Energy Profiles of Reactions

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Presentation transcript:

Energy Profiles of Reactions Collision Theory and Energy Profiles of Reactions Maxwell-Boltzmann and Kinetics Energy Profile Arrhenius Equation Conclusions

Collision Theory Main Concept: Not all collisions are successful. To get over the activation energy barrier, the colliding species need sufficient energy. Also, the orientations of the reactant molecules during the collision must allow for the rearrangement of reactant bonds to form product bonds.

- Unimolecular reactions occur because collisions with solvent or background molecules activate the molecule shown by a Maxwell-Boltzmann thermal distribution of particle energies

- Collision models provide explanation for order of elementary reactions and temperature dependence of rate constant - In most reactions, only a small fraction of collisions leads to reaction; successful collisions have sufficient energy to overcome activation energy barriers AND orientations that allow bonds to rearrange in the required manner

- Maxwell-Boltzmann distribution describes distribution of particle energies; this distribution provides a qualitative estimate of the fraction of collisions with sufficient energy to lead to a reaction, and also how that fraction depends on temperature

Energy Profile of Reactions Main Concept: The magnitude and temperature dependence of the rate of reaction is contained quantitatively in the rate constant.

- Elementary reactions typically involve breaking some bonds and forming new ones - We can view the complex set of motions involved in rearrangement as occurring along a single reaction coordinate

- The energy profile shows energy along this path, which proceeds from reactants, through a transition state, to products.

- The Arrhenius equation - The Arrhenius equation* can show temperature dependence of the rate of an elementary reaction and is interpreted as the activation energy needed to reach the transition state. * Equation itself is not important, but understanding the concept of it Bigger Ea = Bigger Rate Changes w/ Temperature