Chapter Overview:
CO2 in the atmosphere - change over time
the earth's energy balance
the shapes of molecules
molecular vibrations and infrared absorptions
the carbon cycle - CO2 sources and sinks in the atmosphere
the concept of a mole
mass and mole calculations
greenhouse gases and global warming
Atmospheric Carbon Dioxide
atmosphere of early earth probably had high concentration of CO2
some CO2 was absorbed into the oceans as limestone
CaO (lime) + CO2 ----> CaCO3 (limestone)
large amounts of CO2 are converted by plants into carbohydrates (photosynthesis)
CO2 + H2O ----> (CH2O)n + O2
(n is usually 6 for sugars, cellulose, and wood)
the earth's CO2 concentration has stabilized at around 200 - 300 ppm
changes in CO2 concentration over time correlate with temperature (Fig. 3.1)
The Earth's Energy Balance
see Figure 3.2
solar energy arrives at the earth, some is reflected, some absorbed
the earth also radiates heat out (cools off)
a large part of the earth's radiated heat is reabsorbed in the atmosphere (~84%)
the reabsorption is caused by molecules that can absorb infrared light, which leads to
increased vibrational energy in the molecule (they heat up - the greenhouse effect)
the greenhouse effect appears to be increasing (see Figs. 3.3 and 3.4)
The Shape of Molecules
the electrons in a molecule are attracted to the nuclei but they repel one another
a molecule with two bonds arranges those bonds in a line - pointed away from one another
similar effects predict the structure of electrons around any atom in a molecule
2 groups of electrons - linear (180° angles away from one another)
3 groups of electrons - trigonal planar (120° angles away from one another)
4 groups of electrons - tetrahedral (109° angles away from one another)
a typical atom that obeys the octet rule by making 4 bonds will be tetrahedral
e.g., CH4, CCl4, CH2Cl2, etc.
atoms that make less than 4 bonds:
H2O - the two bonds and the two lone pairs are tetrahedral
and the H-O-H shape is described as bent (not linear)
CO2 - the two double bonds (4 electrons each) are arranged in a line
O3 - the middle oxygen has one double bond, one single bond, and one lone pair
so the 3 groups of electrons are trigonal and the O-O-O molecule is bent
Molecular Vibrations
bonds can vibrate, with a frequency that matches that of infrared radiation
absorption of IR causes a bond to increase its vibrational energy
CO2 has four distinct vibrations: 2 stretching modes and 2 bending modes
an IR spectrum shows which wavelengths are absorbed
for CO2, absorptions at 15 µm (bending) and 4.2 µm (stretching)
most molecules have bonds that can vibrate on absorbing IR radiation, and thus
can contribute to the greenhouse effect (e.g., CH4, H2O, NO)
some molecules do not absorb IR radiation (symmetric diatomics like O2 and N2)
individual atoms don't absorb IR radiation (no bonds to vibrate, e.g., Ar)
The Carbon Cycle
see Fig. 3.8
carbon takes on many forms, which interconvert chemically or biologically
generation of CO2: respiration of living things and decomposition of organic matter
(about 55 billion metric tons per year from each source)
absorption of CO2: photosynthesis by plants (about 110 billion metric tons per year)
the combination of these sources and sinks are in close balance
however, mankind also generates about 5 bmt of CO2 per year by burning fuels,
plus releases another 1 - 2 bmt per year in burning forests
the oceans provide a flexible sink and source of CO2
estimate that the oceans generate 90 bmt and absorb 93 bmt of CO2 per year
the net effect is still an increase in total CO2 in the atmosphere every year (about 3 bmt)
The Mole
the standard unit for counting up atoms or molecules
to make a convenient detectable amount
1 mole = 6 x 1023 units (atoms or molecules)
Avogadro's number (6 x 1023)
1 mole of atoms weighs the atomic mass in grams
1 mole of C atoms weighs 12 grams (see the periodic table - actually 12.011 g)
1 mole of O atoms weighs 16 grams
1 mole of CO2 molecules weighs 44 grams (12 + 16 + 16)
Calculations Using Molar Mass
balanced equations represent how many molecules react with one another
since moles are standard numbers of molecules, then balanced equations
also represent how many molecules react with one another
C + O2 ----> CO2
1 atom C + 1 molecule O2 ----> 1 molecule CO2
1 mole C + 1 mole O2 ----> 1 mole CO2
12 grams C + 32 grams O2 ----> 44 grams CO2
of every 44 g of CO2 , 12 g is C or carbon is 12/44 = 27% of CO2 by weight
the carbon cycle estimated 3 bmt of carbon added to the atmosphere
3 bmt of C = 27% of weight of CO2 added to the atmosphere
3 bmt / 0.27 = 11 bmt of CO2
try the Sceptical Chymist 3.13
another example - combustion of one gallon of gasoline (assumed to be all octane)
C8H18 + 12.5 O2 ----> 8 CO2 + 9 H2O
1 mole 12.5 moles 8 moles 9 moles
1 gallon of gasoline weighs 2,620 grams
2,620 grams per gallon / 114 grams per mole = 23 moles of octane per gallon
C8H18 + 12.5 O2 ----> 8 CO2 + 9 H2O
23 moles 287.5 moles 184 moles 207 moles
molar mass of octane = (8 x 12) + (18 x 1) = 114 grams per mole
molar mass of oxygen = (2 x 16) = 32 grams per mole
molar mass of carbon dioxide = 12 + (2 x 16) = 44 grams per mole
molar mass of water = (2 x 1) + 16 = 18 grams per mole
C8H18 + 12.5 O2 ----> 8 CO2 + 9 H2O
23 moles 287.5 moles 184 moles 207 moles
2620 grams 9200 grams 8096 grams 3726 grams
1 gallon ~1700 gallons ~1100 gallons ~ 1 gallon
Greenhouse Gases and Global Warming
methane is a major contributor to the greenhouse effect because it absorbs IR strongly
(it has more different vibrations than CO2)
sources of methane: natural gas seepage and release during drilling
anaerobic decay of organic matter, including landfills
(aerobic decay - with oxygen - gives CO2)
decomposition of cellulose (plant matter) by cows, sheep, termites
overall changes in climate are difficult to predict
possible reactions: study the situation some more
undertake some remediation starting immediately
prepare to accommodate to global warming