Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110

General Chemistry

Summer 2006

Lecture Notes::Lec 22_30 June

© R. Paselk 2006
 
     
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The Representative Elements, cont.

Group V, cont.

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 VII

Chemistry

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)
HF 565 -510 -159
HCl 427 -366 -96
HBr 363 -334 -81
HI 295 -291 -64

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.


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Last modified 30 June 2006