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03.envirchem.aos104f08.sld9

03.envirchem.aos104f08.sld9 - Section 3 Environmental...

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Unformatted text preview: Section 3 Environmental Chemistry 1 Environmental Chemistry • Definitions • Chemical Reactions ! Stoichiometry ! Photolytic Reactions • Enthalpy and Heat of Reaction • Chemical Equilibria ! pH ! Solubility 2 Introduction • Almost every pollution problem has some chemical basis ! From a chemical transformation or reaction or from the chemical properties of waste products • Some problems involving chemical reactions: ! Greenhouse gases ! Ozone Hole ! Urban smog ! Acid deposition ! Water pollution 3 Definitions • Atom ! 1 atomic mass unit (amu) = 1/12 the mass of one carbon-12 atom (1.66053886 × 10 –27 kg • Proton (charge = +1, m = 1 amu) • Neutron (charge = 0, m = 1 amu) • Electron (charge = –1, mass ~0) • Atomic weight • Isotope 4 • Molecule • Molecular weight • Mole ! Number of moles = mass/molecular weight ! 1 g-mol = 6.022 × 10 23 molecules ! 1 lb-mol = 2.7 × 10 26 molecules 5 Chemical Reaction Stoichiometry Balance those chemical reaction equations! Example 1 A 1.67 × 10 –3 M glucose solution (C 6 H 12 O 6 ) is completely biodegraded to carbon dioxide and water. How much oxygen is required (mg/ Ὑ )? Balanced equation: C 6 H 12 O 6 + 6O 2 ! 6CO 2 + 6H 2 O 6 ! From the amount of glucose we have in moles, it takes six times that amount in oxygen to decompose the glucose If this is the amount of oxygen in one liter of air, then the concentration in mg/ Ὑ is This oxygen demand given by stoichiometry is called the theoretical oxygen demand. 6 ! 1.67 ! 10 " 3 mol = 0.01 mol 0.01 mol ! ! 32 g mol ! 1000 mg g = 320 mg ! 7 Theoretical Oxygen Demand (TOD): oxygen needed to fully oxidize a quantity of organic material to carbon dioxide and water Biochemical Oxygen Demand (BOD): oxygen required for oxidation of organic wastes carried out by bacteria BOD ~ TOD 8 Photolysis • Photolytic reactions (also known as photodissociation) perform key steps in atmospheric chemistry • Photolysis occurs when a molecule absorbs a photon of light and decomposes • “Photochemical” describes a reaction or set of reactions that derive at least some of the energy needed for the reactions from sunlight 9 Energy in photons used in photochemistry are related to the wavelength of the light: E = energy of the photon (J) h = Planck’s constant (6.6 × 10 –34 J·s) З = frequency (cycles/s = Hz) c = speed of light (3 × 10 8 m/s) Е = wavelength (m) E = h ! = hc " 10 Example Energy per photon of light with wavelength 550 nm E = h ! = hc " E = hc ! = 6.6 " 10 # 34 Js ( ) 3 " 10 8 m s ( ) 1 10 9 m nm $ % & ’ ( ) 550 nm = 3.6 " 10 # 19 J 11 Enthalpy • We may need to determine the energy of a photon based on the enthalpy of the system • Enthalpy ( H ) is determined by the internal energy ( U ) and the product of the pressure ( P ) and volume ( V ) For a process with constant volume: For a process with...
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03.envirchem.aos104f08.sld9 - Section 3 Environmental...

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