Helium
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naturally occurring in natural gas wells as a product of radioactive decay.
5.27 × 10–4% of atmosphere
inert gas shield in welding, breathing gas diluent, gas chromatography carrier gas, lowtemperature refrigerant
the only known substance to lack a S-L-G triple point.
the only known substance that can not be solidified at 1 atm.
Other Noble Gases
Seminal work: Bartlett, 1962
ΔHION O2 = 1175 kJ·mol–1
O2(g) + PtF6(g) → O2[PtF6](s)
ΔHION Xe = 1170 kJ·mol–1
Xe(g) + PtF6(g) → Xe[PtF6](s)
first true compound of a noble gas.
Compounds of Kr, Xe, and Rn now known.
only bonds to F, Cl, O, and N are stable.
compounds: XeF2, XeF4, XeF6, XeO3, XeOF4, MXeF7, M2XeF8 [M=Rb, Cs], XeO4, KrF2
Argon is the most abundant noble gas in Earth’s atmosphere (~1%)
Oxygen
Preparation
1. decomposition of less-stable metal oxides
2. decomposition of other oxygen-containing materials
3. electrolysis of water
4. distillation of liquid air
Chemical reactivity
1. reactions with main-group metals: ionic oxides (basic solutions)
2. reactions with nonmetals: covalent oxides (acidic solutions)
3. reactions with compounds: if the compound is composed of elements that combine with
oxygen, the compound will also react with oxygen to form the respective oxides.
Big Question 3
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Acid Rain
coal + O2 → CO2 + H2O + SO2
> 30 million tons of SO2 released into the atmosphere in the USA annually.
Pure, unpolluted rainwater has pH ≈ 5.5.
“Acid rain” has a pH below this. At pH < ~4.0, all vertebrates, most invertebrates, and many microorganisms are destroyed.
SO2 can be removed from power plant exhaust by “scrubbing”.
CaO + SO2 → CaSO3
Carbon Dioxide
complete combustion of fossil fuels and many other fuels produces CO2, the prototypical greenhouse
gas. (Other naturally occurring greenhouse gases include H2O and CH4.)
Carbon Monoxide
incomplete combustion of carbon compounds produces CO. Much CO is from automobile exhaust.
CO binds strongly to hemoglobin, the protein responsible for O2 transport in blood.
Hb + O2 º HbO2
KO2
Hb + CO º HbCO
KCO
Big Question 3
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Ozone: the “other” form of oxygen
 allotropes: different forms of an element in the same physical state
Ozone is a pollutant in the lower atmosphere, even though we don’t have enough of it in the upper
atmosphere!
In the upper atmosphere, UV light breaks bonds in O2, a process called photodissociation.
Marconi (1901) found evidence of free electrons in the upper atmosphere, formed by photoionization.
Free O atoms from photodissociation can combine with molecular oxygen to form ozone.
Meanwhile, ozone absorbs photons in the 240-310 nm range (UV), experiencing photodissociation.
If there were no ozone in the upper atmosphere, all of these UV photons would reach Earth’s surface.
Ozone Depletion
Freon-11 CFCl3 unreactive, water-insoluble
Freon-12 CF2Cl2 not removed from atmosphere in rainfall
e.g., CF2Cl2 + hν → CF2Cl· + Cl· (190-225 nm, 30 km altitude)
Cl· + O3 → ClO· + O2 (1)
rate = k (Cl)(O3); k = 7.2 × 109 M–1·s–1 @25 °C
2ClO· → 2 Cl· + O2
more Cl atoms free to attack more ozone
(2)
2×(1) 2 Cl· + 2 O3 → 2 ClO· + 2 O2
(2)
2 ClO· → 2 Cl· + O2
net:
Replacing CFCs: HFCs (hydrofluorocarbons; C–H replaces C–Cl)
Downsides:
 Some damage is already done and hard to fix
 Costly to replace CFCs with HFCs
 CFC substitutes less effective refrigerants than CFCs
Big Question 3
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