lecture_14_2009

lecture_14_2009 - 74 Reaction Coordinates, Activation...

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74 Reaction Coordinates, Activation Energies, and Catalysis So far, our discussion of kinetics has focused on the significance of the concentration terms in the rate laws. Now, we need to focus on what the rate constant means in terms of the energetics of molecular configurations that lie between the reactants and products. Let’s just consider a simple chemical reaction in which one bond breaks, and another bond forms. Let’s represent the reacting system symbolically as A + BC AB + C So, the B - C bond breaks, and the A - B bond forms. Suppose we were to make a sketch of the energy of the system between reactants and products. To start the thought process, let’s assume that the process is sequential: 1. the B – C bond breaks, and then 2. the A – B bond forms Our energy diagram would look like the following: So, this diagram represents the energy profile followed by the reaction if the bond-making and bond-breaking processes are sequential. So the total energy that must be invested above the energy of the reactants is the activation energy , E a A + BC A + B + C AB + C Energy to break B - C bond Energy to form A - B bond E a
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75 For the vast majority of chemical reactions, bond-breaking and bond-making are not sequential, but are simultaneous. So, because we do not have to go all the way to atoms, the activation energy for simultaneous bond-breaking and bond- making is lower than indicated above. The green curve shows this. We denote the configuration in which bonds are made and broken simultaneously by
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lecture_14_2009 - 74 Reaction Coordinates, Activation...

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