KineticsCh15

KineticsCh15 - Kinetics Ch 15 Kinetics Thermodynamics and...

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Kinetics Ch 15
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Kinetics Thermodynamics and kinetics are not directly related Investigate the rest of the reaction coordinate Rate is important!
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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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A  B 10 18 24 28 31 33 What is happening to the rate of the reaction as time progresses? Why?
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Rate = Δ[B]/Δt = -Δ[A]/Δt Rate = Δ[product]/Δt = -Δ[reactant]/Δt A  B Rate of Reaction
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2 N2O5 (g) 4 NO2 (g) + O2 (g) Rate = Δ[O2]/ Δt Stoichiometry important! Rate = Δ[NO2]/ 4 Δt = - Δ[N2O5]/ 2 Δt 2 N2O5 (g) 4 NO2 (g) + O2 (g) Defining Rate Rate is defined arbitrarily by one pdt or rxt To be self consistent, Example:
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Another Example Data Calculated Rates
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Collect concentration data for reactants and products, then graph Effect of stoichiometry Average rate Instantaneous rate
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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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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
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Relationship Between Rate and Concentration 2 NO2 (g) + F2 (g) 2 NO2F (g) Rate = Δ[NO2F]/ 2Δt = -Δ[F2]/ Δt = -Δ[NO2]/ 2Δt Rate α [NO2] and [F2] Rate = k [NO2]x [F2]y k = rate constant x and y are the orders of reaction , these are determined experimentally – not from stoichiometry!
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2 NO2 (g) + F2 (g) 2 NO2F (g) Rate = k [NO2]x [F2]y From experiment, x = 1 , y = 1 Rate = k [NO2] [F2] = Rate Law 1st order in NO2 , 1st order in F2, 2nd order overall One way to determine the rate law is from initial rates.
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H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) I3- (aq) + 2 H2O (l) Expt # [H2O2 ] [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 [H2O2]x [I-]y [H+]z
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H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) I3- (aq) + 2 H2O (l) Expt # [H2O2 ] 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
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KineticsCh15 - Kinetics Ch 15 Kinetics Thermodynamics and...

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