Meteo 465/565 -- The Middle Atmosphere
Chemical Principles

1.  If we have 35 chemicals that are important in the stratosphere, then we have a very large number of combinations that are possible.  How can we tell which reactions will happen?  The first thing we can look at is energy.

Generally, we consider only enthalpy.  However, in reality, we must also consider entropy.  Fortunately, the change in entropy is usually small compared to the change in enthalpy.

Sometimes, excited states of atoms and molecules are involved.  In fact, some of the most important reaction sequences, such as the production of the hydroxyl radical, OH, are initiated by excited state atom chemistry.

Light can also be important, as you have already learned.

The amount of energy that is gained from a reaction is not related to the rate that the reaction proceeds.  There are other effects.

2.  The collision rate (per unit volume per unit time) is proportional to the concentrations of the two reactants, their cross sections, and some kinetic factors.  We might think that knowing the energy and the collision frequency is enough. It is not.  We also need to know the orientation of the molecules as they come toward each other, the "steric factor".  This factor can modify the reaction rate substantially.

3.  We have three main reaction types:

a.  unimolecular (first order)                A -> B + C

b.  bimolecular (second order)            A + B -> C + D

c.  termolecular (third order)               A + B + M -> AB + M

All of these are important for the stratosphere, although it should be no surprise that in the mesosphere, termolecular reactions are much less important than they are in the lower stratosphere.