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Topic 7 Equilibrium

Topic 7 Equilibrium. Dynamic equilibrium The position of equilibrium Equilibrium constants Le Chateliers principle Equilibrium in chemical processes. When A and B mixes the product C starts to form C reacts “back” to form A and B The reaction A +B  C and

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Topic 7 Equilibrium

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  1. Topic 7 Equilibrium • Dynamic equilibrium • The position of equilibrium • Equilibrium constants • Le Chateliers principle • Equilibrium in chemical processes

  2. When A and B mixes the product C starts to form • C reacts “back” to form A and B • The reaction A +B C and C A + B goes on at the same time • At equilibrium the rate for both reactions is the same • http://www.chm.davidson.edu/ronutt/che115/K/K_Solutions.htm

  3. 7.1 Dynamic equilibrium • Chemical equilibrium is a dynamic equilibrium. It may look like nothing is happening but reactions occur all the time. A + B C + D • means a reversible reaction, but sometimes ↔ or  is used

  4. In an equilibrium all of the species, A, B, C and D, is present. • Equilibrium doesn’t mean “equal”

  5. Physical equilibrium H2O(l) H2O(g)

  6. 7.2 Position of equilibrium A + B C + D Forward rate = kf[A][B] Reverse rate = kr[C][D] • At equilibrium: Forward rate = Reverse rate • kf/kr=Kc =[C][D] / [A][B] Kc:The Equilibrium constant Kc: temperature dependent

  7. a A + b Bc C + d DKc = [C]c[D]d / [A]a[B]b • H2 + I22 HI Kc= [HI]2 / [H2][ I2] • 4 NH3 + 5 O2 4 NO + 6 H2O Kc=[ H2O]6[NO]4 / [NH3]4[O2]5

  8. Units • Unit: Kc has not a fixed unit. Sometimes Kc can be without unit. • E.g. Kc =[ H2O]6[NO]4 / [NH3]4[O2]5 M6M4/M4M5= M

  9. When Kc >>1, the reaction goes almost to completion • When Kc<<1, the reaction hardly proceeds.

  10. Le Chatelier’sPrinciple If a chemical system at equilibrium experiences a change in concentration, temperatureor volume/ pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

  11. Change in concentrationofreactants Co(H2O)62+ + 4 Cl- + energyCoCl42- + 6 H2O If youaddmoreHCl the concentrationofCl-increases The sizeof the denominatorincreaseswhichcauses a change in the quotientwhichwillinduce a change in the position of the Equilibrium. To keepKcconstant the concentrationof the productshave to increase Thiswillshift the equilibrium to the right and the solution willturnblue Kc= [CoCl4][H2O]6= [Co(H2O)6][Cl-]4

  12. Change in concentrationofproducts Co(H2O)62+ + 4 Cl- + energyCoCl42- + 6 H2O If youaddmore H2O the concentrationof H2O increases The sizeof the nominatorincreaseswhichcauses a change in the quotient, whichwillinduce a change in the position of the equilibrium To keepKcconstant the concentrationof the reactantshave to increase Thiswillshift the equilibrium to the left and the solution willturnpink Kc= [CoCl4][H2O]6= [Co(H2O)6][Cl-]4

  13. Change in temperature- endothermicreaction A change in temperatureaffects the valueofKc If the Kcincreases or decreasesdepends on the DH of the reaction Youcantreat the temperature as a reactant/product and reason accordingly: If the temperature is increased the equilibrium has to change position so the extra heat is consumed. When the reaction is endothermic, as above, thiswillshift the equilibrium to the right and the solution will turnblue The shift to the right willincrease the concentrationofproducts (nominator) whichwillincrease the valueofKc Co(H2O)62+ + 4 Cl- + energyCoCl42- + 6 H2O Kc= [CoCl4][H2O]6= [Co(H2O)6][Cl-]4

  14. Change in temperature- exothermicreaction H2(g) + I2(g)  2HI(g) + energyDH = -51.0 kJ/mol If the temperature is increased the equilibrium has to change position so the extra heat is consumed. When the reaction is exothermic thiswillshift the equilibrium to the left The shift to the leftwillincrease the concentrationofreactants (the denominator) Kc= [HI]2 [H2][I2] whichwilldecrease the valueofKc

  15. Change in pressure Changes in pressuremay be seen as a change in concentrationofbothreactants and products. If the reaction has the same numberofgaseousparticles as reactants and products a change in pressurewillaffect them in the same fashion. 2 HI (g) H2 (g) + I2 (g) Changing the pressure for thisreactionaffects the rate ofreactionbut not the position of the equilibrium

  16. Change in pressure If the reaction has different numberofgaseousparticles as reactants and products a change in pressurewillaffect them like this: N2 (g) + 3H2 (g) 2NH3 (g) Increasing the pressure for thisreaction is like increasing the concentrationsof all the species. In the expression: thiswillresult in that the denominatorincreases morethan the nominatorbecause it has more terms. Thus the ratiowill be decreasedand the equilibrium has to shift to the right and producemore NH3 to maintain the valueofKc. Kc= [NH3]2 [N2][H2]3

  17. Le Chatelier’sPrincipleChanges in Temperature, Pressure and Concentration will affect the equilibrium. A new equilibrium must be found.

  18. Observe that a catalyst doesn’t change the equilibrium but the equilibrium may be reached more rapidly.

  19. The Haber process N2(g) + 3 H2(g)2 NH3(g)DH = -92 kJ/mol With Iron catalyst. How to build an ammonia factory ?

  20. The Contact process 2SO2(g) + O2(g) 2SO3(g) DH= -196kJ/mol With V2O5 catalyst

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