Chemical Kinetics Chapter 12 Catalysts Sections 8

Catalysts A substance that speeds up a reaction without being used itself. All biological reactions are assisted by enzymes Enzymes increase the rate of reactions. Homogeneous Catalysts-same phase as reacting molecules Heterogeneous Catalysts – different phase than reacting molecules.

How does a catalyst work? Provides a new energy pathway with a lower activation energy. Ea is changed but E is not.

How does a catalyst work? There is a larger fraction of effective collisions

Heterogenous Catalyst Involves the adsorbtion of a substance on the surface of a solid catalyst. Example – The hydrogenation of ethylene; catalyst is platinum, palladium or nickel C H C H + H2 Involves four steps Adsorption and activation of the reactants Migration of the adsorbed reactants on the surface Reaction of the adsorbed substances Escape or desorption of the products

Heterogenous Catalyst Adsorption and activation of the reactants

Heterogenous Catalyst Migration of the adsorbed reactants on the surface

Heterogenous Catalyst Reaction of the adsorbed substances

Heterogenous Catalyst Escape or desorption of the products

Homogeneous Catalysts NO catalyzes ozone production in the troposphere NO + ½ O2  NO2 NO2  NO + O O2 + O  O3 3/2 O2  O3 NO catalyzes ozone destruction in the upper atmosphere NO + O3  NO2 + O2 O + NO2  NO + O2 O + O3  2O2 NO is a catalyst because it is not consumed

Homogeneous Catalysts Chlorine from freon (CCl2F2) catalyzes the destruction of ozone Cl + O3  ClO + O2 O + ClO  Cl + O2 O + O3  2O2 October 25, 2012

End of Chapter Exercises #65, 67, and 69

End of Chapter Exercises #65 One mechanism for the destruction of ozone in the upper atmosphere is NO + O3  NO2 + O2 O + NO2  NO + O2 O + O3  2O2 Which species is a catalyst? Which species is an intermediate Ea for the uncatalyzed reaction is 14.0 kJ/mole. Ea for the same reaction when catalyzed is 11.9 kJ/mole. What is the ratio of the rate constant for the catalyzed reaction to that for the uncatalyzed reaction at 25 C? Assume that the frequency factor A is the same for each reaction.

End of Chapter Exercises #67 Assuming that the mechanism for the hydrogenation of C2H2 given in Section 12.8 is correct, would you predict that the product of the reaction of C2H4 with D2 would be CH2D—CH2D or CHD2—CH3? How could the reaction of C2H4 with D2 be used to confirm the mechanism for the hydrogenation of C2H4 given in Section 12.8

End of Chapter Exercises #69 A famous chemical demonstration is the “magic genie” procedure, in which hydrogen peroxide decomposes to water and oxygen gas with the aid of a catalyst. The activation energy of this (uncatalyzed) reaction is 70.0 kJ/mol. When the catalyst is added, the activation energy (at 20.0 C) is 42.0 kJ/mol. Theoretically, to what temperature (C) would one have to heat the hydrogen peroxide solution so that the rate of the uncatalyzed reaction is equal to the rate of the catalyzed reaction at 20. C? Assume the3 frequency factor A is constant and assume the initial concentrations are the same.