|Lecture Notes::Lec 22_30 June
© R. Paselk 2006
The Representative Elements, cont.
Group V, cont.
- Oxygen: We have already studied much of the chemistry of oxygen in a variety of guises (oxides, oxyacids, water, etc.) .
- Two common allotropes, oxygen (O2) and ozone (O3). Both reactive gases, but ozone is an even more powerful oxidizing agent, doing immense amounts of damage in cities, but is protective in the stratosphere as an ultraviolet absorber.
- -2 (oxide) is the predominant oxidation state, but can also have -1 (peroxide) and -1/2 (superoxide). Positive oxidation states occur only in compounds with fluorine and are of mostly formal interest to us in the oxidation state rules.
- Oxygen is isolated from liquid air by fractional distillation, having a boiling point (-183 °C) slightly above that of nitrogen (-196°C).
- It is the most common element in the Earth's crust, and the third most common element in the Universe (after hydrogen and helium).
- It comprises about 21% of the Earth's atmosphere by volume.
- Nearly all free oxygen is a result of biological processes.
- As a powerful oxidizing agent free oxygen is used by the majority of organisms to provide energy via respiration.
- As a powerful oxidizing agent oxygen is also very toxic, and organisms have to have a variety of protective systems to survive in its presence.
- Most mineral ores are either oxides or sulfides.
- Many oxide ores are a result of biomineralization as a result of metabolic processes of microorganisms.
- Most human energy is obtained via combustion with oxygen, both metabolically and technologically.
- Sulfur: Sulfur is found as the free element, in oxide minerals (e.g. gypsum, CaSO4*2H2O) and combined with many metals as sulfides.
- Elemental sulfur exists in S8 rings under laboratory conditions. (Sulfur does not form strong pi bonds like oxygen, so the dimer is not stable, preferring the sigma bonds of the eight-membered ring.) At higher temperatures the rings open to form long chains, explaining the transition from the watery liquid formed upon melting to the viscous, "plastic" sulfur, that occurs upon further heating and then a liquid form again at yet higher temps. The Plastic sulfur may be captured via quick cooling (e.g. pouring into cool water) to get an elastic allomer, which gradually reverts to the normal brittle sulfur (recall demo).
- Sulfur oxides
- Sulfur dioxide dissolves in water to form slight amounts of sulfurous acid. The acid itself cannot be isolated, but its salts (sulfites and hydrogen sulfites can be).
- Sulfur trioxide combines with water to form sulfuric acid, the most important chemical of commerce.
- Sulfuric acid in its concentrated form has an immense affinity for water, and will extract the elements of water from carbohydrates etc.
- Selenium and Tellurium behave much like sulfur, selenides and tellurides forming sulfide analogs with metals, as well as combining with hydrogen and hydrocarbons to form noxious and toxic compounds analogous to hydrogen sulfide and mercaptans.
- Selenium is an essential micronutrient. Its biochemistry is only partially understood, but it is used to form an amino acid (#21) which is the selenium analog of cysteine, a sulfur containing amino acid, which is essential for the functioning of some enzymes.
Chemical Aside: Precambrian Chemistry - A Possible Chemical Explanation for Slowed Evolution in the Proterozoic
(to explore this further see A.D. Anbar and A.H. Knoll. Proterozoic Ocean Chemistry and Evolution: A Bioinorganic Bridge? Science 297 [16 August 2002] pp 1137-1141)
Most of Earth's history is in the precambrian. Remarkably, life seems to have originated early in the precambrian, as much as 3.8 Billion years ago (Ba). Its perhaps not surprizing that life would take a few hundred million years or so to create and tune up the immense complexity of metabolism and the genetic machinery of protein biosynthesis. But once that had occured, which must have been before the split between the three major branchs of life, why did evolution proceed so slowly? In particular I'd like to address the apparently slow evolution of the eucaryotes between about 2000 Ma and 600 Ma. A recent explanation is based on chemistry that we have been exploring in lecture and lab. In very rough outline the idea is that eukaryotes need trace amounts of heavy metals like Fe, Mo, Cu, etc. for their enzymes to function, and in particular need Fe and Mo for efficient use of nitrogen in the form of nitrate.
The argument is that at a certain level of oxygen, continental metal sulfides from the mantle will be oxidized to sulfate, which can then be reduced to sulfide by bacterial sulfate reduction which will then precipitate out the heavy metals as sulfides. For example (using simplified chemistry, and lactate as a metabolite):
Terrestrial: FeS(s) + 2 O2(g) Fe2+ + SO42-
Ocean floor (bacterial): 2 CH3CHOHCOO- + SO42- 2 CH3COO- + 2 CO2 + 2 H2O + S2-
Fe2+ + S2- FeS(s)
Obviously various geological factors (plate techtonics, mountain building, etc. also would play a role, as discussed in class, but it is interesting that such simple chemistry could have such an impact.
To explore the precambrian further you might like to visit my Precambrian web pages at the HSU Natural History Museum.
Group VI consists of Fluorine, Chlorine, Bromine, Iodine, and Astatine. All of the members of this group are non-metals (we will ignore astatine as its longest lived isotope has a half-life of about 8 hours, making its chemistry of little interest).
The halogens all occur as reactive, diatomic molecules, and are thus not found in the free state on Earth. They all form -1 halide ions (X-), which are found in sea water and various minerals.
Oxyacids: hypoXite, Xite, Xate, hyperXate. Acidity increases with oxygens. Why? Charge spread over larger species on the anion, stabilizing the salt vs. the acid in the dissociation equilibrium. Also expect acidity to decrease down Periodic table as halogens become more metalic.
Hydrogen halides: Acidity. Strong - weak: HI > HBr > HCl >> HF. Why? Bond energies decrease HI to HF, but enthalpies of hydration increase as well, as seen in the table:
Data from Zumdahl, Chemistry (6th ed.)
||Bond Energy (kJ/mol)
||Enthalpy of Hydration of X- (kJ/mol)
||Entropy of Hydration of X- (J/mol*K)
Note that the bond energies and enthalpies of hydration almost cancel in their effects, so the main differences must be due to entropy. So what of HF? It is a weak acid because the small size and high charge density of fluoride ion results in strong binding of water - bound water has decreased entropy.
HF reacts with silica to give silicon fluoride. It is thus used to etch glass, dissolve silica sands for soil studies, and remove silicates from mineral samples in geology studies.
© R A Paselk
Last modified 30 June 2006