kinetics - Rate Laws An expression of how the reaction rate...

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1 Kinetics Slide 1 Rate Laws An expression of how the reaction rate depends on the concentrations of all substances in the reaction For a reaction: rate = k A w B x C y D z Where k is the rate constant and w , x , y and z are the orders Expression can contain reactants, products, catalysts, etc. but usually only starting materials Kinetics Slide 2 Rate Constant k Determined experimentally; typically represented as k obs A constant for a given reaction under a given set of conditions; numerical value and units depend on identity of reactants and orders; essentially a ‘fudge factor’ that allows us to link rate to concentrations Theoretically complicated; described by the Arrhenius equation: RT E a Ae k = A = frequency factor, E a = Activation energy
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2 Kinetics Slide 3 Reaction Orders Can ONLY be determined experimentally; Orders do NOT correlate with coefficients in balanced equation! Can be an integer, 0, a fraction, positive OR negative! Each species in the reaction has an order; the order tells us indirectly the role that species has in influencing the rate of the reaction. The overall order of a reaction is the sum of the individual orders found in the reaction. 2O 3 (g) 3O 2 rate = k [O 3 ] 2 [O 2 ] -1 2SO 2 (g) +O 2 (g) 2SO 3 (g) rate = k [SO 2 ][SO 3 ] -1/2 Kinetics Slide 4 Reaction Orders: Numerical Consequences Zero Order : rate = k [A] 0 ; concentrations have no effect on rate until equal to zero; First Order : rate = k [A]; double the concentration, rate increases by a factor of 2; Second Order : rate = k [A] 2 ; double the concentration, rate increases by a factor of 4 (or 2 2 ); Third Order : rate = k [A] 3 ; double the concentration, rate increases by a factor of 8 (or 2 3 ); Negative Orders : rate = k [A] -x = k /[A] x ; increasing the concentration will cause the reaction to slow down; usually seen when products show up in the rate expression; Fractional Orders : rate = k [A] 1/x ; indicative of very complex kinetics.
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3 Kinetics Slide 5 Zero Order Reaction Kinetics Slide 6 Pseudo Order Reactions Consider the following reaction: Where rate = k [CH 3 Br][I - ] What if you made the concentration of CH 3 Br so high that it essentially NEVER changed, at least during the time frame of the measurement? For this case, k [CH 3 Br] = constant = k’ obs So rate = k’ obs [I - ] We would then say that this reaction is a pseudo first order reaction A pseudo order reaction is one in which the concentrations of one or more species in the reaction are essentially held constant so they do not show up in the rate expression CH 3 Br + I - CH 3 I + Br -
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kinetics - Rate Laws An expression of how the reaction rate...

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