The Temperature Dependence of Reaction Rates
Chemical Activation
Only collisions with enough energy react to form products. The energy of the system changes as the reactants approach each other.
The Reaction Coordinate is the distance along the path to your goal. The energy of interaction is your height up the mountain.
A chemical potential of interaction usually looks like pushing a boulder over a hill. This is why the isomerization of the unstable CH3NC does not occur very fast at low temperature, even though energy is released upon reaction.
A typical organic reaction mechanism is the SN2 reaction, as seen above in
the replacement of the Br- leaving group with the OH-
nucleophile. Here is a movie of an SN2
reaction.
The reaction potential depends greatly on the reaction mechanism. Kinetics can
tell us the nature of this potential surface and thus the reaction pathway.
We need a way to measure the activation energy of a reaction. The activation
energy is different for the forward and reverse reactions, and the difference is the
reaction exoergicity.
Where does the chemical activation energy come from anyway? It comes from
the thermal excitation of the molecules.
The fraction of highly excited molecules is exponentially dependent on the
temperature. So then should the reaction rate.
The formula that describes the temperature dependence of the rate constant is
attributed to Ahhrenius:
The relative rate constants of the same reaction at different temperatures looks
something like the vapor pressure as a function of temperature.
Interpreting the Arrhenius Factors: A and Ea
Collision Frequency and the Approach Factor
The figure above shows the importance of molecular orientation in an effective collision.
Only one of the five orientations shown for the collision between NO and NO3
leads to product. (Nitrogen is Blue, Oxygen is Red)
In the effective orientation, those atoms collide that will become bonded in the product.
Molecular steric effects (like only certain collisions leading to
reaction products) influence the value of the the Arrhenius A (pre-exponential) factor,
but not the Energy of Activation.
The A factor is equal to the high temperature limiting rate constant for the
reaction.
Catalysts can greatly influence the energy of activation. They change the nature of the
activated complex.
Enzymes are great catalysts because they can increase the A factore and decrease the
energy of activation at the same time, both increasing the reaction rate without increasing
the temperature.
PJ Brucat // University of Florida