reactionkinetics

reactionkinetics - 2-1 1 Free energy diagrams describe the...

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3-1 1 2-1 Free energy diagrams describe the course of a reaction Reaction Coordinate (relative geometry of reacting atoms) A + B reactants A–B product [ A - - - B ] transition state 2 stable stable unstable Reaction Coordinate 2-1 Transition states live for about 10 –14 s; They cannot be directly studied 3 where K is the equilibrium constant R = 1.98 cal/(mol·K) T in degrees Kelvin = (… + y f ,Y + z f ,Z ) – ( a f ,A + b f ,B + …) 2-1 Thermodynamics of aA + bB + … … + yY + zZ K = = …[Y] y [Z] z [A] a [B] b exp (– G °/ RT ) 4 2-1 Thermodynamics of aA + bB + … … + yY + zZ Reaction Coordinate a A + b B + … … + y Y + z Z … + y Y + z Z < 0, K > 1 products favored > 0, 0 < K < 1 reactants favored 5 2-1 Thermodynamics of aA + bB + … … + yY + zZ = T where = (… + y f ,Y + z f ,Z ) – ( a f ,A + b f ,B + …) = (… + yS° f ,Y + zS° f ,Z ) – ( aS° f + bS° f ,B + …) 6 2-1 Thermodynamics of aA + bB + … … + yY + zZ Reaction Coordinate a A + b B + … … + y Y + z Z … + y Y + z Z < 0 product bonds stronger > 0 reactant bonds stronger
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3-2 7 2-1 Thermodynamics of aA + bB + … … + yY + zZ Reaction Coordinate a A + b B + … … + y Y + z Z … + y Y + z Z < 0 products more ordered > 0 reactants more ordered 8 Calculate the equilibrium percentages of butane and 2-methylpropane at 373 K H 3 C CH 3 H 3 CC H 3 3 = –1.64 kcal = –3.678 cal/K 9 G° and K as functions of T for butane 2-methylpropane 300 500 700 Temperature [K] 0 –1 1 [kcal] 1 2 0 3 4 5 K 10 Equilibrium percentages of butane and 2- methylpropane as functions of T Percent butane 300 500 700 Temperature [K] 20 40 0 60 80 100 20 40 0 60 80 100 Percent 2-methylpropane 11 Are products or reactants favored at high temperature? HH H H = –28 kcal/mol 12 2-1 Kinetics of aA + bB + … … + yY + zZ Rate laws describe how the rate v depends on reactant concentration Example v = k [A] a [B] b • Such a reaction is said to be a th-order in [A], b th-order in [B], etc.
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reactionkinetics - 2-1 1 Free energy diagrams describe the...

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