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TODAY A new chapter: Equilibrium Writing equilibrium expressions

TODAY A new chapter: Equilibrium Writing equilibrium expressions observing LeChatelier’s Principle Calculating equilibrium constants, K. TODAY A new wrinkle on Equilibrium: Q. Q, from the Q continuum, Star Trek ,The Next Generation No, not that Q . TODAY

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TODAY A new chapter: Equilibrium Writing equilibrium expressions

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  1. TODAY • A new chapter: Equilibrium • Writing equilibrium expressions • observing LeChatelier’s Principle • Calculating equilibrium constants, K • TODAY • A new wrinkle on Equilibrium: • Q • Q, from the Q continuum, • Star Trek ,The Next Generation • No, not that Q ..

  2. TODAY • A new chapter: Equilibrium • Writing equilibrium expressions • observing LeChatelier’s Principle • Calculating equilibrium constants, K • TODAY • A quick recap of Monday’s concepts • Magnitudes of Keq • Q: the reaction quotient • Q: how to use it

  3. A quick recap on equilibrium expressions: [products] Keq [reagents] For this reaction: [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O But if reaction goes BOTH to right and to left, which are reagents and which are products? By convention” reagents are species to the left of arrow products are species to the right of arrow

  4. A quick recap on equilibrium behavior: You observed this chemical system: [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O When you added excess Cl- : [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O When you added excess H2O: [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O

  5. The series of reactions observed: 1. CuCl2(H2O)2+ 3H2O [CuCl(H2O)5]+ + Cl- 2. [CuCl(H2O)5]+ + H2O [Cu(H2O)6]2++ Cl- 3. [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]2++ H2O [CuCl(H2O)52+] To calculate Keq [Cu(H2O)62+][Cl] 0.80 M Need Keq concentrations 0.28, NO UNITS! 0.40M x7.1 M Pure liquids and solids don’t appear in Keq expression:

  6. Combining reactions equals multiplying K’s (a) CuCl2(H2O)2+ 3H2O [CuCl(H2O)5]+ + Cl- K1eq (b) [CuCl(H2O)5]+ + H2O [Cu(H2O)6]2++ Cl- K2eq Kneteq Net CuCl2(H2O)2+ 4H2O [Cu(H2O)6]2++ 2 Cl- [CuCl(H2O)52+ [Cl] [Cu(H2O)62+][Cl] K1eq x K2eq [CuCl(H2O)52+] CuCl2(H2O)2 [Cu(H2O)62+][Cl]2 K1eq x K2eq Kneteq CuCl2(H2O)2

  7. Are Your Eyes Misleading You? What is in the graduated cylinder? Visible electronic spectra mixture can appear GREEN Abs [CuCl(H2O)5+] [Cu(H2O)62+] 700 nm 400 nm wavelength

  8. Are Your Eyes Misleading You? What is in the graduated cylinder? Recall equilibrium concentrations: [CuCl(H2O)5+] 0.8 M Keq 0.28 0.4 M x7.1M [Cu(H2O)62+][Cl] Visible electronic spectra If Keq ~ 1, product concentrations are similar to reagent concentrations Abs mixture can appear GREEN [CuCl(H2O)5+] [Cu(H2O)62+] 700 nm 400 nm wavelength

  9. How large must Keq be for reaction to be “complete”? Consider this reaction: [Ni(H2O)6]2++ 6 NH3 [Ni(NH3)6]2++ 6H2O Keq 2.0 x 108 [Ni(NH3)6]2++ 3 “en” [Ni(en)3]2++ 6 NH3 Keq 7.3 x 109

  10. What is Keq for this reaction: ? [Ni(H2O)6]2++ 3 “en” [Ni(en)3]2++ 6 H2O [Ni(H2O)6]2++ 6 NH3 [Ni(NH3)6]2++ 6H2O K1 = 2.0 x 108 [Ni(NH3)6]2++ 3 “en” [Ni(en)3]2++ 6 NH3 K2 = 7.3 x 109 K = K1x K2 [Ni(H2O)6]2++ 3 “en” [Ni(en)3]2++ 6 H2O K = K1x K2 = (7.3 x 109)(2.0 x 108) = 1.5 x 1018

  11. What happens if the concentrations are equal: [CuCl(H2O)5+]= 0.8 M, [Cu(H2O)62+]= 0.8 M, [Cl] = 0.8M for this reaction: [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O You investigate by calculation: [CuCl(H2O)5+] 0.8 M 1.25 [Cu(H2O)62+][Cl] 0.8 M x0.8 M But you know Keq = 0.28 ≠ 1.25: what does this mean? It’s not at equilibrium!!

  12. So under these concentration conditions: [CuCl(H2O)5+]= 0.8 M, [Cu(H2O)62+]= 0.8 M, [Cl] = 0.8M [CuCl(H2O)5+] 1.25 > 0. 28 = Keq [Cu(H2O)62+][Cl] This tells you one definite thing: there’s too much in numerator, or, there’s too much product How will reaction system species behave? [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O [CuCl(H2O)5+] decreases, [Cu(H2O)62+] increases, [Cl] increases

  13. This is Q!!! Ratio of Concentrations under Non-Equilibrium conditions [products] Q [reagents] aA + bB cC + dD [C]c[D]d Q [A]a[B]b Q: the Reaction Quotient

  14. The reaction quotient Q can be determined for any set of concentrations Possible outcomes [C]c[D]d 1. Q Keq [A]a[B]b [C]c[D]d 2. Q > Keq [A]a[B]b [C]c[D]d 3. Q < Keq [A]a[B]b

  15. Example problems to be used with reaction: [Cu(H2O)6]2++ Cl- [CuCl(H2O)5]++ H2O Keq = 0.28 A. [CuCl(H2O)5+]= 0 M, [Cu(H2O)62+]= 0.4 M, [Cl] = 0.4 M B. [CuCl(H2O)5+]= 1 M, [Cu(H2O)62+]= 1 M, [Cl] = 0.5 M C. [CuCl(H2O)5+]= 0.01 M, [Cu(H2O)62+]= 0.01 M, [Cl] = 0.01 M Compare this result with earlier equimolar at 0.8M !!

  16. Calculations, calculations, calculations, Many types: 1. Calculating K from equil. concentrations 2. Calc’gKeq from initial and changed concentrations 3. Calc’g final concentrations from initial, change and Keq 4. Calculating a new K from adding 2 reactions

  17. The ICEbox method For these types: 2. Calc’gKeq from initial and changed concentrations 3. Calc’g final concentrations from initial, change and Keq

  18. Lots of K’s Keq – a general equilibrium constant Kc - equilibrium constant in concentrations Ksp - equilibrium constant for solubility Kb - equilibrium constant for OH- formation Kp - equilibrium constant in partial pressures Ka - equilibrium constant for H+ dissociation Kf - formation constant for metal complexes Not to be confused with:k – rate constants

  19. The K “Zoo” Keq Kf Ksp Kp Ka Kc Kb little k

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