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Effects of Stresses on Equilibrium

Effects of Stresses on Equilibrium. “Stress” = something that causes a reaction (forward or reverse) to speed up or slow down. Ex: A + B ↔ C + D If forward rate increases C + D produced at greater rate, (amounts increase) A + B used up faster, (amounts decrease). Le Chatelier’s Principle.

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Effects of Stresses on Equilibrium

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  1. Effects of Stresses on Equilibrium • “Stress” = something that causes a reaction (forward or reverse) to speed up or slow down. Ex: A + B ↔ C + D • If forward rate increases • C + D produced at greater rate, (amounts increase) • A + B used up faster, (amounts decrease)

  2. 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 • “Shift” = forward or reverse reaction speeds up • Possible Stresses: • Change in Conc. • Change in Pressure • Change in Temperature

  3. Change in Concentration Will affect only gas and aqueous INCREASE Concentration Reaction shifts in direction that will consume extra amount A + B ↔ C + D (Increase [A], shifts ) DECREASE Concentration 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 as that will not change their concentrations.

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

  6. http://www.kentchemistry.com/links/Kinetics/LeChatelier.htm • Ex: N2(g) + 2O2(g) ↔ N2O4(g) 3 moles gas1 mole gas • Increase Pressure: Shifts • Decrease Pressure: Shifts

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

  8. Le Chat’s Overview Part 1 • http://www.youtube.com/watch?v=7zuUV455zFs&safe=active • Le Chat’s Overview Part 2 http://www.youtube.com/watch?v=XhQ02egUs5Y&safe=active

  9. How might you optimize the production of ammonia in the Haber reaction? N2(g) + 3H2(g) ↔ 2NH3(g) + 91.8kJ What could we do to: concentration, temp. and pressure? To shift right Increase [N2], [H2], Decrease [NH3] Decrease Temp Increase Pressure Haber reaction http://www.youtube.com/watch?v=NWhZ77Qm5y4&safe=active

  10. Common Ion Effect • 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. • Ex: Saturated solution of AgCl AgCl(s) ↔ Ag+1(aq) + Cl-1(aq) • Stress: Add NaCl (s). • It breaks apart in water forming Na+1 and Cl-1 • Common Ion = Cl-1 • Shift • This causes a decrease in the solubility of AgCl

  11. Effect of Catalyst on an Equilibrium System • When you add a catalyst it increases the rate of the forward and reverse reactions equally. • The is no overall “shift” • Lowers activation energy the same amount for the forward and reverse reaction

  12. Practice Regents Questions Given the a system at equilibrium: N2(g) + 3H2(g) ↔ 2NH3(g) + energy Which changes occur when the temperature of this system is decreased? (1) The conc. of H2(g) increases and the conc. of N2(g) increases. (2) The conc. of H2(g) decreases and the conc. of N2(g) increases. (3) The conc. of H2(g) decreases and the conc. of NH3(g) decreases. (4) The conc. of H2(g) decreases and the conc. of NH3(g) increases.

  13. Given the equation representing a reaction at equilibrium: N2(g) + 3H2(g)↔2NH3(g) + energy Which change causes the equilibrium to shift to the right? (1) decreasing the concentration of H2(g) (2) decreasing the pressure (3) increasing the concentration of N2(g) (4) increasing the temperature

  14. Given the system at equilibrium: 2POCl3(g) + energy ↔ 2PCl3(g) + O2(g) Which changes occur when O2(g) is added? (1) The equilibrium shifts to the right and the concentration of PCl3(g)increases. (2) The equilibrium shifts to the right and the concentration of PCl3(g)decreases. (3) The equilibrium shifts to the left and the concentration of PCl3(g) increases. (4) The equilibrium shifts to the left and the concentration of PCl3(g) decreases.

  15. Given the reaction at equilibrium: N2(g) + 3H2(g) -->2NH3(g) + 91.8 kJ What occurs when the conc. of H2(g) is increased? (1) The rate of the forward reaction increases and the concentration of N2(g) decreases. (2) The rate of the forward reaction decreases and the concentration of N2(g) increases. (3) The rate of the forward reaction and the concentration of N2(g) both increase. (4) The rate of the forward reaction and the concentration of N2(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: H2(g) + I2(g) + heat ↔ 2 HI(g) What will be the result of an increase in temperature? (1) The equilibrium will shift to the left and [H2] will increase. (2) The equilibrium will shift to the left and [H2] 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/essentialchemistry/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

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