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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
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

questions
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]
atmospheric pressure
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
measuring atmospheric pressure

vacuum

h

A

B

MEASURING ATMOSPHERIC PRESSURE

Measurement of atmospheric pressure with the mercury barometer:

mass m a of the atmosphere
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

pressure gradient force
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

barometric law variation of pressure with altitude
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

sea level pressure can t vary over more than a narrow range 1013 50 hpa
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)

vertical profiles of pressure and temperature mean values for 30 o n march
VERTICAL PROFILES OF PRESSURE AND TEMPERATUREMean values for 30oN, March

From 1000 hPa to

0.01 hPa:

Stratopause

Tropopause

regions of the atmosphere
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
vertical transport buoyancy
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’

example venus vs the earth
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)?