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Unformatted text preview: UNIT 1: Chemical Kinetics 1.1 Kinetics : the study of reaction rates • can establish how fast a reaction occurs • can determine how long it will take for the concentration of a reactant to reach a certain value • can elucidate reaction mechanisms (the sequence of steps that make up overall reaction) 1. Reaction rates (section 14.1 - 14.2) Rate = the change in concentration of a specific reactant or product per unit time. A question in the study of smog: how long will it remain in the air? – you need to study the reaction rate of species produced in smog – NO2 is an important culprit in smog, making HNO3 and O3 Here is an overall reaction for the decomposition of NO2: 2 NO2 (g) → 2 NO (g) + O2 (g) Concentrations (mol/L) NO O2 NO2 Time (s) At equilibrium, conc. are not changing. Measured concentration of O2 and NO2 in the first 50 s: at t = 0.00 s at t = 50.0 s 0.00105 M [NO2] = 0.0100 M [NO2] = 0.0079 M [O2] = 0.00 M [O2] = find the rates of change of reactant and products concentrations (a) Rate of disappearance of NO2 = 1.2 Δ[NO 2 ] [NO 2 ]f -[NO 2 ]i = Δt t f -t i 0.0079 − 0.0100 = = −4.2 x 10 −5 M/s = − 4.2 x 10 −5 mol/(L ⋅ s) 50.0 − 0.00
NOTE: Units of rate depend on units of time interval!! (b) Rate of appearance of O2 = Δ[O 2 ] 0.00105 – 0.00 = = 2.1 x 10 −5 mol/(L ⋅ s) Δt 50.0 – 0.0
This rate is smaller because only one molecule of O2 is produced for every two NO2 that disappear (curve rises up only a little) but this means that the reaction has different rates depending on what species is followed… SO… By convention: (1) rates are positive, so for reactants, where concentration is decreasing with time, rates are multiplied by -1. (2) rates are divided by the number of moles of that species in the balanced reaction so that all rate values are the same, regardless which species is used for the calculation Therefore, the reaction rate is = (–1/2) x rate of disappearance of NO2 = (-1/2)(-4.2x10-5 ) = 2.1x10-5 M/s = rate of appearance of O2 = 2.1x10-5 M/s 2. Rate Laws (section 14.3) A rate law defines how the rate of a reaction depends on the concentration of all species. 1.3 Reaction rate (Δ[ ]/Δ t) changes with time : • rate slows down as less reactant is remaining • need to specify time interval when reporting a rate [ ] (mol/L) Time (s) A. Method of Initial Rates In order to have a simple rate expression, where we have rates dependent on the concentration of reactants only, do measurements under initial reaction conditions (no products formed yet, so no backwards reaction). For overall reaction: aA + bB → cC + dD the rate depends on : • rate constant (k; this tells you how fast the reaction is) • the concentration of reactants (determined experimentally) Reaction Rate = k [A]m [B]n Order of reaction : m + n (NO PRODUCTS!) Different from reaction stoichiometry !! The order of the reaction tells you what effect a change in concentration will have on the initial reaction rate: If exponent is 1: doubling the concentration of that species 1 → [2A]1 =2[A]1), doubles the reaction rate ([A] If exponent is 2: doubling the concentration of that species 2 → [2A]2 =4[A]2), quadruples the reaction rate ([A] If exponent is 0: doubling the concentration of that species 0 → [2A]0), has no effect on the reaction rate ([A] If exponent is -1: doubling the concentration of that species -1 → [2A]-1 =(1/2)[A]–1), the reaction rate falls to 1/2 ([A] ...
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- Spring '09