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

Iabs = 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)

Van de Graaff Generator
Escape Velocity
Thermal Conductivity in Daily Life
Applications of Interference
AC Generator or Dynamo
Applications of Michelson Interferometer
Basic Theory of Interference or Condition for Constructive and Destructive Interference
Cornu’s Method for Determination of Young’s Modulus Using Interference Phenomenon
Magnetism and Magnetic Effect of Electric Current– NIOS

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