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21

PHOTOCHEMISTRY AND THE

THEORY OF CHEMICAL REACTIONS

So far we have considered chemical reactions in which the energy necessary to

surmount the activation barrier is obtained from the environment in the form of heat

translated as molecular motion. These are thermal reactions. Light of frequencyνis

also able to impart energyE=hνto reactant molecules so that they can surmount

the barrier. These arephotochemical reactions.

21.1 EINSTEIN’S LAW

Like any chemical reaction, a photochemical reaction involves excitation of the

reactant species up and over an activation energy barrier before transmission to

the product state. In a photochemical reaction, theactivation energyis supplied by

incident light instead of ambient heat. The Bohr–Einstein concept of a particle of

light (a photon) carrying aquantumof energyE=hνleads one to suppose that the

energy of an incident light particle is concentrated in one reactant molecule only,

rather than being distributed evenly over an entire collection of molecules. This

mechanism of energy transmission means that if the quantumEis sufficiently large,

all struck molecules will acquire the activation energy and the number of molecules

reacting will be equal to the number of photons striking and retained by the system.

This is Einstein’s law.

molecules reacting=photons absorbed

Concise Physical Chemistry,by Donald W. Rogers

Copyright©C2011 John Wiley & Sons, Inc.

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