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CHEMISTRY CONCEPTS (LAST CLASS)

CHEMISTRY CONCEPTS (LAST CLASS). CHEMICAL THERMODYNAMICS : steps don’t matter  final state – initial state. CHEMICAL KINETICS : rates depend on series of elementary reactions that make up the reaction mechanism A + B  C k 1 A + D  B k 2. QUESTIONS.

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CHEMISTRY CONCEPTS (LAST CLASS)

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  1. CHEMISTRY CONCEPTS (LAST CLASS) CHEMICAL THERMODYNAMICS: steps don’t matter  final state – initial state CHEMICAL KINETICS: rates depend on series of elementary reactions that make up the reaction mechanism A + B  C k1 A + D  B k2

  2. QUESTIONS • Which would have the larger atomic radius: C or O? • What is the oxidation state of the carbon in methanol? • For the reaction A + B ↔ C + D, what would be the equilibrium constant if [A] = 2[D] and [C]=[B]

  3. Chapter 2. ATMOSPHERIC PRESSURE “SEA LEVEL” PRESSURE MAP http://weather.unisys.com

  4. ATMOSPHERIC PRESSURE Pressure is the weight exerted by the overlying atmosphere: • Average sea-level pressure (SLP): • ≡101.325 kPa • ≡1 atm • ≡1.013 25 bar • ≡1013.25 millibars (mbar, mb) or hectopascals (hPa) • ≈760.001 mm-Hg, 0 °C ≡760 torr • ≈1033.227  cm–H2O, 4 °C • ≈14.695 948  psi

  5. vacuum h A B MEASURING ATMOSPHERIC PRESSURE Measurement of atmospheric pressure with the mercury barometer:

  6. MASS ma OF THE ATMOSPHERE Mean pressure at Earth's surface: 984 hPa Radius of Earth: 6378 km P=F/A Total number of moles of air in atmosphere: Mol. wt. of air: 29 g mole-1 = .029 kg mole-1

  7. PRESSURE-GRADIENT FORCE Fp = [P(z)-P(z+dz)]A P(z+dz) Low Pressure P(z) High Pressure Fg = mg slab of surface area A Pressure gradient force goes from high to low pressure

  8. BAROMETRIC LAW(variation of pressure with altitude) • Consider elementary slab of atmosphere: P(z+dz) P(z) hydrostatic equation unit area Ideal gas law: Assume T = constant, integrate: Barometric law Ma= .02897 kg/mole

  9. SEA-LEVEL PRESSURE CAN’T VARY OVER MORE THAN A NARROW RANGE: 1013 ± 50 hPa Consider a pressure gradient at sea level operating on an elementary air parcel dxdydz: P(x) P(x+dx) Pressure-gradient force Vertical area dydz Acceleration For DP = 10 hPa over Dx = 100 km, g ~ 10-2 m s-2 a 100 km/hr wind in 1 h! Effect of wind is to transport air to area of lower pressure a dampen DP On mountains, however, the surface pressure is lower, as the pressure-gradient force along the Earth surface is balanced by gravity: P(z+Dz) P-gradient • This is why weather maps show “sea level” isobars; • The fictitious “sea-level” pressure at a mountainous site assumes an isothermal air column to be present between the surface and sea level (at T of surface site) gravity P(z)

  10. VERTICAL PROFILES OF PRESSURE AND TEMPERATUREMean values for 30oN, March From 1000 hPa to 0.01 hPa: Stratopause Tropopause

  11. REGIONS OF THE ATMOSPHERE • Troposphere: • generally homogeneous, characterized by strong mixing • decreasing T with increasing altitude from heat-radiating surface • near surface boundary layer exists (over the oceans ~1km depth), BL often cloud topped and can trap emissions • Tropopause: • serves as a “barrier” that causes water • vapourto condense to ice • “tropopause folding” where strat air intrudes • into lower levels  exchange mechanism • Stratosphere: • increasing T with altitude due to OZONE • causing heating from absorption of UV • Mesosphere: • absence of high levels of radiation absorbing • species and thus a T decrease • upper mesosphere and higher defines the • exosphere from which molecules and ions • can escape the atmosphere • Thermosphere: • rarified gases reach temperatures as high as 1200C by absorption of high energy radiation

  12. BAROMETRIC LAW: THE SEA-BREEZE EFFECT ~1 km ~10 km

  13. VERTICAL TRANSPORT: BUOYANCY Imagine object of same density as fluid: P-gradient z+Dz Object (r) Fluid (r’) Now look at force imbalance when density of object differs from surrounding fluid: z Gravity If object is lighter than fluid  accelerate upwards Note: Barometric law assumed a neutrally buoyant atmosphere with T = T’

  14. EXAMPLE: VENUS VS THE EARTH • Compare the atmospheres of Venus and Earth: • VENUS EARTH • g, m/s2 8.9 9.8 • Planet radius(km) 6100 6400 • Surface pressure, atm 91 1  • Temperature, T 700 250 • How does the mass of Venus’ atmosphere compare to the Earth’s? (is it larger/smaller and roughly by how much?) • How does the scale height of Venus’s atmosphere compare to Earth’s (remember Venus’ atmosphere is mostly CO2)?

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