Lec XXIX

Chem 1200

Angel C. de Dios

Kinetics VII

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Equations you may need for the third exam:

Collision Theory

The Arrhenius equation can help us build a model of chemical reactions at the molecular level. 

Specifically, our model must account for the temperature dependence of rate constants, as well as the energy of activation.

One remaining part in addition to having a collision with enough energy is having a collision with the right orientation.

As we try to explain what we observe in kinetics experiments, this is collision theory.

Reactant molecules need to collide explaining the rate law we observe. In addition, these collisions need to be of sufficient impact or energy, explaining the temperature dependence of rate constants. Lastly, these collisions must be of the right orientation. Here is an example. The Cl atom is required to hit the HI molecule on the H end in order for the collision to produce HCl and and an I atom.

Thus, not all collisions are fruitful. They require a minimum energy. Moreover, the collision needs to happen at the right orientation. Thus, not all collisions with sufficient energy lead to products, only those that have the right orientation. This orientation requirement becomes even more stringent with complex molecules.

The last thing we will discuss: catalysts. Like intermediates, a catalyst does not show up in a balanced chemical equation. Unlike intermediates, catalysts are present at the beginning of a reaction. Catalysts do not appear in the balanced equation because these are regenerated. Catalysts are therefore not consumed. A catalyst increases the rate of the reaction by providing an alternative reaction mechanism with a lower activation energy.

The numbers on the above plot (1, 2, 4, 10) are values for the Michaelis constant, This constant (when we assume that the equilibrium and steady approaches are equivalent) is proportional to the dissociation constant of the complex. A higher value for the Michaelis constant therefore corresponds to a weaker binding between enzyme and substrate.

KD is the dissociation constant of the ES complex, that is, KD = [E][S]/[ES]. The higher KD is, the weaker the binding is between E and S.

When the second step is rate determining, that is, kcat is smaller than kr, the Michaelis constant KM is equal to KD, where KD is the dissociation constant of the ES complex.