Lecture15 - Kinetics Rates and Mechanisms of Chemical...

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1 Chapter 16 Kinetics: Rates and Mechanisms of Chemical Reactions Kinetics: Rates and Mechanisms of Chemical Reactions 16.1 Factors That Influence Reaction Rates 16.2 Expressing the Reaction Rate 16.3 The Rate Law and Its Components 16.4 Integrated Rate Laws: Concentration Changes over Time 16.7 Reaction Mechanisms: Steps in the Overall Reaction 16.8 Catalysis: Speeding Up a Chemical Reaction 16.5 The Effect of Temperature on Reaction Rate 16.6 Explaining the Effects of Concentration and Temperature Review of Previous Lecture Reaction mechanisms determined by rate limiting step (look for rate limiting step…will be unimolecular or bimolecular process) Intermediates can be connected to reactant concentations (i.e. formed from reactants) – leads to higher order reactions Reaction order with respect to reactants determined by “Isolation” Experiments…in which one reagent is kept in excess and other reagent concentration varied to determine its concentration dependence or order. Reaction rate constants relate probability that the molecules have energy in the correct atomic motions (modes) to react Increased temperature increases the energy into kinetic, rotational, and vibrational degrees of freedom --- increases the probability that the molecules will have energy in the correct atomic motions to react Temperature effects can be quantitatively connected through Arrenhius equation: k = Ae -Ea/RT ; A = frequency of sampling activation barrier and exp(- Ea/RT) gives probability of having sufficient energy to go over barrier Activation Energy Diagrams… give relative energies for reactants, activation energy or transition state, and products Reaction Kinetics: Basic Concepts Need to know if reaction will occur or not (spontaneous) Æ Δ G Need to know how fast a reaction will occur Æ kinetics Factors that affect reaction rates: Collision frequency (concentrations of reactants) Orientation factors (must have correct aligment) Energy of collision must be sufficient to overcome energy barriers to reaction (e.g. bond breaking) Kinetics provide microscopic information on how reactions occur reaction mechanisms = control
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2 Flour “Bomb”: Demo of Effects of Collision Frequency Expect Collision Frequency to increase with surface area (for heterogeneous reactions) Expect reaction rate to increase with temperature for exothermic reactions Flour “Bomb”: Demo of Effects of Collision Frequency • Look for increase in reaction rate with surface area Æ collision frequency •Look for Temperature effects in the colour of the flame and expansion Æ PV = nRT…..PV work being done leads to rapid cooling (thank goodness) Quantifying Collision Frequency Number of molecules undergoing collision with a surface = (N A n/V) 2/3 = number of molecules per unit area Volume of a sphere = (4/3) π r 3 Surface area of sphere = 4 π r 2 Surface area/unit volume = 3/r …...for the same total volume, one has
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