Photochemistry sem 5 509

photochemistry ,photolysis ,photochemical reactions ,comparison between photochemical and thermal react ,difference between photochemical and thermal react ,quantum yeild ,quantum effficiency ,high quantum yeild ,low quantum yeild

1. Photochemistry CECH-509 UNIT-4A

2. 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.

3. 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.

4. 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.

5. 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

6. 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.

7. 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.

8. 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.

9. 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.

10. Laws of photochemistry :-

11. 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

12. 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 .

13. 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 .

14. High quantum yield :- Two or more molecules decomposes per photon - ɸ > 1 then the reaction has high quantum yield . Causes :- 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 → A 2 + B secondary reaction One photon reaction has decomposed two molecules, q uantum yield of overall reaction is 2.

15. 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 → 2 A primary reaction A + B 2 → A B + B secondary reaction B + A 2 → AB + A chain reaction q uantum yield of overall reaction is very high.

16. High quantum yield :- examples :- Decomposition of HI. Hydrogen- chlorine reaction.

17. 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.

18. 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 A 2

19. 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.

20. Low quantum yield :- examples :- Dimerization of anthracene Combination of hydrogen and bromine molecule