KineticsCh15

KineticsCh15 - Kinetics Ch 15 Kinetics Thermodynamics and...

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1 Kinetics Ch 15 Kinetics • Thermodynamics and kinetics are not directly related • Investigate the rest of the reaction coordinate • Rate is important! Chemical Kinetics Kinetics – the study of the rates of chemical reactions Rate of reaction – change in concentration per unit time rate = Δ conc / Δ time • Rate is generally not constant. It changes over the course of a reaction •A B
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2 A B 10 18 24 28 31 33 What is happening to the rate of the reaction as time progresses? Why? Rate = Δ [B]/ Δ t = - Δ [A]/ Δ t Rate = Δ [product]/ Δ t = - Δ [reactant]/ Δ t A B Rate of Reaction 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) Rate = Δ [O 2 ]/ Δ t Stoichiometry important! Rate = Δ [NO 2 ]/ 4 Δ t = - Δ [N 2 O 5 ]/ 2 Δ t 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) Defining Rate • Rate is defined arbitrarily by one pdt or rxt • To be self consistent, • Example:
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3 Another Example Data Calculated Rates • Collect concentration data for reactants and products, then graph • Effect of stoichiometry • Average rate • Instantaneous rate Rate Law • Study rates to understand mechanism of reaction • True rate depends on forward and reverse reactions (remember equilibrium?) • But we can write rate law based on reactants – Many reactions functionally irreversible – Use initial rates (reverse rate is negligible)
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4 Rate Laws • Two forms of rate law • Differential Rate Law (Rate Law) – How rate depends on concentration of reactants – Experiment: Initial Rates of multiple trials • Integrated Rate Law – How concentrations of species depend on time – Experiment: One trial sampled at multiple times Relationship Between Rate and Concentration •2 NO 2 (g) + F 2 (g) 2 NO 2 F (g) • Rate = Δ [NO 2 F]/ 2 Δ t = - Δ [F 2 ]/ Δ t = - Δ [NO 2 ]/ 2 Δ t • Rate α [NO 2 ] and [F 2 ] • Rate = k [NO 2 ] x [F 2 ] y • k = rate constant x and y are the orders of reaction , these are determined experimentally – not from stoichiometry! 2 (g) + F 2 (g) 2 NO 2 F (g) • Rate = k [NO 2 ] x [F 2 ] y • From experiment, x = 1 , y = 1 • Rate = k [NO 2 ] [F 2 ] = Rate Law •1 st order in NO 2 , 1 st order in F 2 , 2 nd order overall • One way to determine the rate law is from initial rates.
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5 H 2 O 2 (aq) + 3 I - (aq) + 2 H + (aq) I 3 - (aq) + 2 H 2 O (l) Expt # [H 2 O 2 ][ I - H + ] Initial rate M/s 1 0.010 0.010 0.0005 1.15 x 10 -6 2 0.020 0.010 0.0005 2.30 x 10 -6 3 0.010 0.020 0.0005 2.30 x 10 -6 4 0.010 0.010 0.001 1.15 x 10 -6 Rate = k [H 2 O 2 ] x [I - ] y [H + ] z H 2 O 2 (aq) + 3 I - (aq) + 2 H + (aq) I 3 - (aq) + 2 H 2 O (l) Expt # [H 2 O 2 ] Rel [I - ] Rel [H + ] Rel Initial rate M/s Rel 1 0.010 1 0.010 1 0.0005 1 1.15 x 10 -6 1 2 0.020 2 0.010 1 0.0005 1 2.30 x 10 -6 2 3 0.010 1 0.020 2 0.0005 1 2.30 x 10 -6 2 4 0.010 1 0.010 1 0.001 2 1.15 x 10 -6 1 Rate = k [H 2 O 2 ] x [I - ] y [H + ] z Rate = k [H 2 O 2 ][I - ] Example Problem •2 NO 2 (g)
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KineticsCh15 - Kinetics Ch 15 Kinetics Thermodynamics and...

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