Laws of Photochemistry (Principles of Photochemistry)

Laws of Photochemistry:

There are two Photochemical Principles:

Grotthus-Draper Principle of Photochemical Activation.
Stark-Einstein Law of Photochemical Equivalence.

Let us explain each type:

Grotthus-Draper Principle of Photochemical Activation (or First Law of Photochemistry): “When light falls on an object, a part of it is reflected, a part of it is transmitted and the rest is absorbed. It is only the light absorbed by the object which is effective in bringing about a chemical reaction.”

However, it is not necessary that absorbed light will always bring about a chemical reaction. It may be converted into kinetic energy of the absorbing molecules and only heat effects are produced or it may be re-emitted as fluorescence or phosphorescence.

According to Beer-Lambert’s law

I_abs = I₀ (1 – e^-ℇcx)

Stark-Einstein Law of Photochemical Equivalence (or Second Law of Photochemistry): “It states that in a photochemical reaction, each light-absorbing molecule absorbs only one quantum of radiation which causes that particular reaction.”

If ν is the frequency of the absorbed light, then the quantum of energy absorbed per molecule is given by

E = hν
and energy absorbed per mole is given by
E = N hν ……….(i)

Where N = Avogardo number = 6.02 x 10²³
h = Plank’s constant = 6.62 x 10^-34 Js

The quantity E (energy absorbed per mole) is called an Einstein.

Equation (i) can also be written as E = Nh (c/λ) [∵ ν = c/λ]

Where λ = wavelength
c = velocity of light = 3 x 10⁸ ms^-1

Note: For n moles, equation (i) is given by E = n N hν = nNh(c/λ)