CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Lecture Summary, 7 May 2015

Transition state theory: We now need to describe how the collision of two particular molecules can lead to a productive reaction.  We intuitively know that even if the correct two molecules collide with the correct energy, not every collision will lead to forward progress along the reaction coordinate, perhaps because the molecules do not collide in the correct way.  We therefore need a way to quantify how the three dimensional orientation of our reactants in the collision will lead to a reaction.  We do this through transition state theory.

Transition state theory starts with an experimental observation, the Arrhenius equation, which relates the observed rate constant to an experimental energy, the "activation energy (Ea)," that the system must overcome:

  k = A exp[-Ea/RT]

The reaction coordinate at the top of this activation energy is therefore a useful and interesting state for us to look at.  We are going to call this the "transition state" or "activated complex;" it is a configuration of our reactants that our system must pass through in order to move from reactant to product.  Because the transition state must be in equilibrium with both reactants and products:

  A + B <--> [AB]* --> P

Then we can use thermodynamic equilibrium to take a guess what this transition state might look like.