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???• Oct 25 The atmosphere, part 1 Ch. 8• Oct 27 Midterm … No lecture• Oct 29 The atmosphere, part 2 Ch. 8• Nov 1 Light, blackbodies, Bohr Ch. 9• Nov 3 Postulates of QM, p-in-a-box Ch. 9• Nov 5,8 Hydrogen atom Ch. 9• Nov 10,12 Multi-electron atoms Ch.10• Nov 15 Periodic properties Ch. 10• Nov 17 Periodic properties Ch. 10• Nov 19 Valence-bond; Lewis structures Ch. 11• Nov 22 Hybrid orbitals; VSEPR Ch. 11, 12• Nov 24 VSEPR Ch. 12• Nov 26 MO theory Ch. 12• Nov 29 MO theory Ch. 12• Dec 1 Putting it all together• Dec 2 Review for exam

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 earthPretty 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 OHOH + CH4 H2O + CH3

CH3 + O2 CH3OOCH3OO + NO NO2 + CH3OCH3O + O2 HCHO + HO2

HCHO + UV + 2O2 2HO2 + COCO + OH + O2 CO2 + HO2

NO2 + UV NO + OO + O2 O3

HO2 + NO NO2 + OH

Troposphere

NO2 + UV NO + OO + O2 O3

NO + O3 NO2 + O2

HO2 + NO NO2 + OH

Stratosphere

O2 + UV O + OO + O2 O3

NO + O3 NO2 + O2

O + NO2 NO + O2

Interesting note: the role of NO and NO2 in the troposphere is quite differentfrom 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

• 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

OH + NO2 HNO2

CH3COO + NO2 PAN

The radical chains are broken by reactions which form products such as nitric acid and PAN:

O3 + UV O* + O2

O* + H2O 2 OHOH + CH4 H2O + CH3

CH3 + O2 CH3OOCH3OO + NO NO2 + CH3OCH3O + O2 HCHO + HO2

HCHO + UV + 2O2 2HO2 + COCO + OH + O2 CO2 + HO2

Radicals!

• The NO comes from the reaction of N2 with O2 at high temperatures: – N2 + O2 2 NO H0

f(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 reducesuch air pollution by using better engine designs and technology

OrganicsO3

OH

H2O

H2O

Particles -soot, PM2.5

Urban films -grime on windows

Chemistry affects all aspects of the lifetimesand fates of pollution compounds.

Air pollution

NOx

For example, the uptake of water by organic particles to form haze on a polluted summer day: the oxidation state matters!

% Relative humidity

0 20 40 60 80 100

mo

les

H 2O

/ m

ole

of

org

anic

0

1

2

3

4

5

dodecaneoctanoloctanoic acid1,5-pentane-diolmalonic acid

Increasing O : C ratio

Oleic acid, and similar fatty acidcompounds, are present in animal and plant cells. These compoundsare observed in atmospheric particlesdue to ablation from leaves and from cooking

% Relative humidity

0 20 40 60 80 100

Mol

es w

ater

/ m

ole

olei

c ac

id

0

1

2

3

4

Water uptake by oleic acid before and after oxidation by ozone

CH3(CH2)7CH=CH(CH2)7COOH

Before exposure

After exposure

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 warmenough to sustain life becauseof a greenhouse effect due towater vapour and some CO2.

By increasing the concentrationsof greenhouse gases, we arealtering this effect

Most of the observed warming in the past 50 years is attributable to human activities