38-kinetics - Ch 15 Kinetics 15.1 15.2 15.3 15.4 15.5 15.6...

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Unformatted text preview: Ch. 15: Kinetics 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 Week 16 Rate Laws Example Nitrogen monoxide reacts rapidly with oxygen to give nitrogen dioxide. The initial rate of the reaction is measured 3 times at the same temperature, but with different initial concentrations of NO and O2 . Determine the rate law, rate constant, and order of rxn. Expt # 1 2 3 [NO], M 1.0x10-4 1.0x10-4 2.0x10-4 [O2], M 1.0x10-4 3.0x10-4 3.0x10-4 Initial Rate, M/s 2.8x10-6 8.4x10-6 3.4x10-5 Show on board… 1 Week 16 CHEM 1310 - Sections L and M 2 Reaction Rates Rate Laws Forms of Rate Laws Integrated Rate Law Rate Laws: A Summary Reaction Mechanisms Steady-State Approximation A Model for Chemical Kinetics Catalysis CHEM 1310 - Sections L and M Molecularity of a Reaction Unimolecular (a single reactant) A → B + C (a decomposition) NH4 NO3 (l) NO2 (g) + 2 H2O (g) Reaction Mechanisms Reaction mechanisms are a detailed series of elementary steps and rates which are combined to yield the overall chemical reaction Bimolecular (2 reactants) A + B → C + D (most common) NO2 (g) + CO (g) NO (g) + CO2 (g) NO2 (g) + CO (g) NO (g) + CO2 (g) Termolecular (3 reactants) A + B + C → products (less likely) Week 16 CHEM 1310 - Sections L and M 3 Week 16 What is the reaction mechanism for the above reaction? CHEM 1310 - Sections L and M 4 Reaction Mechanism NO2 (g) + CO (g) NO (g) + CO2 (g) Rxn Mechanism Example Consider the rxns below. Determine: Molecularity of each elementary step Equation for the overall reaction Reaction intermediates Cl2 Cl + CHCl3 CCl3 + Cl 2 Cl HCl + CCl3 CCl4 NO3 is formed and consumed; it is a reactive intermediate. NO2 (g) + NO2 (g) Bimolecular Steps NO3 (g) + CO (g) NO2 (g) + CO2 (g) NO (g) + NO2 (g) + CO2 (g) 5 + NO (g) NO2 (g) + NO2 (g) + CO (g) Week 16 All products and reactants are in the gas phase. Week 16 CHEM 1310 - Sections L and M 6 CHEM 1310 - Sections L and M 1 Chemical Equilibrium 2A+B C+D Overall Reaction Reaction Rates k1 [A]2 k1 Chemical Equilibrium at equilibrium: forward rate = reverse rate A+A A2 A+A k-1 Reverse k1 [A]2 = k-1 [A2] Similarly… A2 + B k2 C+D A2 + B k-2 C+D A2 Suppose the reaction mechanism is: k1 A2 A+A A2 + B k2 C+D Forward k2 [A2 ] [B] Then the reverse reactions and rates would be: Reaction Rates k-1 A2 A+A k-1 [A2] A2 + B Week 16 k-2 C+D k-2 [C] [D] 7 Week 16 k2 [A2 ] [B] = k-2 [C] [D] CHEM 1310 - Sections L and M 8 CHEM 1310 - Sections L and M Chemical Equilibrium How can the equilibrium constant be deduced from the rate constants? Rate-Determining Step The particular elementary step that dictates the overall reaction rate is the rate-determining step. Case #1: When the rate-determining step occurs first, the first step is slow and determines the rate of the overall reaction. Forward reaction rate = k f [A]a [B]b Reverse reaction rate = k r [C] [D] c d 1.) k f [A]a [B]b = k r [C]c [D]d 2.) Week 16 k f [C] [D] = = K eq k r [A]a [B]b CHEM 1310 - Sections L and M 9 c d Case #2: When the rate-determining step occurs after one or more fast steps, mechanisms are often signaled by a reaction order greater than two. The slow step determines the overall rate of the reaction. Week 16 CHEM 1310 - Sections L and M 10 RDS: First Rxn Slow Case #1: When the RDS occurs first, the first step is slow and determines the rate of the overall reaction. RDS: First Rxn Slow 1.) NO 2 + F2 æk1 NO 2 F + F æÆ 2.) NO 2 + F æk 2 NO 2 F æÆ Step 1 is the RDS rate = k1[NO 2 ][F2 ] (slow) (fast) E n e r g y slow fast F + NO2F NO2 + F2 Reaction Progress NO2 F Week 16 CHEM 1310 - Sections L and M 11 Week 16 CHEM 1310 - Sections L and M 12 2 RDS: Second Step Slow Transition States & Intermediates Transition States E n e r g y slow fast N2O 2 + O2 2NO 2NO2 Reaction Progress E n e r g y fast slow fast intermediates Reaction Progress Week 16 CHEM 1310 - Sections L and M 13 Week 16 CHEM 1310 - Sections L and M 14 Effect of T on Rxn Rates Arrenhius Equation k = Ae Ea RT ln k An Arrenhius Plot 18.5 18 17.5 17 16.5 16 0.0025 0.0027 0.0029 0.0031 0.0033 0.0035 Change in Activation Energy ΔE = Eaforward - Eareverse Transition State E n e r g y lnk = lnA - Ea RT 1/T (K-1) Ea f Slope = - Ea / R Ea r The Activation Energy (Ea) is the minimum collision energy that reactants must have in order to form products Week 16 CHEM 1310 - Sections L and M 15 ΔE = negative exothermic Reaction Progress CHEM 1310 - Sections L and M 16 Week 16 Change in Activation Energy What is a Catalyst? A chemical that… provides a lower energy path, but it does not alter the energy of the starting material and product changes the energy of the transition(s), in the reaction has no effect on the thermodynamics of the overall reaction 17 Week 16 CHEM 1310 - Sections L and M 18 ΔE = Eaforward - Eareverse Transition State E n e r g y Ea f Ea r ΔE= positive endothermic Reaction Progress Week 16 CHEM 1310 - Sections L and M 3 Effect of a Catalyst on Ea What to Know - Ch. 15 Rate Law Expressions Differential vs. Integrated Order of Reaction Calculation of Reaction Rate and Half-Life Mechanisms Activation Energy & Arrhenius Eqn Catalysis General definition and concepts Impact on Activation energy Week 16 CHEM 1310 - Sections L and M 19 Week 16 CHEM 1310 - Sections L and M 20 4 ...
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This note was uploaded on 08/23/2011 for the course CHEM 1310 taught by Professor Staff during the Spring '11 term at University of Florida.

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