Chapter 18 Gases and the atmosphere Chemical reactions in the atmosphere Ozone cycle Ozone depletion Greenhouse effect Global Warming Pollution in the troposphere 1 Composition of the atmosphere mole fraction moles of i xi total moles N2 and O2 represent 99 of atm For other components use parts per million ppm ppm xi 106 Eg Neon xNe 0 00001818 Ne concentration 18 18 ppm See table 18 1 mole fraction x 106 ppm ppm x 10 6 mole fraction 2 Composition of the atmosphere If you know the barometric pressure you can determine the partial pressure PBAR 0 987 atm PTOT PNe xNe PTOT 18 18 10 6 0 987 atm 17 94 10 6 atm Which atmospheric gas has the highest partial pressure Pressure and altitude Altitude km 0 50 100 200 P atm 1 10 3 10 6 10 13 collision frequency sec 1 1010 107 103 1 Low pressure means fewer collisions When collision frequency is low chemical reactions occur less frequently 4 Temperature Profile High energy radiation is absorbed Ions formed Density of gases is small Warming caused by ozone cycle Ultraviolet light absorbed Life where we live Weather 5 Regions of Atmosphere Based on temperature profile 1 Troposphere T as altitude Life where we live 2 Stratosphere T Ultraviolet light absorbed 3 Mesosphere T Density of gases low 4 Thermosphere T High energy radiation absorbed Present life forms could not survive above ground if all solar radiation reaches earth 6 Distribution of Radiation Shorter wavelength light cannot penetrate atmosphere as efficiently as longer wavelength light due to absorption Reactions in the atmosphere Photochemical Reactions reactions that involve the absorption or emission of a photon light Electronic excitation NO2 g h NO2 g means more reactive Photodissociation bond broken by light light absorbed O2 h O O Light must have sufficient energy to break bonds Bond energy for O2 495kJ mol What is the maximum wavelength light capable of photodissociating O2 Photoionization removal of an outer valence electron from a molecule by absorption of a photon 8 N2 h N2 e Penetration of Solar Radiation N 2 9 Thermosphere Ionosphere 10 Warming of the Stratosphere Ozone concentration peaks in stratosphere 25km concentration 10ppm Presence of ozone causes temperature increase with altitude in stratosphere Because ozone absorbs radiation UV 11 Ozone O3 O O O O O O Structure 2 resonance forms bent bond angle 117 bond length 1 28 O2 is 1 21 in agreement with VSEPR predictions Properties Light blue gas Pungent odor smell near electrical discharges Hf 142kJ mol reactive less stable than O2 12 Why is ozone so important Because ozone blocks UV Ozone cycle formation of O3 O2 h 2O 242nm O O2 O3 uv blocking by O3 O3 h O2 O 320nm The small amount of O3 in stratosphere reflects the delicate balance between creation and destruction Solar radiation is converted to heat 13 Problem Ozone depletion What causes it Chlorofluorocarbons CFCs Examples CFCl3 CF2Cl2 freon 11 freon 12 Properties Uses relatively inert aerosol propellants easily liquefied coolants for refrig Non combustible solvents for cleaning volatile polymer manufacturing evaporate and diffuse upward easily 14 How CFC destroys Ozone CF2Cl2 h CF2Cl Cl 240nm Cl O3 ClO O2 ClO O Cl O2 NET O3 O 2O2 Cl atom from CFC catalyzes O3 destruction speeds up reaction but is not used up 1 Cl atom destroys 2 million O3 molecules Global concentration of O3 has been on the decline since 1980 15 Greenhouse Effect Energy comes to earth from the sun Some is reflected by atmosphere Some is absorbed Most of uv and visible reaches the earth s surface causes warming Earth radiates heat back out mainly infrared energy 16 Greenhouse effect H2O and CO2 absorb and re emit IR light Absorption of infrared radiation keeps heat from being released back into space Without the greenhouse effect Earth would be 33 colder water ice CO2 added to the environment will cause 17 temperature change on earth The Carbon Cycle Sources of atmospheric CO2 Natural volcanic eruption sources decaying organic materials respiration Man made sources fuel combustion deforestation cement manufacturing Sinks for CO2 Formation of rock Dissolves in oceans Photosynthesis 18 Chemistry in the Troposphere Industrial Smog and Acid Rain Photochemical Smog Acid Rain Sulfur in atmosphere Sources bacterial decay of organic matter volcanic gases forest fires fossil fuels combustion industrial process In atmosphere 2SO g O2 g 2SO3 g SO3 g H2O g H2SO4 Natural Rain Acid Rain pH 6 pH of 4 to 4 5 effects pH of soil and water corrodes metals Fe dissolves stone marble limestone CaCO3 s H2SO4 aq CaSO4 aq CO2 g H2O 19 Photochemical Smog NO NO2 CO hydrocarbons O3 In auto engine N2 O2 2NO H 181kJ In air 1 2NO O2 2NO2 NO2 h 2NO O O O2 M O3 M 400nm 2 NO2 H2O HNO3 burns eyes Ozone good in the stratosphere but not in the troposphere Diminishes respiratory capabilities Reacts with NO to form NO2 and O2 Photodissociates to form reactive oxygen radicals which then react with hydrocarbons etc 20 Review What is ppm Trends of pressure temperature going up in atmosphere Photochemical reactions What does ozone do What causes green house effect and how Pollution acid rain and smog 21 Chapter 11 part 2 Properties of Liquids Viscosity Surface Tension Phase Changes energy of phase changes Dynamic Equilibrium Vapor pressure Phase diagram 22 Structure Effects Function Functional Group Boiling point Structure Hydrocarbon MW 72amu 36 C CH3CH2CH2CH2 CH3 Aldehyde MW 72amu 75 C Ketone MW 72amu 79 C amine MW 73amu 78 C CH3CH2CH2CH2 NH2 ether MW 74amu 34 C CH3CH2 O CH2CH3 Alcohol MW 74amu 117 C CH3CH2CH2CH2 OH carboxylic acid MW 74amu 141 C O CH3 CH2 CH2 C H O CH3 CH2 C CH3 O CH3 CH2 C OH 23 Viscosity Resistance of a liquid to flow is called viscosity It is related to the ease with which molecules can move past each other Viscosity increases with stronger intermolecular forces and decreases with higher temperature 24 Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid Surface molecules have fewer interactions Energy is minimized by minimizing the surface area 25 Properties of Liquids Review Viscosity resistance to flow IM forces viscosity Surface Tension energy needed to increase surface area IM forces surface tension 26 Energy of phase changes Endothermic Exothermic It requires energy to disrupt intermolecular forces Energy is released when intermolecular interactions are formed vaporization sublimation melting fusion condensation deposition freezing 27 Heat