PHOTOCHEMISTRY

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PHOTOCHEMISTRY PRESENTED BY D.THIRUPPATHI DEPARTM
ENT OF CHEMISTRY VIVEKANANDA COLLEGE TIRUVEDAKAM
WEST 625 234
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Photochemistry deals with the study of chemical
reactions caused by the absorption of light
radiations in the visible and UV regions. (?
800 to 200 nm).
Electromagnetic Spectrum and Visible Light
Infrared Microwaves
Gammarays
UV
Radio waves
X-rays
Visible light
Wavelength (nm)
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• A reaction which takes place by absorption of the
  visible and ultraviolet radiations is called a
  photochemical reaction.

Photosynthesis of HCl gas H2 (g) Cl2 (g) h?
? 2 HCl (g)
For example
Photosynthesis Plants use solar energy to
convert carbon dioxide and water into glucose and
oxygen. Human formation of vitamin D by exposure
to sunlight.

• The reactions which are caused by heat and in
  absence of light are called thermal or dark
  reaction.

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Difference between Photochemical and
Thermochemical reactions
S.N Photochemical Reactions Thermochemical Reactions
1 These involve absorption of light radiations These reactions involve absorption or evolution of heat.
2 The presence of light is the primary requirement for the reactions to take place These reactions can take place in dark as well as in light
3 Temperature has a very little effect on the rate of photochemical reactions Temperature has a significant effect on the rate of a thermochemical reaction
4 ?G for photochemical spontaneous reactions ve ?G for a thermochemical reaction is always negative
5 Photochemical activation is highly selective Thermochemical activation is not selective in nature
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Laws of Photochemistry

• Grotthus-Draper Law (First law of
  photochemistry)
• Only the light which is absorbed by a molecule
  can be effective in producing photochemical
  changes in the molecule.
• Stark-Einsteins Law ( Second Law of
  Photochemistry)
• It states that for each photon of light
  absorbed by a chemical system, only one molecule
  is activated for a photochemical reaction. The
  energy absorbed by one mole of the reacting
  molecules is given ENh?. This energy is called
  one Einstein.

Where, N is Avogadro number, h is Planks
constant (6.625 x 10-34 Js) ? is frequency of
radiation (i.e. ? C/?)
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Fluorescence and phosphorescence in terms of
excitation of electrons
S0 S1 ½ S2 -1/2 S S1 S2 ½
(-1/2) 0 2S 1 2 x 0 1 1 S1 S2
1/2 (1/2) 1 2S1 2x1 1 3
h?
h?
Singlet ground state So
Singlet excited state S1 (pair of electorns
with Opposite spins but each in different
orbital)
Triplet excited state T1 (pair of electrons
with parallel spins in different Orbitals)
The excited species can return to the ground
state by losing all of its excess energy by any
one of the paths shown in Jablonski diagram.
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Jablonski diagram
Radiative paths are represented by straight
arrows and non-radiative paths by curly lines.
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Chemiluminescence is the emission of light as the
result of a chemical reaction.
A B ? Products light
Bioluminescence is the production and emission
of light by a living organism. It is a form
of chemiluminescence. Bioluminescence occurs
widely in marine vertebrates and invertebrates,
as well as in some fungi, microorganisms
including some bioluminescent bacteria and
terrestrial invertebrates such as fireflies.
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PHOTOSYNTHESIS
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Carbon moves from atmosphere to plants to animals
and back to atmosphere.
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PHOTOSYNTHESIS RESPIRATION

CO2 H2O ? O2 SUGARS SUGARS O2 ?
H2O CO2
O2
CO2
O2
CO2
MOST LIVING ORGANISMS
PLANTS, ALGAE, BACTERIA
SUGARS
H2O
H2O
Sunlight energy
USEFUL CHEMICAL ENERGY (ATP)
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Summary of Photosynthesis

1. Light energy absorbed by chlorophyll a drives the
   reactions of photosynthesis.

2. Converts light energy into chemical energy to
make organic compounds.
3. CO2 and H2O used to produce C6H12O6
(glucose) and O2 (gas).
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Thank You
Only post by Maruthupandi M, Madurai,
TN,Indian