Photochemistry Part 3, for UG & PG Students
- 1. Shri Shivaji Education Society Amravatis Shri Shivaji Arts, Commerce & Science College Motala, Dist. Buldana
- 2. Mr. Bhaskarrao Subhashrao Bhise Assistant Professor Shri Shivaji Arts, Commerce & Science College Motala Dist. Buldana
- 3. • Beers law • Law of Photochemistry • Quantum Yield Content:-
- 4. Beers Law Beer ( 1852) extended the concept of Lamberts Law to the concentration of solution. Mr. B. S. Bhise Asst. Proff. Shri Shivaji College Motala Statement:- When a beam of monochromatic light passes throw a solution of absorbing substance, the rate of decrease of light with thickness of solution is directly proportional to intensity of incident light as well as concentration of solution.
- 5. Mathematical Expression:- - 𝑑𝐼 𝑑𝑥 Ꞓ I x C - 𝑑𝐼 𝑑𝑥 = α 𝐼. 𝐶 ---------------- ( 1) Rearrange equation ( 1 ) 𝑑𝐼 𝐼 = −α𝐶 𝑑𝑥 ------------------ ( 2) Integration eq. ( 2 ) between the limit I=I0 at x=0 and I=I at x=x, we get ∫ 𝑑𝐼 𝐼 = -α𝑐∫ dx Where, I= Intensity of incident light X= Thickness of medium α= Constant I I0 x 0 ------------------- ( 3) OR ln 𝐼 𝐼0 =- α𝐶 𝑥 OR 𝐼 𝐼0 =e --------- ( 4) −α𝐶 𝑥 ---------- ( 5) I=I0e -α𝐶 𝑥 -------- ( 6 )
- 6. According to equation the intensity of beam of monochromatic light decreases with increase in the ‘ x ‘ and concentration ‘ C ‘ of absorbing substance. Equation ( 4 ) can be written as 2.303𝑙𝑜𝑔 𝐼 𝐼0 =-α C x 𝑙𝑜𝑔 𝐼 𝐼0 = α 2.303 C x 𝑙𝑜𝑔 𝐼0 𝐼 = α 2.303 C x 𝑙𝑜𝑔 𝐼0 𝐼 = Є C x --------- ( 7 ) --------- ( 8 ) --------- ( 9 ) --------- ( 10 ) Where, Є= α 2.303 and called as molar absorption coefficient or molar absorptivity or molar extenction coefficient.
- 7. Limitation of Beers Law :- 1. The presence of impurities taht absorption. 2. Devation may occur solution may coloured, association, dissociates, hydrolysis, complex formation etc. 3. If solution highly concentrated 4. Deviation may be occur monochromatic light not used. 5. Devation may occur solution form polymerization. 6. Deviation may ocuur solutin is turbid, air-bubble or impurity 7. Deviation also obtain some experiment condition i.e. Ph, temperature,ionic strength. 8. At high concentration departures from beers law. 9. Deviation may occur if sample cells or curvettes not matched. 10. Deviation also occur if width of slit is not uniform.
- 8. Application of Beers- Lamberts Law Application of Beers –Lamberts Law 1. Determination of unknown concentration of a compound. 2. Comparison known and unknown solution concencentration. 3. Characterize a compound/solution by drawing absorbance spectrum. 4. Measurment of absorbance by using spectrophotometer.
- 9. Laws of Photochemistry There are two basic laws governing photochemical reaction a) Grothus- Draper Law & b) Stark- Einstein law A) Grothus- Draper Law This is first law of photochemistry and known as photochemical activation. Grothus ( 1817) and Draper ( 1841) is termed as Grothus-Draer Law. “ According to this law only light which is absorbed by a system can bring photochemical reaction. B) Stark-Einstein Law This law proposed by Stark ( 1908) and A. Einstin ( 1912) applied concept of quantum energy to photochemical reaction. This law also called as second law of photochemistry. “ According to this law photochemical reaction each molecule of reacting substance absorbed one quantum of radiation which leads to photochemical change. The energy ‘ E’ absorbed by each molecules given by.
- 10. E=hv -------------- ( 1) The energy absorbed per mole is given by E=N hv -------------- ( 2) E= N h 𝐶 𝜆 :. V= 𝐶 𝜆 OR Where, E= Energy in Einstein v=Frequency N=Avogadros number ( 6.0225 x 𝟏𝟎𝟐𝟑 ) h= Planks constant ( 6.6256 x 𝟏𝟎−𝟐𝟑 J sec C= Velocity of light ( 3 x 𝟏𝟎𝟖 m/sec) 𝜆= Wave length in meter
- 11. Quantum Yield It is defined as , the number of molecules reacting per quantum energy absorbed. Or The number of moles reacting per Einstein of energy absorbed. It is denotes by 𝝓. 𝝓= 𝑁𝑜.𝑜𝑓 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑟𝑒𝑎𝑐𝑡𝑖𝑛𝑔 𝑖𝑛 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒 𝑁𝑜.𝑜𝑓 𝑞𝑢𝑎𝑛𝑡𝑎 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒. 𝝓= 𝑁𝑜.𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑟𝑒𝑎𝑐𝑡𝑖𝑛𝑔 𝑖𝑛 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒 𝑁𝑜.𝑜𝑓 𝐸𝑖𝑛𝑠𝑡𝑒𝑖𝑛𝑠 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒. Experimental Determination of Quantum Yield:- Determination of quantum yield of photochemistry reactions, there are two types as follow. 1. Determination of number of moles reacted It is determined by analytical methods used in chemical kinetics. Knowing the concentration of reactant before and after the reaction, number of moles can be calculated.
- 12. 2. Determination of number of Einsteins Absorbed Determination of quantum yield by given diagram- 1. Source of light:- The light used may be sunlight, arc lamp. Mercury vapour lamp, Discharge tube, Tungsten lamp. 2. Monochromator:- It is made up of gelatin or coloured glass or transparent plates with metal films. It absorbed unwanted wave length of light and transmit required wave length. Prisms and gratings also used as monochromator.
- 13. 3. Reaction Cell:- It is made up of glass or quarts with optically planes windows for the entrance and exit of light. It is placed in thermostate to keep constant temperature. 4. Detector:- 1. The light coming from reaction cell and fall on detector. It measure the intensity of transmitted light. 2. First light passed through empty cell and measure its intensity ( Ia). Then light passed through reaction cell and measure intensity of transmitted light ( I). The difference between two reading give intensity of absorbed light. 3. Measurment of intensity of light by using thermopile or Chemical actinometer. a) Thermopile:- 1. It is multi-junction thermocouple consist of Ag and Bi connected with a moving coil galvanometer. The metal strips are blacked with lamp black or platinum black. 2. The radiation fall on metal strips is absorbed completely and convert into heat. Increases temperature and generate current and measure difference temperature. 3. The current produced is proportional to intensity of light. 4. Thermopile are calibrated with standerd light sources.
- 14. b) Chemical Actinometer:- 1. It consist of solution which affected by light. Light fall on solution photochemical reaction start up. 2. Photochemical reaction is directly proportional to intensity of light absorbed. 3. Common chemical actinometer is “Uranyl Oxalate” 4. Uranyl Oxalate contain 0.05 M oxalic acid and 0.01 M urynyl sulphate in water. 5. The chemical reaction take place as follow, 6. UO2 +2 + hv (UO2 +2)* 7. 2C2O4 + (UO2 +2)* CO + CO2 + H2O + UO2 +2 8. In the first step Chemical Actinometer absorb light and get activated, the activated chemical actinometer transfer its energy to reactant and fall down ground state. Oxalic acid decompose to products. 9. Remaining oxalic acid can be titrated against with KMnO4 solution. 10. The knowing no. of moles reacted and number of einsteins absorbed , quantum yield can be calculated. 11. The quantum yield obtained for some important photochemical reaction are given in table.
- 15. Quantum yield some imported photochemical reaction. Reaction Quantum Yield ( 𝞍) 2 NH3 N2 +3H2 2HI H2 + I2 2HBr H2 + Br2 H2 + Cl2 2HCl CO + Cl2 COCl2 SO2 + Cl2 SO2Cl2 2NO2 2NO + O2 H2S H2 + S 3O2 2O3 CH3COCH3 CO + C2H6 Maleic Acid Fumaric Acid H2 + Br2 2HBr 0.2 2 2 104 - 106 103 1 0.7 1 3 0.3 0.04 0.01
- 16. The evident from previous table the law of photochemical equivalence is directly strictly valid for few reactions. Various photochemical reaction can be devided into three categories as follow 1. The quantum yield is a small interer such as 1, 2 or 3 High Quantum Yield reaction Ex.Combination of SO2 and Cl2, Dissociation of HI or HBr, Ozonolysis of oxygen. 2. The Quantum yield is less than 1 called as Low quantum yieldreaction. Ex. Dissociation of NH3, NO2 or acetone vapour and transformation of malic acid to fumaric acid. 3. The Quantum yield is extremely high. Ex. Combination of CO and Cl2, H2 and Cl2.
- 17. High and low quantum yield explained by Bondenstein following two processes. 1. Primary Processes:- Consider molecule A is absorbed light and get exited in higher energy level A* ( Ground State) A + hv A* ( Exited State) The molecule absorb light get excit but light dissociated to get down and formation of atoms or free radiacals. 2. Secondary Processes:- 1. This process is not related absorption of light. 2. Formation of free radicals in the primary process reaction carry chain reaction or deactivation of excited species. 3. As a result quantum yield increases or decreases and treaction does not follow photochemical equivalence.
- 18. 3. Reason for high quantum Yield a) Chain Reaction:- In primary process formation of free radicals and continue chainreaction in secondary process to increases in quantum yield. Photochemical reaction of combination of H2 and Cl2 have very high quantum yield. Chain reaction as follow, i) Cl2 + hv Cl. + Cl. Primary Process ii) Cl. + H2 HCl + H. Secondary Process iii) H. + Cl2 HCl + Cl. Secondary Process The secondary process (ii) and (iii) alternately addition one by one to formation of number of HCl molecules. Stop the chain reaction by using activated hydrogen and activated chlorine atom or impurity of oxygen like O2. b) Intermediate formed product may be act as catalyst. c) Secondary reaction may be exothermic and released energy start the reaction.
- 19. Diagram-2 Diagram-1 d) Collision of activated molecules with other molecules i.e. transfer of energy from activated molecules into another and get activated to form products. Reason for low quantum yield:- 1. Insufficient energy for activate molecules and does not activate . 2. Primary process may be reversed 3. Dissociated fragments may be recombination to form original molecule ( Starting) 4. Activated/Excited molecule loss before energy by collision with non excited mole. 5. The excited molecule deactivate before formation of products.