10.9: Activation Energies
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The majority of chemical transformations do not amount to only breaking or forming bonds. The two processes go together, either simultaneously or sequentially. Since breaking bonds requires energy and forming bonds returns energy, most chemical reactions proceed with either a net output or a net input of energy. A few reactions can occur with no net energy imbalance either way.
Lilkewise, bond breaking typically leads (runs ahead of) bond formation. An old bond must be at least partially broken before a new one can begin to form. The energy cost of breaking the old bond must be supplied before we can get a return from bond formation. This is the reason for the activation energy requirement of many chemical reactions. The more the bond formation process lags behind bond breaking, the higher the activation energy requirement. However, one must be careful not to try to relate bond dissociation energies with activation energies. Unless an extremely simple process involving small atoms or molecules is under consideration, the two parameters are very different. One such process might be the breaking of the H-H bond in the hydrogen molecule to produce hydrogen atoms, or the breaking of the Cl-Cl bond in the chlorine molecule to produce chlorine atoms. Provided these processes are taking place in the gas phase and the bonds are being broken homolytically, the activation energy is the same as the bond dissociation energy.
But a large number of chemical reactions take place in solution, or with bond formation taking place as bond breaking is also occurring. A transition state is reached as two (or more) reactants approach each other and their potential energy rises to a maximum. At the potential energy maximum they have become so closely associated in a configuration that is so highly disorted that any further change will cause them to either revert to reactants, or to form products. Any factors (other than the bond dissociation energies) that stabilize or destabilize the transition state, such as involvement of the solvent (solvation) or the development (or relief) of steric strain can affect the energy level associated with this transition state. In a single step reaction the energy associated with the transition state is also the activation energy. In a multistep reaction, where several transition states are possible for each step, the energy associated with the highest transition state is the activation energy of the overall reaction. The activation energy can also be viewed as a kinetic parameter if it is defined as the minimum kinetic energy with which the reactants should approach each other for reaction to occur.
We will come back to further elaborate on some of these points as we expand on the study of different types of reactions and examine the meaning of the Hammond postulate.