Presentation on theme: "Chapter 12: Chemical Kinetics"— Presentation transcript:
1 Chapter 12: Chemical Kinetics Reaction RatesRate Laws: Differential and IntegratedExperimental Determination of the Rate LawIntegrated Rate Laws & Concentration/Time ProblemsCollision Theory of ReactionsReaction MechanismsPredicting Rate LawsTemperature Dependence of the Rate Constant (Arrhenius)Catalysis
2 As the reaction proceeds, it gets slower because the rate depends on [NO2].
4 Initial Rate (mole/L s) Example 1:Determine the rate law and the value of the rateconstant for the following reaction.2 NO Br2 2 NOBrExperiment[NO]0[Br2]0Initial Rate (mole/L s)11.1 x 10-51.2 x 10-50.3721.9 x 10-51.3 x 10-50.6933.5 x 10-51.244.0 x 10-53.0 x 10-53.4
5 Example 2: Determine the rate law and the value of the rate constant for the following reaction in whichOH- is a catalyst.OCl I- OI Cl-Experiment[OCl-]0(mole/L)[I-]0[OH-]0Rate(mole/L s)1.0040.00201.004.8x10-425.0x10-432.4x10-440.504.6x10-450.259.4x10-4
6 Example 3:The concentration of H2O2 was monitored over time for the following reaction at 25°C:H2O2 (aq) H2 (g) + O2 (g)Find the rate law and the value of the rate constant for this reaction at 25°C.Time (s)[H2O2](mole/L)0.0001.0000.9100.7800.5900.3700.2200.1300.0820.050
7 Example 4: The decomposition of N2O5 to NO2 and O2 is first order with a rate constant of4.80 x 10-4 /s at 45°C.If the initial concentration of N2O5 is 1.65 x 10-2 mole/L, what is the concentration after 825 s?How long would it take for the concentration of N2O5 to decrease to 1.00 x 10-2 mole/L if its initial concentration wa that given in (a)?
8 Example 5: For the following decomposition reaction, AB A + B Rate = k[AB]2k = 0.20 L/mole sHow long will it take for [AB] to reach onethird of its original value of 1.50 M? Whatis [AB] after 10.0 seconds?
9 Zero Order Half Life t1/2 = [A]0 / 2k Note that this half life decreases as the reactionproceeds.There is less reactant toConsume and it runs at thesame rate so it takes lesstime.Time (s)[A]184.108.40.2063.0.403.5.203.75.103.88.050
10 First Order Half Life t1/2 = 0.693 / k The first order half life is constantfor a particular reaction andtemperature.As the initial concentrationdecreases, the decrease in rateis compensated for by lessreactant to consume.Time (s)[A]1.624.0.8048.0.4072.0.2096.0.10120.0.050
11 Second Order Half Life t1/2 = 1 / [A]0 k Note that this half life increasesas the reaction proceeds.The rate decreases so stronglywith concentration that it stilltakes longer to use up half ofthe reactant even though theamount of reactant consumed isless for each half life.Time (s)[A]1.610.2.8030.6.4071.4.20153.10
13 Example 6: A decomposition reaction has a rate constant of yr-1.What is the half life of this reaction?How long does it take for the concentration of the reactant to reach 12.5% of its original value?
14 Example 7: It took 143 s for 50.0% of a particular substance to decompose. If the initialconcentration was M and thedecomposition reaction follows secondorder kinetics, what is the value for therate constant?
15 Example 8: For the reaction A products, successive half-lives are observed to be10.0, 20.0, and 40.0 min for anexperiment in which [A]0=0.10M.Calculate the concentration ofA at the following times.80.0 min30.0 min
16 Example 9: (Book #34) O + NO2 NO + O2 [NO2] in large excess, 1.0x1013molecules/cm3Find the order of the reaction with respect to [O].Reaction is known to be first order in [NO2]. Find the value of k.Time (s)[O] atoms/cm35.0x1091.0x10-21.9x1092.0x10-26.8x1083.0x10-22.5x108
17 Example 10: Consider the following mechanism for the decomposition of hydrogen peroxide catalyzed by I-.Step 1: H2O2 + I H2O + IO- ; fast equilibriumStep 2: IO- + H2O2 H2O + O2 + I- ; slowa) Write a balanced equation for the overall reaction.b) List any catalysts.c) List any intermediates.d) Write the rate law for the overall reaction.
18 Example 11: Consider the following mechanism for the production of phosgene:Step 1: Cl2 (g) Cl (g); fast equilibriumStep 2: Cl (g) + CO (g) COCl (g); fast equilibriumStep 3: COCl (g) + Cl2 (g) COCl2 (g) + Cl (g); slowStep 4: 2 Cl (g) Cl2 (g); fasta) Write a balanced equation for the overall reaction.b) List any catalysts.c) List any intermediates.d) Write the rate law for the overall reaction.
21 Example 12: The activation energy of the following reaction is 3.5 kJ/mole and the change inenthalpy is kJ/mole. Calculatethe activation energy of the reversereaction.OH + HCl H2O + ClHint: Sketch the reaction energy diagram.
22 Example 13: For the reaction A2 + B2 2 AB, the activation energy of the forward reaction is125 kJ/mole and that of the reversereaction is 85 kJ/mole. Assuming thereaction occurs in one step:Draw a reaction energy diagram.Calculate DH for the reaction.Sketch a possible transition state.
23 Figure 12.2: Collisions with E > Ea at two temperatures.
24 Example 14: The isomerization reaction: CH3NC CH3CN has an activation energy of 161 kJ/mole.If the rate constant at 600K is 0.41/s,what is it at 1000K?
25 Example 15: The rate constant of a reaction is 4.50x10-5 L/mol•s at 195°C and3.20 x 10-3 L/mol•s at 258°C.What is the activation energy of thisreaction?
26 Catalysts Work by Lowering Ea: They provide a different mechanism!
27 More collisions result in reaction when Ea is lower.
28 Homogeneous Catalysis In the stratosphere, NO catalyzes thedestruction of ozone, O3.NO (g) + O3 (g) NO2 (g) + O2 (g)O (g) + NO2 (g) NO (g) + O2 (g)Overall:O3 (g) + O (g) 2 O2 (g)
29 Heterogeneous Catalysis C2H4 + H2 C2H6 with a metal catalyst a. reactants b. adsorption c. migration/reaction d. desorption