Expert: Dr. Jeffery Raymond - 10/18/2011

Question
Dear Professor,
I am an Italian with a study curriculum in humanities who is trying to remediate his ignorance in natural sciences.
According to the collision theory of chemical kinetics, an activation energy is needed for a reaction to take place.
I would like to ask you whether my understanding is correct if I have understood that this activation energy -given for example by collisions, or by light absorption in photochemical reactions- is the quantized energy necessary for electrons in the reactants to make quantum leaps to higher energy levels or sublevels.
Hoping you can answer me, i thank you with all my heart.
Yours sincerely,
David

Answer
In general, yes. There are 3 typical energy quanta that can effect this.  Rotational energy is lowest energy jump, and often is not pivotal to reactions... except when microwave energy is used to super-rotate something.  Even then, that rotational energy is quickly converted into vibrational and kinetic energy and has the same effect as heating a system.  The next highest type of quanta is the vibrational energy levels, typically associated with infrared radiation.  These are the primary sublevels of energy for an electron in an orbital.  For some reactions where heat is the Ea (activation energy) you may think of this as a combination of vibrational jumps allowing an electron to work its way up to a higher electronic orbital (reactive orbital) and then allowing two orbitals to combine to form a new bond.  The highest energy quanta associated with most chemical reactions is one of ultraviolet or visible light.  This absorption of a high energy photon allows a immediate jump to the new electronic orbital and will allow for a different reactive state from that provided by heat.

The tricky thing here is the heat/temperature of the system.  That is to say the kinetic energy of the molecules.  The kinetic energy allows for direct collisional energy transfer.  This can be elastic (where momentum is conserved) or inelastic (where the energy of collision gets added to the an electrons energy and causes it to jump to a higher orbital.  

So, in summary, reactions occur because of destablization of electonic orbitals through one or more changes in quantum number/state. This can happen a couple ways and can be sped by kinetic energy in the system.

A final note, and perhaps beyond the scope of the question, is that changes in quantum state have rules that are followed based on how you destablize an orbital.  Some states cannot be reached readily through heat, while some are not allowed with light.  This means that two reactive species may form some products with heat and different products with light as the shape and flavor of the reactive state may be different depending on how the energy is obtained.