Download Chemical Kinetics, Reaction Rate - General Chemistry, Notes | CHEM 162 and more Study notes Chemistry in PDF only on Docsity! 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 Chapter #15 – Chemical Kinetics A Model for Chemical Kinetics (Zumdahl Section 15.8) • Concentrations of reactants affect rates • So does temperature • So do catalysts Obviously, a rate constant is not really a universal constant, but is constant only for a fixed set of experimental conditions. How do we explain the dependence of reaction rates on temperatures and catalysts? i.e. Where do Rate Constants come from? Arrhenius believed that for molecules to react upon collision, they must become "activated," and the parameter Ea has become known as the Activation Energy Only some collisions occur with enough energy to overcome the activation barrier Figure 15.9 no n- re ac tiv e re ac tiv e Figure 15.10 N um ber of C ollisions Recall properties of gasses from Chapter 5 Reactants are at thermal equilibrium which is defined by the system temperature non-reactive fraction reactive fraction A clue comes from the strong temperature dependence of many rate constants: Note that this temperature dependence is not linear In 1877, Svante Arrhenius suggested that k varies exponentially with 1/T k = Ae-Ea/RT Ea is a constant with dimensions of energy A is a constant with the same dimensions as k T1 = 500 K k1 = 9.51 × 10-9 mol-1L s-1 T2 = 600 K k2 = 1.10 × 10-5 mol-1L s-1 (big change !!!) ln (k2/k1) = (Ea/R)[(1/T1 - 1/T2)] ln (1.1 × 10-5/9.51 × 10-9) = (Ea/8.3145 J/mol K)*[(1/500 K - 1/600 K)] rearrange and solve: Ea = 176 kJ/mol Example: 2HI(g) H2(g) + I2(g) Rate = k[HI]2 Catalysis It is not always practical or convenient to increase reaction rates by increasing the temperature. A Catalyst is a substance that speeds up a reaction without being consumed during by the reaction Catalysis - The use of catalysts to speed up reactions without changing the temperature Homogeneous catalysts- catalysts that are in the same phase (e.g. solution or gas) as the reacting molecules Heterogeneous catalysts- catalysts that are in a different phase from the reacting molecules Catalysts • Catalysts are used in a huge variety of ways because they can enhance reaction rates by many orders of magnitude! • In general, they work by lowering the activation barrier to a reaction. Note that the energies of the reactants and products do not change, only the energy barrier changes. The transition state is stabilized by the catalyst!
