Chemistry of Natural Resources

Chapter 2 - Ozone

Chapter Overview:

atomic structure

molecular structure and bonding

electromagnetic radiation

absorption of light

atmospheric chemistry and the ozone hole

chlorofluorocarbons

Atomic Structure

neutrons - neutral (uncharged)

protons - positively charged - same mass as a neutron

electrons - negatively charged - small mass (1/1838 as much as proton or neutron)

nucleus - contains protons and neutrons (essentially all the mass of an atom)

electrons occupy space around the nucleus (essentially all the volume of an atom)

atomic number - number of protons in the nucleus

atomic number uniquely defines the element

typically # of protons = # of electrons (charges balance, atom is neutral)

(if charges don't balance, the atom is charged, an ion )

The Periodic Table of the Elements

arrangement based on similarity of chemical and physical properties (Mendeleev)

energy levels - quantum theory - energy is quantized

electrons can exist only in specific energy states with fixed capacities

first energy level can only hold 2 electrons, next 8 electrons, etc.

valence electrons - electrons in the outermost (highest) energy state of an atom

the chemistry of an atom is primarily determined by the number of valence electrons

compare hydrogen (1 valence electron) and sodium (1 valence electron)

atomic weight sum of protons plus neutrons (electrons negligible)

isotopes - different forms of the same element with different atomic weights

e.g., hydrogen and deuterium, carbon-12, carbon-13, carbon-14

Molecular Structure

bonding - atoms like to have a full outermost shell of electrons

options: give away valence electrons, get extra electrons, share electrons

examples - H2 (sharing of two electrons), H2O (sharing of 8 electrons),

Na+ Cl- (transfer of one electron)

Lewis structures - represent electrons as dots about atoms

a bond is a pair of bonding (shared) electrons, represented as a line

octet rule - arrange electrons around atoms so as to fulfill an octet

including shared electrons (bonds) and unshared electrons

hydrogen does not follow the octet rule, only wants two electrons

examples - N2, O2, O3

Electromagnetic Radiation

includes all forms of oscillating electromagnetic fields: light, x-rays, radiowaves, etc.

wavelength ( lambda ) - distance between equivalent spots on the wave

frequency ( nu ) - number of waves moving past a given point per second

electromagnetic radiation moves at the speed of light ( c ) c = lambda x nu

the human eye can only detect a small part of the electromagnetic spectrum

visible light lambda ~ 400 - 700 nm

photons - particles of light energy (zero mass, moving at speed of light)

energy of a photon is related to its frequency E = h n = h c / l

Absorption of Light

when the energy of a photon exactly matches the energy levels available in a molecule,

the photon can be absorbed and its energy imparted to the molecule

radiowaves - very low photon energies, detection requires special equipment

microwaves - low photon energies, correspond to molecular rotational energies

infrared - intermediate energy, correspond to molecular vibrations

visible - high energy photons, correspond to electronic energy levels

ultraviolet - very high energy, correspond to electronic bond strengths

x-rays - extremely high energy, correspond to nuclear energy levels

Effects of Ultraviolet Light

oxygen (O2) absorbs UV at wavelengths below 242 nm and dissociates

O2 ----(hn)----> 2 O

ozone (O3) absorbs UV at wavelengths below 320 nm and dissociates

O3 ----(hn)----> O2 + O

between the above two processes, most of the UV from sunlight is absorbed

a small amount of UV above 320 nm comes through, but it is the least harmful

part of the UV spectrum

biological effects of UV are generally measured by DNA modification

compare Figs 2.4 and 2.5 - calculate where maximum danger of UV is

at 340 nm, high intensity but low sensitivity

at 300 nm, low intensity but high sensitivity

Atmospheric Chemistry

ozone concentration reaches a steady-state through the Chapman cycle of reactions:

(1) O2 ----(hn)----> 2 O

(2) O + O2 ------> O3

(3) O3 ----(hn)----> O2 + O

(4) O + O3 ------> 2 O2

O atoms are highly reactive and react almost immediately ( << 1 sec)

ozone is more stable and a typical molecule exists for about 100 - 200 seconds

note that steps 1 & 3 create O atoms, steps 2 & 4 remove O atoms

step 2 creates O3 , steps 3 & 4 remove O3

overall the maximum concentration of ozone occurs in the stratosphere (10 - 50 km up)

this is where step 1 is significant

other reactions that affect the ozone concentration:

uv photolysis of water H2O ----(hn)----> H + OH

H + O3 --------> OH + O2

OH + O3 --------> H + 2 O2

note that H and OH regenerate one another while they convert O3 to O2

i.e., they are catalysts for the removal of ozone

a more accurate description of the previous chain reaction is:

uv photolysis of water H2O ----(hn)----> H + OH

H + O2 --------> HO2

HO2 + O3 --------> OH + 2 O2

OH + O3 --------> HO2 + O2

in this case, HO2 and OH regenerate one another while they convert O3 to O2

the difference is that H atoms are reactive enough that they combine with O2 first

then HO2 carries out the catalytic decomposition (not H atoms)

free radicals - molecules with an odd number of electrons

can't follow octet rule, generally reactive and unstable

other free radicals also tend to affect ozone concentrations

NO + O3 --------> NO2 + O2

NO2 + sunlight --------> NO + O

O + O2 --------> O3

(the SST flies in the stratosphere and generates excess NO and NO2 there)

Chlorofluorocarbons (CFCs)

compounds composed of only carbon, fluorine, and chlorine (no hydrogen)

used as refrigerants, propellants, solvents, etc. trade-name Freon (DuPont)

useful because they are stable, nontoxic, nonflammable, almost inert

after use, they are released to the atmosphere

in 1985 ~ 0.6 ppb atmospheric concentration, increasing ~ 0.4% per year

only way to destroy CFCs is by UV photolysis in upper atmosphere

but this also provides a catalytic pathway for removal of more ozone

CF2Cl2 -----(hn)-----> CF2Cl + Cl

Cl + O3 --------> ClO + O2

ClO + O3 --------> Cl + 2 O2

note Figure 2.11 - where ozone is low, ClO is unusually high and vice versa

1987 Montreal Protocol - cut in CFC production worldwide (U.S. phase-out 1995)

The Antarctic Ozone Hole

graph on page 35 - measurements of total ozone over South Pole in October

winter in Antarctica (June - Sept) is coldest place on earth (-90°) and has no sunlight

water is frozen into ice clouds in the stratosphere

these ice crystals catalyze additional chemistry

return of the sun in October melts ice crystals, releases burst of reactive molecules that can serve as catalysts for ozone removal (e.g., Cl atoms)

weather conditions at the North Pole are similar but not as severe

Solving the Problem

replacements for CFCs

HFCs - hydrofluorocarbons

have no Cl atoms to interact with ozone

C-H bond is easily broken, HFCs can be intentionally degraded readily

adding more ozone (see the cartoon, p 65)

removing chlorine atoms (and their sources)