Exploring the World of Photochemistry with Mr. Ajit K. Dhas at Deogiri College, Aurangabad
Dive into the intriguing realm of photochemistry as Mr. Ajit K. Dhas from the Department of Chemistry at Deogiri College, Aurangabad, unravels the interactions between matter and light radiations. Discover the differences between thermal and photochemical processes, explore the laws of photochemistry, and understand the significance of photochemistry in various natural phenomena like the synthesis of ozone, vitamin D, and other essential processes.
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Topic :- Photochemistry - I Mr. Ajit K. Dhas Department of Chemistry Deogiri College Aurangabad 1
B. Sc. III (V Semester) Photochemistry - I
Outline Interaction of radiation with matter, Difference between thermal and photochemical processes, Laws of photochemistry, Grothus - Drapper law, Stark- Einstein law,
Introduction Definition: Photochemistry is the branch of chemistry which is mainly concerned with the interaction between matter & light radiations. Or It can also be defined as the study of the interaction of UV-Visible radiations with matter resulting into a physical change or into a chemical reaction.
Difference between thermal and photochemical processes Thermal reaction/process Involve absorption or evolution of heat. Can take place even in absence of light (dark reactions). Temperature has significant effect on the rate of a thermochemical reaction. The free energy change G is always negative. Spontaneous hence decrease in free energy. Can be accelerated by the presence of a catalyst. Photochemical reaction/processes Absorption or evolution radiations(UV or Visible). Light is necessary hence also called as light reactions. Intensity of light has significant effect on the rate of a photochemical reaction. The free energy change G may be positive or negative. of light reactions Can be accelerated by the presence of a photosensitizer. photosensitizer acts in a different way than acatalyst. e.g. peroxides. However a e.g. majority of chemical reactions are belonging to this class, such as decomposition of Calcium carbonate into calcium oxide and carbon dioxide at high temperature, decomposition of water molecules at high temperature. e.g. Photosynthesis, Synthesis of vitamin D under the skin on exposure to light, Bioluminescence, Synthesis of ozone in earth stratosphere from photochemical reaction of oxygen, Photo-degradation, Photo-alkylation, etc.
Significance of Photochemistry to understanding Various natural phenomenon. 1. 2. Synthesis of Ozone: In the earth stratosphere from photochemical reaction of oxygen. 3. Synthesis of Vitamin D: Natural synthesis of vitamin D in sunlight below the skin. 4. Phototherapy: Treatment of jaundice in new born babies. 5. Phototropism: Phototropism means the orientation of the growth of a plant towards the direction of light. Plant in a sunlight shows more growth compare to the plant in the shadow. 6. Photoperiodism: Physiological processes such as flowering in the plant is affected by length of exposure to sunlight. e.g. blooming in Sunflower 7. Photodynamic: Treatment of diseases with the help of photosensitive drugs 8. Bioluminescence: Glowing of Fire flies Photosynthesis: Synthesis of glucose by the plants.
Laws of Photochemistry Photochemistry branch is governed by following important laws: 1. Grotthus-Draper Law 2. Stark-Einstein s Law
Laws of Photochemistry Grotthus-Draper Law/ Law of Photochemical activation/ First law of Photochemistry : Only the light which is absorbed by a molecule can be effective in producing photochemical changes in the molecule. Reflected or transmitted light does not produce any chemical change. Calculation of absorbed light, Iabsorbed= Io It Where, Iabsorbed = Intersity of absorbed light Io=Intensity of initial radiations It=Intensity of transmitted radiations
Laws of Photochemistry Stark-Einstein s Law/ Law of photochemical equivalence/ 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. Or One quantum of radiations activate one molecule for a photochemical reaction. Energy absorbed by a molecule is equal to Planck's equations, Therefore, Energy = E = h = hc/ Where, E=Energy associated with a photon, h= Planck's Constant= 6.626 10-34 J.S or 6.626 10-27erg.S =Frequancy c=Speed of the radiations =Wavelength For 1 mole, Energy = E = Nh = Nhc/ = 1 Einstein Where, N=Avagadro s number= 6.023 1023
Laws of Photochemistry Stark-Einstein s Law: Energy = E = h = hc/ Where, E=Energy associated with a photon, h= Planck's Constant= 6.626 10-34 J.S or 6.626 10-27erg.S =Frequancy c=Speed of the radiations =Wavelength For 1 mole, Energy = E = Nh = Nhc/ = 1 Einstein Where, N=Avagadro s number= 6.023 1023 E= 6.023 1023 X 6.626 10-34 E=3.985x10-10 E= 3.985 x 10-10 X 3 x 108 / Jmol-1 E=11.97 x 10-2 / Jmol-1 =11.96 x 10-5/ Jmol-1 =11.96 x 10-2 / Jmol-1 = 1 Einstein The energy possessed by one mole of substance or the energy absorbed by one mole of reacting molecule is called one Einstein.
Problem 1 : 1. Calculate energy of radiation with wavelength 1000 A0. Solution: According to Planck's equation, E=h = Nhc Given, N= Avogadro's number= 6.023 1023 h= Pancks Constant= 6.626 10-34 J.S or 6.626 10-27erg.S C=Speed of radiations= 3 108 m/s or 3 1010 cm/s = 1000 A0 =1000 10-10 m= 1000 10-8 cm Therefore, E = 6.023 1023 6.626 10-27 3 1010 1000 10-8 E = 11.96 1012 erg/mole or 11.96 105 Joule/mole
Problem 2 : 1. If the value of an Einstein is 72 kcal, Calculate the wavelength of the light. Solution: According to Planck s equation, E=h = Nhc Given, N= Avogadro's number= 6.023 1023 h= Pancks Constant= 6.626 10-34 J.S C=Speed of radiations= 3 108 m/s E= 72 kcal = 72 x 4.184 x 103 = 301.24 x 103 J Therefore, = 6.023 1023 6.626 10-34 3 108 301.24 x 103 = 0.3974 x 10-6 = 3974 x 10-10 m=3974 A0
Problem 3: Calculate the value of the Einstein of energy for radiation of wavelength 4000 A0. Solution: Energy = E = h = Nhc/ = 1 Einstein h= Planck's Constant= 6.626 10-34 J.S c= 3 x 108 m/s = 4000 A0 = 4000 x 10-10 m = 4 x 10-7 m Therefore, Einstein = 6.023 1023 X 6.626 10-34 X 3 x 108 / 4 x 10-7 Einstein=11.96 x 10-2 / 4 x 10-7Jmol-1 = 2.99 X 105 J/mole = 0.7146 x 105 Cal/mole = 71.46 Kcal/mole
Interaction of radiation with matter When light radiations are allowed to fall on matter, some of the radiations are absorbed, some get reflected, some get transmitted and some get scattered. In photochemistry, we study the absorption and emission of light by matter. It consists of the study of various Photophysical Processes and Photochemical Processes (reactions).
Interaction of radiation with matter Photophysical Processes: e.g. Fluorescence, Phosphorescence, IC, ISC. Photochemical Processes (reactions):
Electronic Excitation UV and Visible light has a sufficiently high energy therefore, it can produce change in electron energy level i.e. called electronic excitation.
Electronic Excitation According to MOT in molecules electrons are filled in bonding & non bonding orbitals. The electronic transition occur from HOMO (Highest Occupied Molecular Orbitals)to LUMO (Lowest Unoccupied molecular orbitals).
Electronic excitations A triplet state is more stable than the p* A n t i b o n d i n g singlet state. Because in triplet state, electrons are unpaired - lesser inter electronic repulsion B o n d i n g p take place, while in singlet state electrons are G r o u n d S t a t e S 0 S i n g l e t S t a t e S 1 E T r i p l e t S t a t e T 1 paired, causing inter electronic repulsion and is unstable. Calculation of spin multiplicity of electrons= 2S+1, Where S= Electron Spin For Singlet state= 2x0+1=1 Since S=0 For Singlet state because electron spin is paired For Triplet State= 2x1+1=3 Since S=1 For Triplet state because electron spin unpaired
Selection Rule Selection Rule: 1. Spin selection rule: S = 0 allowed 2. Laporte selection rule: Spin selection Rule: Transition without change in multiplicities are allowed transitions while transitions with change in multiplicities are not allowed
Chemical bond energies: 100 1000 kJ/mol Light energies: 150- 600 kJ/mole Therefore UV and visible radiations are capable to produce chemical changes within the molecule. 600 kJ/mol 300 150 ULTRAVIOLET VISIBLE INFRARED 200 nm 400 nm 800 nm
