Major Greenhouse Gases and its Characteristics Relative Current Atm. Annual greenhouse greenhouse Principal Gas Concentration Concentration efficiency contribution sources of gas (ppm) Increase (%) (CO2 = 1) (%) Fossil fuels, CO2 355 0.4 1 57 deforestation Foams Foams, CFCs 0.00225 5 15 25 Aerosols, refrigerants Wetland, rice, CH4 1.675 1 25 12 livestock, fossil fuel Nitrous Fertilizer, fossil 0.31 0.2 230 6 Oxide fuel Controversy is what this means for future Double CO2 ∆T = 6oC Efforts f Global W i Eff t of Gl b l Warming sheets, Melting of the Greenland and Antarctica ice sheets leading to substantial increase in sea levels. Intergovernmental panel on climate change (IPCC) reports sea-level rise up to 7 meters by 2040 if ice sheets continue to melt at this rate. Siberian permafrost has frozen carbon (1600 billion tonnes) in the soil twice atmosphere. -twice the amount in atmosphere Sea absorbs CO2 and increase in CO2 in atmosphere leads to increase in ocean acidity. With rising temperature, air circulation, ocean currents and rain patterns would change causing violent weather. 4 Summary The Earth’s Atmosphere Chemistry of Stratosphere Global W i d Cli t Change Gl b l Warming and Climate Ch 5 Photochemical Process Study of chemical reactions which are caused by the absorption of light radiation (or photons). In thermal chemical reactions, the activation energy, Ea results intermolec lar collisions from the random intermolecular collisions. In photochemical reactions, Ea is acquired by the absorption of photons f li ht i t d ith ti l l th of light associated with particular wavelength. Examples: - Formation of O3 from O2 in stratosphere - Synthesis of Vitamin D 6 Jablonski Diagram depicting various photochemical processes 7 p On absorption of p , j p photon, e- jumps from S0 electronic GS to S1, S2, S3 etc.., excited singlet states. A + hv A* For every singlet state, Sn, there exists a corresponding triplet state, Tn except T0 Corresponding Tn state is lower in energy than Sn. The activated molecule, A*, returns to the GS by dissipating its energy through various processes. Non-radiative t (1) N iti di ti transitions (2) Radiative transitions and (3) Chemical reactions 8 Non-Radiative Transitions • Internal Conversion (IC) - Transitions between states of same spin S2 to S1 T2 to T1 S3 to S1 T3 to T1 S4 to S1 etc.. - Radiation less transitions - E of excited molecule is dissipated in the form of heat thro’ intermolecular collisions. - life-time less than 10-11 s life time s. • Inter System Crossing (ISC) - Transitions between states of different spins S1 to T1 S2 to T2 S3 to T3 and so on. 9 Radiative Transitions • Fluorescence (F) - Transitions involving return of singlet ES S1 state to GS S0 S1 to S0 - Accompanied by emission of radiation is ll d in - S1 to S0 i allowed, i spectroscopy - life time is less than 10-8 s. • Phosphorescence (P) - Transitions from triplet ES T1 to GS S0 T1 to S0 - Accompanied by emission of radiation. spectroscopy. - Triplet T1 to S0 is forbidden transition in spectroscopy - life time is 10-3 s due to spin inversion. 10 • Chemical Reaction - Excited state lose energy by undergoing chemical reaction S1 to S0 is very fast T1 to S0 is slow, hence favorable for chemical reaction. Primary Photochemical Processes 11 • Photodissociation - Molecular fragmentation upon absorption of radiation Optical Dissociation - Direct excitation to a dissociative state Predissociation - Excitation to a stable state - Non-radiative transition 12 Laws of Photochemical Processes: (1) Grotthus-Draper Law (principle of photochemical activation) Only light which is absorbed by a system can bring about a photochemical change. (2) Start-Einstein Law (principle of quantum activation) Molecule is activated by absorption of one quantum of radiation in the primary step of the photochemical process. Suppose ν is the frequency of the radiation absorbed, then the corresponding quantum of energy absorbed per molecule will be, E = hv mole, Quantum of energy absorbed per mole E = N A hv = N A hc λ 13 , gy p g E, Energy absorbed per mole of the reacting substance is called Einstein. Quantum Yield: as, Quantum yield of photochemical process is defined as No. of molecules that react No. of moles that react φ= = q No. of quanta of radiation absorbed No. of Einsteins of radiation absorbed Quantum yield for the product formation is, No. of molecules of product formed No. of moles of product formed φ= = No. of quanta of radiation absorbed No. of Einsteins of radiation absorbed 14 The energy of monochromatic radiation (in terms of no. of Einsteins absorbed) can be measured by Actinometer. Quantum yields of photochemical reactions Photochemical λ (nm) φ Reaction 2NH3 N2 + 3H2 210 0.2 2NO2 2NO + O2 405 07 0.7 H2 + Cl2 2HCl 400 104 – 106 CO + Cl2 COCl2 400-436 103 3O2 2O3 170 190 170-190 3 15 Chemiluminescence: Emission of light as a result of chemical reaction and is f h t h i l i reverse of photochemical reactionti Fe Luminol 3-Amino phthalate - Used by forensic scientists by collecting blood samples. - Haemoglobin is Fe containing protein which acts as catalyst for the reaction.