75-Rhenium Traditional Cache
Hidden : 1/29/2012
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: micro (micro)

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Geocache Description:

This cache was hidden to help fulfill the requirements for Boreal Walker's
Periodic Table of Elements Challenge
GC2P5TJ


Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. The free element has the third-highest melting point and highest boiling point of any element. Rhenium resembles manganese chemically and is obtained as a by-product of molybdenum and copper refinement. Rhenium shows in its compounds a wide variety of oxidation states ranging from -1 to +7. Discovered in 1925, rhenium was the last stable element to be discovered. It was named after the river Rhine in Europe. Nickel-based superalloys for use in jet engines contain up to 6% rhenium, making jet engine construction the largest use for the element, with chemical industry catalytic uses being next-most important. Because of the low availability relative to demand, rhenium is among the most expensive industrial metals, with an average price of approximately US$4,575 per kilogram, on 1 August 2011.
History
Rhenium (Latin: Rhenus meaning: "Rhine") was the last element to be discovered having a stable isotope (other new radioactive elements have been discovered in nature since then, such as neptunium and plutonium). The existence of a yet undiscovered element at this position in the periodic table had been first predicted by Dmitry Mendeleev. Other calculated information was obtained by Henry Moseley in 1914. It is generally considered to have been discovered by Walter Noddack, Ida Tacke, and Otto Berg in Germany. In 1925 they reported that they detected the element in platinum ore and in the mineral columbite. They also found rhenium in gadolinite and molybdenite. In 1928 they were able to extract 1 g of the element by processing 660 kg of molybdenite. The process was so complicated and expensive that production was discontinued until early 1950 when tungsten-rhenium and molybdenum-rhenium alloys were prepared. These alloys found important applications in industry that resulted in a great demand for the rhenium produced from the molybdenite fraction of porphyry copper ores. In 1908, Japanese chemist Masataka Ogawa announced that he discovered the 43rd element and named it nipponium (Np) after Japan (which is Nippon in Japanese). However, later analysis indicated the presence of rhenium (element 75), not element 43. The symbol Np was later used for the element neptunium.
Characteristics
Rhenium is a silvery-white metal with one of the highest melting points of all elements, exceeded by only tungsten and carbon. It is also one of the densest, exceeded only by platinum, iridium and osmium. Its usual commercial form is a powder, but this element can be consolidated by pressing and sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact solid having a density above 90% of the density of the metal. When annealed this metal is very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K; tungsten-rhenium alloys are also superconductive around 4-8 K, depending on the alloy. Rhenium metal superconducts at 2.4 K.
Isotopes
Rhenium has a stable isotope, rhenium-185, which nevertheless occurs in minority abundance, a situation found only in one other element (indium). Naturally occurring rhenium is 37.4% 185Re, which is stable, and 62.6% 187Re, which is unstable but has a very long half-life (~1010 years). This lifetime is affected by the charge state of rhenium atom. The beta decay of 187Re is used for rhenium-osmium dating of ores. The available energy for this beta decay (2.6 keV) is one of the lowest known among all radionuclides. There are twenty-six other recognized radioactive isotopes of rhenium.
Compounds
Rhenium has nine known oxidation states: -1, 0, +1, +2, +3, +4, +5, +6 and +7. The oxidation states +7, +6, +4, and +2 are the most common. The most common rhenium compounds are the oxides and the halides exhibiting a broad oxidation number spectrum: Re2O7, ReO3, Re2O5, ReO2, and Re2O3 are the known oxides, and ReF7, ReCl6, ReCl5, ReCl4 and ReCl3 are a few of the known halogen derivatives. Known sulfides are ReS2 and Re2S7. Skeletal formula of rhenium hydride described in the text. Rhenium hydride Reaction of rhenium with hydrogen produces the negatively charged hydride [ReH9]2- ion, which is isostructural with [TcH9]2-. It consists of a trigonal prism with Re atom in the center and six hydrogen atoms at the corners. Three more hydrogens make a triangle lying parallel to the base and crossing the prism in its center. Although those hydrogen atoms are not equivalent geometrically, their electronic structure is almost the same. The coordination number 9 in this complex is the highest for a rhenium complex. Two protons in it can be replaced by sodium (Na+) or potassium (K+) ions. Rhenium is most available commercially as the sodium and ammonium perrhenates. It is also readily available as dirhenium decacarbonyl; these three compounds are common entry points to rhenium chemistry. Various perrhenate salts may be easily converted to tetrathioperrhenate by the action of ammonium hydrosulfide. It is possible to reduce the dirhenium decacarbonyl Re2(CO)10 by reacting it with sodium amalgam to Na[Re(CO)5] with rhenium in the formal oxidation state -1. Dirhenium decacarbonyl may be oxidatively cleaved with bromine to give bromopentacarbonylrhenium(I), then reduced with zinc and acetic acid to pentacarbonylhydridorhenium: Re2(CO)10 + Br2 ? 2 Re(CO)5Br Re(CO)5Br + Zn + HOAc ? Re(CO)5H + ZnBr(OAc) Bromopentacarbonylrhenium(I) may be decarbonylated to give the rhenium tricarbonyl fragment either by refluxing in water: Re(CO)5Br + 3 H2O ? [Re(CO)3(H2O)3]Br + 2 CO or by reacting with tetraethylammonium bromide: Re(CO)5Br + 2 NEt4Br ? [NEt4]2[Re(CO)3Br3] + 2 CO Rhenium diboride (ReB2) is a hard compound having the hardness similar to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride. Rhenium was originally thought to form the rhenide anion, Re- , in which it has the -1 oxidation state. This was based on the product of the reduction of perrhenate salts, such as the reduction of potassium perrhenate (KReO4) by potassium metal. "Potassium rhenide" was shown to exist as a tetrahydrated complex, with the postulated chemical formula KRe·4H2O. This compound exhibits strongly reducing properties, and slowly yields hydrogen gas when dissolved in water. The lithium and thallous salts were also reported. Later research, however, indicates that the "rhenide" ion is actually a hydridorhenate complex. "Potassium rhenide" was shown to be in fact the nonahydridorhenate, K2ReH9, containing the ReH92- anion in which the oxidation state of rhenium is actually +7. Other methods of reduction of perrhenate salts yield compounds containing other hydrido- complexes, including ReH3(OH)3(H2O).
Occurrence
Rhenium is one of the rarest elements in Earth's crust with an average concentration of 1 ppb; other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust. Rhenium is probably not found free in nature (its possible natural occurrence is uncertain), but occurs in amounts up to 0.2% in the mineral molybdenite (which is primarily molybdenum disulfide), the major commercial source, although single molybdenite samples with up to 1.88% have been found. Chile has the world's largest rhenium reserves, part of the copper ore deposits, and was the leading producer as of 2005. It was only recently that the first rhenium mineral was found and described (in 1994), a rhenium sulfide mineral (ReS2) condensing from a fumarole on Russia's Kudriavy volcano, Iturup island, in the Kurile Islands. Kudryavy discharges up to 20–60 kg rhenium per year mostly in the form of rhenium disulfide. Named rheniite, this rare mineral commands high prices among collectors.
Production
Ammonium perrhenate Commercial rhenium is extracted from molybdenum roaster-flue gas obtained from copper-sulfide ores. Some molybdenum ores contain 0.001% to 0.2% rhenium. Rhenium(VII) oxide and perrhenic acid readily dissolve in water; they are leached from flue dusts and gasses and extracted by precipitating with potassium or ammonium chloride as the perrhenate salts, and purified by recrystallization. Total world production is between 40 and 50 tons/year; the main producers are in Chile, the United States, Peru, and Kazakhstan. Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons per year. Prices for the metal rose rapidly in early 2008, from $1000–$2000 per kg in 2003-2006 to over $10,000 in February 2008. The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures: 2 NH4ReO4 + 7 H2 ? 2 Re + 8 H2O + 2 NH3
Applications
The F-15 engine uses rhenium-containing second-generation superalloys Rhenium is added to high-temperature superalloys that are used to make jet engine parts, making 70% of the worldwide rhenium production. Another major application is in platinum-rhenium catalysts, which are primarily used in making lead-free, high-octane gasoline.

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