Midterm Matters
• any appeals regarding the test must be
communicated to Dr. Gentleman by
THURSDAY, NOVEMEBER 4
• dgentlem@utsc.utoronto.ca
Next week: Lab #4
What’s coming up???
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Oct 25
Oct 27
The atmosphere, part 1
Midterm … No lecture
Ch. 8
• Oct 29
The atmosphere, part 2
Ch. 8
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Light, blackbodies, Bohr
Postulates of QM, p-in-a-box
Hydrogen atom
Multi-electron atoms
Periodic properties
Periodic properties
Valence-bond; Lewis structures
Hybrid orbitals; VSEPR
VSEPR
MO theory
MO theory
Putting it all together
Review for exam
Ch. 9
Ch. 9
Ch. 9
Ch.10
Ch. 10
Ch. 10
Ch. 11
Ch. 11, 12
Ch. 12
Ch. 12
Ch. 12
Nov 1
Nov 3
Nov 5,8
Nov 10,12
Nov 15
Nov 17
Nov 19
Nov 22
Nov 24
Nov 26
Nov 29
Dec 1
Dec 2
More about our atmosphere
• Last time we discussed some aspects of
stratospheric chemistry
– This is mostly all about ozone
• Today we will look at some chemistry
which happens in the troposphere (below
about 12 km altitude)
Solar illumination on earth
Pretty much all the energy which drives the
earth comes as radiation from the sun
Light absorbed by ozone in
ozone layer
• In the troposphere most chemistry starts
with ozone and UV light … and often ends
with CO2
– For example, oxidation of methane, whose balanced
equation is: CH4 + 2 O2  CO2 + 2 H2O
O3 + UV  O* + O2
O* + H2O  2 OH
OH + CH4  H2O + CH3
CH3 + O2  CH3OO
CH3OO + NO  NO2 + CH3O
CH3O + O2  HCHO + HO2
HCHO + UV + 2O2  2HO2 + CO
CO + OH + O2  CO2 + HO2
NO2 + UV  NO + O
O + O2  O3
HO2 + NO NO2 + OH
Interesting note: the role of NO and NO2 in the troposphere is quite different
from in the stratosphere. At lower altitudes, there is not the correct UV radiation
(not enough energy, since it’s already been absorbed higher up!!)to form
O atoms from O2 … tropospheric O (and so O3) is formed from NO2
Stratosphere
Troposphere
O2 + UV  O + O
O + O2  O 3
NO2 + UV  NO + O
O + O2  O 3
NO + O3  NO2 + O2
O + NO2  NO + O2
NO + O3  NO2 + O2
HO2 + NO NO2 + OH
• So we see that NO reacts with radicals to make
NO2, which reacts with sunlight and oxygen to
make ozone
• Ozone reacts with sunlight and water to make
hydroxyl radicals (OH)
• Hydroxyl radicals react with most substances to
make oxidized products
• These reactions all involve formation and
destruction of free radicals… they are examples
of chain reactions
O3 + UV  O* + O2
O* + H2O  2 OH
OH + CH4  H2O + CH3
CH3 + O2  CH3OO
CH3OO + NO  NO2 + CH3O
CH3O + O2  HCHO + HO2
HCHO + UV + 2O2  2HO2 + CO
CO + OH + O2  CO2 + HO2
Radicals!
The radical chains are broken by reactions which
form products such as nitric acid and PAN:
OH + NO2  HNO2
CH3COO + NO2  PAN
• The NO comes from the reaction of N2
with O2 at high temperatures:
– N2 + O2  2 NO
DH0f(NO) = + 90.2 kJ/mol
(what would le Chatalier say?)
• Cars, trucks, buses, etc.
• Power plants and factories
• Lightning
• Hydrocarbons (such as methane) come
from transport, power plants, ruminant
animals, rice paddies, trash dumps, …
• Urban air pollution involves the same
chemistry as we just saw, but more
intense
• The timing of photochemical smog
generally tracks rush hours, and requires
sunlight and warm enough (> 180 C)
temperatures
Emitted from cars
Formed from NO + radicals
Formed from
NO2 + light
Formed from NO2 + radicals
We can help reduce
such air pollution by
using better engine
designs and technology
Urban films
-grime on windows
Particles
-soot, PM2.5
H2 O
Air pollution
NOx
O3
Organics
H2O
OH
Chemistry affects all aspects of the lifetimes
and fates of pollution compounds.
For example, the uptake of water by organic
particles to form haze on a polluted summer
day: the oxidation state matters!
moles H2O / mole of organic
5
Increasing O : C ratio
4
3
dodecane
octanol
octanoic acid
1,5-pentane-diol
malonic acid
2
1
0
0
20
40
60
% Relative humidity
80
100
Oleic acid, and similar fatty acid
compounds, are present in animal
and plant cells. These compounds
are observed in atmospheric particles
due to ablation from leaves and
from cooking
Water uptake by oleic acid before and
after oxidation by ozone
CH3(CH2)7CH=CH(CH2)7COOH
Moles water / mole oleic acid
4
After exposure
3
2
Before exposure
1
0
0
20
40
60
% Relative humidity
80
100
Amber Asad ( a UTSC student) measuring
how water is sorbed by oleic acid
And the result …
Organic compounds
+
Oxidants (ozone, OH, …)
+
Water
+
Sunlight
• We saw how organic carbon (ie- methane) is
transformed to carbon dioxide in the
atmosphere. Note that CO is formed, along the
way.
• Carbon monoxide is also a product of
“incomplete combustion”, in much the same
types of process as we saw earlier.
• CO is problematic because it is a poison to
oxygen-breathers like us. The way in which
oxygen is transported in the blood is by binding
to haemoglobin – a molecule in our red blood
cells. It turns out that CO is better at binding to
haemoglobin, so oxygen is blocked.
• Sulfuric acid is another important atmospheric
molecule: it is a primary source of acid rain
(along with nitric acid, which we saw before)
and, because it is very, very hygroscopic, it acts
as a condensation site for water vapour
• H2SO4 is formed in the atmosphere by the
oxidation of sulfur-containing compounds
(mostly from biological processes) to SO2, then
to the acid
• SO2 is a product of combustion and also is
emitted by volcanoes
• Once formed, SO2 reacts in the atmosphere:
• SO2 + OH  HSO3
• HSO3 + O2  HO2 + SO3
• SO3 + H2O  H2SO4
• The sulfuric acid may be in the gas phase
(where it will form water droplets) or dissolved
already
• Acid rain in this part of North America is mostly
this acid
Apart from air pollution, human introduction of
hydrocarbons and their oxidation products into
the atmosphere affects the carbon cycle
These compounds, especially ones such as carbon dioxide, methane and
halocarbons, can influence the heat balance in the troposphere by absorbing
heat radiation (infrared light) emitted from the earth (“global warming”).
The earth’s surface is warm
enough to sustain life because
of a greenhouse effect due to
water vapour and some CO2.
By increasing the concentrations
of greenhouse gases, we are
altering this effect
Most of the observed warming in the past 50
years is attributable to human activities