Download Chemical Kinetics - Lecture Slides - General Chemistry | CHEM 162 and more Study notes Chemistry in PDF only on Docsity! 1 Chapter #15 – Chemical Kinetics 15.1) Reaction Rates 15.2) Rate Laws: Introduction 15.3) Determining the Form of the Rate Law 15.4) Integrated Rate Law 15.5) Rate Laws: Summary 15.6) Reaction Mechanisms 15.7) The Steady-State Approximation 15.8) A Model for Chemical Kinetics 15.9) Catalysis Figure 15.1 Example: 2NO2(g) 2NO(g) + O2(g) What happens during a chemical reaction? 300C Reactants are used up and products are formed The reaction takes time The final concentrations approach those on the right-hand side of the reaction equation The rate of change in a molecule’s concentration (i.e. the slope) changes with time 2 The Rate is the Change in Concentration per Unit Time Rate of consumption of NO2 = -∆[NO2]/∆t Rate of production of NO = +∆[NO]/∆t Rate of production of O2 = +∆[O2]/∆t As ∆t approaches zero, the instantaneous rate becomes the tangent: d[NO2]/dt 2NO2(g) 2NO(g) + O2(g) Typically, we’re interested in the rate of the reaction itself. For the reaction: 2NO2(g) 2NO(g) + O2(g) Rate = -(1/2)d[NO2] = (1/2)d[NO] = d[O2] dt dt dt The rate of change of concentration of each species is divided by its coefficient in the balanced chemical equation. Rates of change of reactants appear with negative signs, product rates with positive signs. 5 The Integrated Rate Law Express rate law in the form of [A] vs. t Example: First-Order Reaction (n = 1) ln[A] - ln[A]0 = -kt Rate = -d[A] = k[A] dt 1 [A] ⌠ ⌡ [A] [A]0 = ⌠ ⌡ t 0 -k dtd[A] ln = -kt[A][A]0 [A] = [A]0e-kt For a First-Order Reaction (n = 1): Exponential decay of concentration with time [A] = [A]0e-kt ln[A] - ln[A]0 = -kt To test if a reaction is first order, plot ln[A] vs. t It should be linear, with slope = -k and intercept = ln[A]0 6 The Differential Rate Law Rate = -d[A] = k[A]n dt Assumes reverse reaction is negligible (i.e. forward reaction is irreversible) k = rate constant n = order of reaction in [A] k and n are determined experimentally! Reaction: aA products The Integrated Rate Law Express rate law in the form of [A] vs. t Example: First-Order Reaction (n = 1) ln[A] - ln[A]0 = -kt Rate = -d[A] = k[A] dt 1 [A] ⌠ ⌡ [A] [A]0 = ⌠ ⌡ t 0 -k dtd[A] ln = -kt[A][A]0 [A] = [A]0e-kt 7 Example of a 1st-order reaction 2N2O5 (soln) 4NO2 (soln) + O2 (g) Rate = -d[N2O5]/dt = k[N2O5] (differential rate law) ln[N2O5] = -kt + ln[N2O5]0 (integrated rate law) Data: Figure 15.2 Plotting ln[N2O5] vs time gives a straight line Figure 15.3 Linear! First order in N2O5 Slope = -k = -6.93x10-3s-1
