D + energy A + B ↔ C + D + energy

6. Pressure Changes Affect gases only Affect gases only Count total MOLES of gas Count total MOLES of gas on each side of equation Increase pressure = Decrease volume Increase pressure = Decrease volume Shifts in direction that forms fewer moles of gas Shifts in direction that forms fewer moles of gas Decrease pressure = Increase volume Decrease pressure = Increase volume Shifts in direction that forms greater moles of gas Shifts in direction that forms greater moles of gas NOTE: If moles of gas equal on both sides NO SHIFT

7. Ex: N 2 (g) + 2O 2 (g) ↔ N 2 O 4 (g) Ex: N 2 (g) + 2O 2 (g) ↔ N 2 O 4 (g) 3 moles gas 1 mole gas 3 moles gas 1 mole gas Increase Pressure: Shifts Increase Pressure: Shifts Decrease Pressure: Shifts Decrease Pressure: Shifts http://www.kentchemistry.com/links/Ki netics/LeChatelier.htm

8. How might you optimize the production of ammonia in the Haber reaction? How might you optimize the production of ammonia in the Haber reaction? N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + 91.8kJ What could we do to: concentration, temp. and pressure? To shift right Increase [N 2 ], [H 2 ], Decrease [NH 3 ] Decrease Temp Increase Pressure Haber reaction http://www.youtube.co m/watch?v=NWhZ77Q m5y4&safe=active

9. Common Ion Effect Substance is added to an equilibrium system that has a “common ion” with the reaction. Substance is added to an equilibrium system that has a “common ion” with the reaction. This will increase the concentration of that aqueous ion and will cause a shift. This will increase the concentration of that aqueous ion and will cause a shift. Ex: Saturated solution of AgCl Ex: Saturated solution of AgCl AgCl(s) ↔ Ag +1 (aq) + Cl -1 (aq) Stress: Add NaCl (s). Stress: Add NaCl (s). It breaks apart in water forming Na +1 and Cl -1 It breaks apart in water forming Na +1 and Cl -1 Common Ion = Cl -1 Common Ion = Cl -1 Shift Shift This causes a decrease in the solubility of AgCl This causes a decrease in the solubility of AgCl

10. Effect of Catalyst on an Equilibrium System Increases rate of the forward and reverse reactions equally. Increases rate of the forward and reverse reactions equally. There is no overall “shift” There is no overall “shift” Lowers activation energy the same for forward and reverse reaction Lowers activation energy the same for forward and reverse reaction

11. Le Chat’s Overview Part 1 Le Chat’s Overview Part 1 http://www.youtube.com/watch?v=7zuUV4 55zFs&safe=active http://www.youtube.com/watch?v=7zuUV4 55zFs&safe=active http://www.youtube.com/watch?v=7zuUV4 55zFs&safe=active http://www.youtube.com/watch?v=7zuUV4 55zFs&safe=active Le Chat’s Overview Part 2 Le Chat’s Overview Part 2 http://www.youtube.com/watch?v=XhQ02e gUs5Y&safe=active http://www.youtube.com/watch?v=XhQ02e gUs5Y&safe=active

12. Practice Regents Questions Given the a system at equilibrium: N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy Which changes occur when the temperature of this system is decreased? (1) The conc. of H 2 (g) increases and the conc. of N 2 (g) increases. (2) The conc. of H 2 (g) decreases and the conc. of N 2 (g) increases. (3) The conc. of H 2 (g) decreases and the conc. of NH 3 (g) decreases. (4) The conc. of H 2 (g) decreases and the conc. of NH 3 (g) increases.

13. Given the equation representing a reaction at equilibrium: N 2 (g) + 3H 2 (g)↔2NH 3 (g) + energy Which change causes the equilibrium to shift to the right? (1) decreasing the concentration of H 2 (g) (2) decreasing the pressure (3) increasing the concentration of N 2 (g) (4) increasing the temperature

14. Given the system at equilibrium: 2POCl 3 (g) + energy ↔ 2PCl 3 (g) + O 2 (g) Which changes occur when O 2 (g) is added? (1) The equilibrium shifts to the right and the concentration of PCl 3 (g)increases. (2) The equilibrium shifts to the right and the concentration of PCl 3 (g)decreases. (3) The equilibrium shifts to the left and the concentration of PCl 3 (g) increases. (4) The equilibrium shifts to the left and the concentration of PCl 3 (g) decreases.

15. Given the reaction at equilibrium: N 2 (g) + 3H 2 (g) -->2NH 3 (g) + 91.8 kJ What occurs when the conc. of H 2 (g) is increased? (1) The rate of the forward reaction increases and the concentration of N 2 (g) decreases. (2) The rate of the forward reaction decreases and the concentration of N 2 (g) increases. (3) The rate of the forward reaction and the concentration of N 2 (g) both increase. (4) The rate of the forward reaction and the concentration of N 2 (g) both decrease.

16. Given the system at equilibrium: N2(g) + O2(g) + energy ↔ 2 NO(g) Which changes will result in a decrease in the amount of NO(g) formed? (1) decreasing the pressure. (2) decreasing the concentration of N2(g). (3) increasing the concentration of O2(g). (4) increasing the temperature

17. Given the equilibrium reaction in a closed system: H 2 (g) + I 2 (g) + heat ↔ 2 HI(g) What will be the result of an increase in temperature? (1) The equilibrium will shift to the left and [H 2 ] will increase. (2) The equilibrium will shift to the left and [H 2 ] will decrease. (3) The equilibrium will shift to the right and [HI] will increase. (4) The equilibrium will shift to the right and [HI] will decrease.

18. Given the reaction at equilibrium: the concentration of A(g) can be increased by A. lowering the temperature B. adding a catalyst C. increasing the concentration of AB(g) D. increasing the concentration of B(g)

19. Le Chats: (Honors) http://www.mhhe.com/physsci/chemistry/essentialchemis try/flash/lechv17.swf http://www.mhhe.com/physsci/chemistry/essentialchemis try/flash/lechv17.swf http://www.mhhe.com/physsci/chemistry/essentialchemis try/flash/lechv17.swf http://www.mhhe.com/physsci/chemistry/essentialchemis try/flash/lechv17.swf Crash Course: Equilibrium http://www.youtube.com/watch?v=g5wNg_dKsYY&safe=active Le Chat’s (University of Surrey) http://www.youtube.com/watch?v=dIDgPFEucFM&safe=active