Photo chemistry Power Point Presentation
- 1. PHOTO CHEMISTRY BY Dr.M.Prashanthi Assistant Professor Department of Chemistry GDCW,Karimnagar.
- 2. Photochemistry is the study of chemical reactions that proceed with absorption of light radiations by atoms or molecules and its effect on the substance. Light is a type of electromagnetic radiation, a source of energy. The light must be absorbed by a chemical substance in order for a photochemical reaction to take place. The study of photochemistry helps us to know the changes when a molecule absorbs radiations. INTRODUCTION
- 3. Photochemistry deals with the rate and mechanism of reaction taking place due to exposure of light (UV & Visible). (2000-8000 A0 ). The reactions which can be brought about by light radiations are called photochemical reactions. Chemical reactions occur only when a molecule is provided the necessary “activation energy”. In case of photochemical reactions light provides the activation energy. Introduction
- 4. Thermo chemical reactions Photochemical reactions Involves absorption or evolution of heat These involves absorption of light Can takes place in the dark or in the presence light Presence of light is compulsory Temperature affects the rates of these reactions These reactions are independent of temperature The intensity of light does not affect the rates of these reactions The intensity of absorbed light affects the rates of these reactions The free energy change for these reactions is always negative The free energy change for these reactions may be negative or positive Thermochemical activation is not of selective nature Photochemical activation is highly selective Differences between thermal and photo chemical reactions
- 5. All the Photo chemical reactions are governed by two basic laws known as Laws of Photo chemistry 1.Grotthus Draper Law 2.Stark-Einstein Law Laws of Photochemistry
- 6. “Only those radiations of light which are absorbed by a reacting substance are effective in producing chemical change.” According to this law, all the absorbed light radiations need not be used to bring the chemical reaction. Some are used to increase the kinetic energy of molecule while some are re-emitted. Grothus Draper Law
- 7. Einstein law of photochemical equivalence- In a primary photo chemical process, each atom or molecule is excited by the absorption of one quantum of radiation(photon). One mole of reactant absorbs one Einstein energy of radiation Stark-Einstein Law
- 8. A molecule acquire energy by absorbing photon as, A + hυ ---------> A* Thus energy of photon is, E = hυ υ- frequency of absorbing photon. h = plank’s constant = 6.624 × 10–34 J-sec The energy of 1 mol photon (ie. Einstein) is given by, E = Nhυ But, υ= C/λ. E = . . / 𝑵 𝒉 𝑪 𝝀 N= Avogadro’s no. = 6.023 × 1023 mol– . C= velocity of light = 3 × 108 m/s. A mole of photon can excite a mole of molecule. But if electromagnetic radiations are extremely intense, molecule may absorb two or more photons. A + 2 hυ---------> A* Hence this law is not valid for all condition.
- 9. It is defined as the ratio of number of molecules reacting in given time to the number of quanta absorbed in the same time. Ф = number of molecules participated in chemical reaction number of photons absorbed Quantum efficiency should be equal to unity. This is known as normal QE Quantum yield or Quantum efficiency (Φ)
- 10. The quantum yield lower than one or higher than one is called as abnormal quantum yield or efficiency. On this basis the Photo chemical reactions are classified in to three types- 1.Reactions having very high QY 2.Reactions having small QY 3.Reactions having very small QY Abnormal Quantum Yield
- 11. It is due to the fact that one quantum of radiation brings transformation of more than one molecule of the reactants. Ex. Free radical and Recombination reactions. The activated molecule initiates a chain of reactions as a result, several molecules undergo reactions although one Quantum of light is absorbed leading to high QY Reasons for High Quantum yield (Φ>1)
- 12. Photo chemical combination of H2 and Cl2 to form HCl. The QY of this reaction is found to be very high i.e.in the order of 104 to 106 . This is explained due to setting up of a chain mechanism. H2 + Cl2 ------------> HCl Examples for high QY hυ
- 13. In some reactions deactivation of activated molecule takes place in primary process before they undergo reaction and transfer to product. The product of primary process may react back to form reactants, Or Molecules may receive insufficient energy, thereby reaction does not occur. Reasons for Low Quantum Yield (Φ<1)
- 14. Photo chemical combination of H2 and Br2 to form HBr. The quantum yield of this reaction is about 0.01 to 0.02. H2 + Br2 ------------> HBr Examples for low QY hυ
- 15. Decomposition of HI and HBr The electronic spectrum of HI is found in UV region. Hence primary process is decomposition of HI. (Φ= 2). HI + hυ --------> H* + I* The H* and HI reacts in secondary process as, H* + HI ---------> H2 + I* I* + I* ---------> I2 Overall reaction is 2 HI + hυ --------> H2 + I2 As reaction proceeds quantum yield decreases because of thermal reaction. H* + I2 ---> HI + I* Examples of Photochemical Reactions
- 16. Primary process: HBr + hυ --------> H* + Br* Secondary process: H* + HBr ---------> H2 + Br* and Br*+ Br* ---------> Br2 As reaction proceeds quantum yield decreases because of thermal reaction. H* + Br2 ---> HBr + Br* Decomposition of HBr
- 17. 2HI----->H2 + I2 QY-2.0 2HBr---->H2 + Br2 QY-1.0 H2O2----> H2 + O2 QY-2.1 SO2 + Cl2 ---->SO2Cl2 QY-1.0 Reactions having small QY
- 18. Photo processes are of two types- 1.Photo Physical Processes 2.Photo Chemical Processes Photo Processes
- 19. If the absorbed radiation is not used to cause a chemical change, it is re-emitted light of larger wavelength. The three such photo physical processes which can occur are 1. Fluorescence 2. Phosphorescence 3. Chemi luminescence Photo physical Processes
- 20. By absorbing radiations, electrons are exited. During de-excitations- 1. Excited electron returns to normal state instantaneously called fluorescence. 2. Excited electron returns to normal state after some time called phosphorescence. These two phenomena are called Photo physical Processes. Photo Physical Processes
- 21. It is the phenomenon in which a substance absorbs light of shorter wavelengths (high frequency) and emits a part of it instantaneously at different wavelengths with out causing any chemical change. The substance which exhibits fluorescence is called fluorescent substance. Ex. Calcium fluoride, Quinine sulphate solution etc. Fluorescence
- 22. When a substance absorbs radiation of high frequency and emits light even after the incident radiation is cut off, the process is called phosphorescence. The substance which exhibits phosphorescence is called phosphorescent substance. Ex. Zinc sulphide, Alkaline earth sulphides etc. Phosphorescence is chiefly caused by ultraviolet and visible light. It is generally shown by solid substances. Phosphorescence
- 23. By absorbing radiations, electrons in the substance are excited. During de-excitations, if excited electron returns to normal state after producing some chemical change in the substance, it is called Photochemical Process. These processes takes place directly or indirectly under the influence of light radiations. Photochemical Processes
- 24. Photo chemical process is of following types- 1.Photo sensitization 2.Decomposition 3.Double decomposition 4.Isomerisation 5.Addition reactions 6.Polymerization 7.Displacement Examples of Photo chemical process
- 25. The reactions which take place in presence of sensitizer are called Photosensitized reactions. Sensitizer absorbs the radiation and transfers it to the reacting molecules. Ex. Dissociation of Hydrogen molecule UV light of wavelength 2537 A0 does not dissociate hydrogen molecule. Because it has more energy than dissociation energy. H2 +hυ ----------> No reaction In the presence of Hg vapour(acts as a sensitizer), hydrogen molecule dissociates in to H-atoms. Hg + hυ --------> Hg* Hg* + H2 --------> Hg + 2 H . Photosensitized Reactions
- 26. Photosynthesis reaction is sensitized by chlorophyll. Chlorophyll acts as sensitizer in process of photosynthesis. The interaction of CO2 and H2O in sunlight to form carbohydrates and oxygen. Both CO2 and H2O do not absorb visible light. Chlorophyll absorbs light from visible region and transfer the energy to these molecules. 6CO2 + 6H2 O --------------> C6 H12O6 + 6 O2 Photosynthesis Chll. hυ
- 27. The phenomenon of emission of visible light in a chemical reaction at ordinary temperature is called as chemiluminescence. The light emitted in a chemiluminescence reaction is also called cold light because it is produced at ordinary temperature. Ex. Light emitted by glow-worms Greenish glow of Phosphorous Chemiluminescence is reverse of photochemical reaction because light is given out instead of being absorbed. In Chemiluminescence excited product emits radiations of appropriate wavelength which lies in visible region of spectrum. CHEMILUMINESCENCE
- 28. It is a pictorial representation of the energy transitions caused by the absorption of a UV radiation by a molecule. When a molecule absorbs a UV radiation, it is excited to one of the excited singlet electronic states(S1 or S2). Most promotions are from S0 to the S1 state For each singlet excited state there is a corresponding triplet excited state(T1, T2, etc). M + hυ ------> M* M-molecule in G.S M*-molecule in E.S Jablonski Diagram
- 30. The activated molecule returns to ground state by loss of energy through any of the following processes- 1. Non- radiative transition It involves transition from , S2 → S1 or S3 → S1 Or T2 → T1 or T3 → T1 . It does not involve emission of any radiation. Hence called Non- radiative transition. It only involves emission of heat.
- 31. The energy of activated molecule may be lost through either of the following processes- i. Internal Conversion (IC) In this process energy is lost in the form of heat. It involves transition from, S3 → S2 or S2 → S1 Or T3 → T2 or T2 → T1 . It occurs in less than 10-11 second. ii. Intersystem Crossing (ISC) In this the energy is lost through transition between states of different spins. It involves transition from S3→ T3 , S2→ T2 Or S1→ T1 . Such transitions are forbidden.
- 32. 2.Radiative transition It involves transition from S1→ S0 or T1→ S0 . It involves emission of radiation. Radiative transition can be of two types- Fluorescence :- The emission of radiation from S1→ S0 is called Fluorescence. The emission of radiation occurs in 10-8 second. Phosphorescence:- The emission of radiation from T1→ S0 is called Phosphorescence. It is forbidden transition.
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