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

1

1 2

[ ][ ]

[

R

][ ][ ] [ ][ ]

ateof R1 k A B

d Bk A B k A D

dt

QUESTIONSQUESTIONS

1. Which would have the larger atomic radius: C or O?

2. What is the oxidation state of the carbon in methanol?

3. For the reaction A + B ↔ C + D, what would be the equilibrium constant if [A] = 2[D] and [C]=[B]

Chapter 2. ATMOSPHERIC PRESSUREChapter 2. ATMOSPHERIC PRESSURE

http://weather.unisys.com

““SEA LEVEL” PRESSURE MAPSEA LEVEL” PRESSURE MAP

ATMOSPHERIC PRESSUREATMOSPHERIC PRESSURE

Pressure is the weight exerted by the overlying atmosphere:

2:

F NP units Pa

A m

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 PRESSUREMEASURING ATMOSPHERIC PRESSURE

Measurement of atmospheric pressure with the mercury barometer:

Hg

5

Atmospheric pressure =

= 1.013x10 Pa 1013hPa

= 1013 mb

= 1 atm

B AP P gh

= 760 mm Hg (torr)

vacuum

A Bh

MASS MASS mma a OF THE ATMOSPHEREOF THE ATMOSPHERE

Radius of Earth:6378 km

Mean pressure at Earth's surface:984 hPa

Total number of moles of air in atmosphere:

20 1.8 10 molesaa

a

mN

M

Mol. wt. of air: 29 g mole-1 = .029 kg mole-1

kgg

PRm Surface

a18

2

10129.54

P=F/A

PRESSURE-GRADIENT FORCEPRESSURE-GRADIENT FORCE

P(z)

P(z+dz) Low Pressure

Fg = mg

Fp = [P(z)-P(z+dz)]A

Pressure gradient force goes from high to low pressure

slab of surface area A

High Pressure

BAROMETRIC LAWBAROMETRIC LAW(variation of pressure with altitude)(variation of pressure with altitude)

• Consider elementary slab of atmosphere:

P(z)

P(z+dz)[ ( ) ( )] a a

dPP z P z dz A gAdz g

dz

hydrostaticequation

Ideal gas law:a a

a

PM dP M gdz

RT P RT

Assume T = constant, integrate:

/( ) (0) with 7.4 km ( 250 K) z H

a

RTP z P e H T

M g scale height

Barometric law

/( ) (0) z Ha an z n e

unit area

Ma= .02897 kg/mole

SEA-LEVEL PRESSURE CAN’T VARY OVER MORE SEA-LEVEL PRESSURE CAN’T VARY OVER MORE THAN A NARROW RANGE: 1013 ± 50 hPaTHAN 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)

Vertical area dydz

Pressure-gradient force ( ( ) ( ))d P x P x dx dydz F

Acceleration 1 P

x

For P = 10 hPa over x = 100 km, ~ 10-2 m s-2 100 km/hr wind in 1 h! Effect of wind is to transport air to area of lower pressure dampen P

On mountains, however, the surface pressure is lower, as the pressure-gradient force along the Earth surface is balanced by gravity:

P(z)

P(z+z) P-gradient

gravity

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)

VERTICAL PROFILES OF PRESSURE AND TEMPERATUREVERTICAL PROFILES OF PRESSURE AND TEMPERATUREMean values for 30Mean values for 30ooN, MarchN, March

Tropopause

Stratopause

/( ) (0)

85 !

z HP z P e

z km

From 1000 hPa to 0.01 hPa:

REGIONS OF THE ATMOSPHEREREGIONS OF THE ATMOSPHERETroposphere:

• 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 vapour to 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

BAROMETRIC LAW: THE SEA-BREEZE EFFECTBAROMETRIC LAW: THE SEA-BREEZE EFFECT

reminder :

a

RTH

M g

~1 km~10 km

VERTICAL TRANSPORT: BUOYANCYVERTICAL TRANSPORT: BUOYANCY

Object (z

z+zFluid (’)

Imagine object of same density as fluid:

Note: Barometric law assumed a neutrally buoyant atmosphere with T = T’

' ' buoyant accelerationT T

P-gradient

Gravity

'

'P GF F Vg

Now look at force imbalance when density of object differs from surrounding fluid:

'

'B P G

B

F F F Vg Vg

g

If object is lighter

than fluid accelerate upwards

EXAMPLE: VENUS VS THE EARTHEXAMPLE: 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

a. How does the mass of Venus’ atmosphere compare to the Earth’s? (is it larger/smaller and roughly by how much?)

b. How does the scale height of Venus’s atmosphere compare to Earth’s (remember Venus’ atmosphere is mostly CO2)?