Photochemistry CECH-509 discusses photochemical reactions and the laws that govern them. Photochemical reactions are chemical reactions initiated by the absorption of light. Key points: - Photochemical reactions increase free energy while thermal reactions decrease it. The rate of a photochemical reaction depends on the intensity of absorbed light. - Stark-Einstein law states that each reacting molecule absorbs a single photon, gaining energy to activate and enter the reaction. - Quantum yield refers to the number of molecules reacted or formed per absorbed photon, indicating the reaction's efficiency. Values below 1 indicate a low yield while above 1 is high. Secondary reactions can result in yields above 1.

1. Photochemistry CECH-509
UNIT-4A

5. What are photo-chemical reactions ?
• Photochemical reactions are those reactions which are brought
about by absorption of light of all wavelengths, ultraviolet to
infrared and radio-frequency.
• Reactions which are brought about by wavelength 2000◦A to
4000◦A (visible range ) are important.
• All types of reactions :
oxidation reduction
condensation isomerization
polymerization
Occur by absorption of radiation.

6. Comparison between :
Photochemical reaction
• Reaction is brought about
by the absorption of
radiation.
• Free energy increases.
• The rate of reaction
depends on the intensity of
light absorbed.
Thermal reaction
• Reaction taking palce in dark
starts thermally.
• Free energy decreases.
• At constant concentration of
reactants the rate of reaction
depends on the change in
temperature.

8. • In photo-chemical reactions, the free energy increases.
• Eg,
The conversion of oxygen to ozone.
Decomposition of ammonia.
Photosynthesis of carbohydrates from water and carbon
dioxide in presence of chlorophyll.

11. Laws of photochemistry :-
 Grothus Draper law:-
• The light which is absorbed can be effective in producing a
chemical change.
• If In is intensity of light that enters a medium, It is the intensity
of transmitted light and Ia is the intensity of absorbed light,
Ia = In - It

12. Laws of photochemistry :-
 Grothus Draper law:-
• When light falls on a cell containing reaction mixture, some
light is absorbed and some light is transmitted.
• Hence only the absorbed component of light is capable of
producing the reaction.
• Transmitted light is chemically ineffective.
• Absorbed light will not always cause reactions.
• If the conditions are not favourable then the light energy
remains unused & it may be re-emitted as heat or light.

13. Laws of photochemistry :-
 Grothus Draper law:-
Limitations :-
• It gives no idea about the relation between the absorbed
radiation and the molecules undergoing change.
• It purely works on the qualitative aspect.

14. Laws of photochemistry :-
Stark- Einstein law:-
• Stark and Einstein studied the quantitative aspect of
photochemical reactions by application of quantum theory of
light.
• They noted :
• Each molecule taking part in reaction absorbs only a single
quantum or photon of light.
• The molecule gains one photon equivalent energy is activated
and enters into the reaction.

15. Laws of photochemistry :-
Stark- Einstein law:-
In a photochemical reaction, each molecule of thereacting
substance absorbs a single photon of radiation causing the
reaction and is activated to form the products.

16. Laws of photochemistry :-

17. Quantum yield or quantum efficiency :-
• It has been noticed that not all photochemical reaction obeys
the Einstein law.
• The number of molecules reacted or decomposed is often
found to be markedly different from the number of quanta or
photons of radiation absorbed in the given time.
The number of molecules reacted or formed per photon of
light absorbed is termed as Quantum yield.
Denoted by ɸ

18. Quantum yield or quantum efficiency :-
• ɸ = No. of molecules reacted or formed
No. of photons absorbed
• one molecule decomposes per photon - ɸ = 1
• Two or more molecules decomposes per photon - ɸ > 1 then
the reaction has high quantum yield.
• The number of molecules decomposed is less than one per
photon - ɸ < 1 then the reaction has low quantum yield.

19. Quantum yield or quantum efficiency :-
• ɸ = No. of molecules reacted or formed
No. of photons absorbed
• one molecule decomposes per photon - ɸ = 1
• Two or more molecules decomposes per photon - ɸ > 1 then
the reaction has high quantum yield.
• The number of molecules decomposed is less than one per
photon - ɸ < 1 then the reaction has low quantum yield.

20. High quantum yield:-
• Two or more molecules decomposes per photon - ɸ > 1 then the
reaction has high quantum yield.
 Causes :-
a) Reactions subsequent to the primary reaction:-
• One photon absorbed in a primary reaction dissociates one molecule
of the reactant.
• But the excited atoms that result may start a subsequent secondary
reaction in which further a molecule is developed.
AB + hv → A + B primary reaction
AB + A → A2 + B secondary reaction
One photon reaction has decomposed two molecules,
quantum yield of overall reaction is 2.

21. High quantum yield:-
• Reactions chain forms many molecules per photon:-
• Two oe more reactants, a molecule of one of them absorbs a
photon and dissociates ( primary reaction ).
• The excited atom starts a secondary chain reaction.
AB + hv → 2A primary reaction
A + B2 → AB + B secondary reaction
B + A2 → AB + A chain reaction
quantum yield of overall reaction is very high.

22. High quantum yield:-
 examples :-
I. Decomposition of HI.
II. Hydrogen- chlorine reaction.

23. Low quantum yield:-
• The number of molecules decomposed is less than one per
photon - ɸ < 1 then the reaction has low quantum yield.
 Causes :-
Deactivation of reacting molecules :-
• The excited molecules in primary process may be activated
before they got opportunity to react.
• This is caused by collisions with some inert molecules or by
fluorescence.
A + hv → A* activation
A* → hv + A fluorescence.

24. Low quantum yield:-
Occurrence of reverse primary reaction :-
• Primary reaction generally yields a polymer.
• The product then undergoes a thermal reaction giving back the
reactant molecules.
• The reverse thermal reaction proceeds until the equilibrium is
achieved.
2A A2

25. Low quantum yield:-
 Recombination of dissociated fragments :-
• In primary process the reactant molecules may dissociate to
give smaller fragments.
• These fragments recombine back the reactant.
(AB) + hv → A + B
A + B → (AB)
• The secondary reactants involving fragments to form product
will not occur.
• This will lower the yield.

26. Low quantum yield:-
 examples :-
I. Dimerization of anthracene
II. Combination of hydrogen and bromine molecule