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Week 6

Week 6. Chemical Equilibrium Professor Bob Kaplan University Department of Science. Chemical Reaction. A + B = C + D Reactants: A and B Products: C and D. Molar Concentration. [ A ] = Concentration of A [ B ] = Concentration of B

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Week 6

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  1. Week 6 Chemical Equilibrium Professor Bob Kaplan University Department of Science

  2. Chemical Reaction A + B = C + D Reactants: A and B Products: C and D

  3. Molar Concentration [ A ] = Concentration of A [ B ] = Concentration of B [ C ] = Concentration of C [ D ] = Concentration of D Molarity M = Moles / Liter

  4. Spectrophotometry Molar concentration is often measured by shining a light through a cell filled with solution. More light transmitted  Dilute solution Less light transmitted Concentrated solution. www.wavesignal.com/Forensics/Anlys.html Check out: UV-Vis Spectrophotometry

  5. Concentration vs. Time

  6. Rate of Reaction Rate of the change in concentration Slope of graph: [ A ] vs. t Slope of graph: [ B ] vs. t Slope of graph: [ C ] vs. t Slope of graph: [ D ] vs. t

  7. Rate of Reaction Rate of Change = Δ [ A ] / Δ t Rate of Change = Δ [ B ] / Δ t Rate of Change = Δ [ C ] / Δ t Rate of Change = Δ [ D ] / Δ t

  8. Rate of Reaction Concentration decreases from 0.5 M to 0.1 M Δ [ A ] = ( 0.5 - 0.1 ) = 0.4 M It takes 15 seconds for the concentration to decrease from 0.5 M to 0.1 M. Δ t = 15 sec Rate of Change = Δ [ A ] / Δ t = ( 0.5 - 0.1 ) M / 15 sec = 0.026 M per sec Δ [ A ] / Δ t = 0.026 M per sec

  9. Concentration vs. Time

  10. Rxn Rate  Concentration • Rxn rate is proportional to concentration • Reaction rate will start out high and taper off as [ A ] and [ B ] are depleted. • Slope will level off as [ A ] and [ B ]  Zero *Note: Slope of horizontal line = O

  11. Chemical Equilibrium A + B = C + D Rate of forward reaction A + B  C + D = Rate of reverse reaction C + D A + B

  12. Chemical Equilibrium Rate of forward reaction = Rate of reverse reaction Concentrations of reactants and products are said to be in a : Fixed Steady State

  13. Chemical Equilibrium • E.G. If we bail water out of a boat at the same rate that it manages to seep in, then the depth of the pool of water at the bottom of the boat will never change ! • The equilibrium constant K is one way of expressing these fixed concentrations in relation to each other.

  14. Equilibrium Constant K For the reaction:  A  +  B   =   C  +  D The equilibrium constant K is given by: K =

  15. Chemical Equilibrium • In terms of the forward and reverse reactions, note that at equilibrium, the rates of these respective reactions are equal. • Thus we have an alternative way of expressing K in terms of k1 and k2 as follows. Rate of forward rxn = k1 [ A ] [ B ] = Rate of reverse rxn = k2 [ C ] [ D ]

  16. Chemical Equilibrium k1 [ A ] [ B ] = k2 [ C ] [ D ] k1 / k2 = [ C ] [ D ] / [ A ] [ B ] = K K = k1 / k2 The ratio of reaction rate constants for the forward and reverse reactions is equivalent to the equilibrium constant for the reaction.

  17. Example 2 H2 + O2 --> 2 H2O What if I know: Δ [ H2 ] / Δ t What can I say about: Δ [ H2O ] / Δ t ??? Mole ratio of H2 : H2O = 2 : 2 = 1 : 1 Δ [ H2 ] / Δ t = Δ [ H2O ] / Δ t

  18. Example 2 H2 + O2 --> 2 H2O What if I know: Δ [ H2 ] / Δ t What can I say about: Δ [ O2 ] / Δ t ??? Mole ratio of O2 : H2 = 1 : 2 = 1 / 2 Δ [ O2 ] / Δ t = ½ Δ [ H2 ] / Δ t

  19. General Rate Law a A + b B = c C + d D ( 1 / a ) * Δ [ A ] / Δ t = ( 1 / b ) * Δ [ B ] / Δ t = ( 1 / c ) * Δ [ C ] / Δ t = ( 1 / d ) * Δ [ D ] / Δ t

  20. “Please Return to Seminar…..” If you have come here directly from the SC155 Seminar session, please return to the KU course platform now to continue with the live session of discussion, questions and answers  See you all there !

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